STIMULATED ELECTROMAGNETIC EMISSIONS BY
HIGH-FREQUENCY ELECTROMAGNETIC PUMPING OF THE
IONOSPHERIC PLASMA
T. B. LEYSER∗
Department of Physics and Astronomy, University of California, Los Angeles, U.S.A.
Abstract. A high frequency electromagnetic pump wave transmitted into the ionospheric plasma
from the ground can stimulate electromagnetic radiation with frequencies around that of the ionospherically reflected pump wave. The numerous spectral features of these stimulated electromagnetic
emissions (SEE) and their temporal evolution on a wide range of time scales are reviewed and
related theoretical, numerical, and simulation results are discussed. On long (thermal) time scales
the SEE constitutes a self-organization of the ionospheric plasma which depends on the interaction
of nonlinear processes in a hierarchy of time scales in response to the electromagnetic pumping.
Particularly, the appearance of the rich SEE spectrum is associated with the slow self-structuring
of the plasma density into a spectrum of magnetic field-aligned density striations. The dependence
of the SEE on electron gyroharmonic effects and the presence of density striations suggests that
the existence of a magnetic field in the plasma is important for plasma turbulence to dissipate into
non-thermal electromagnetic radiation during the long time quasi-stationary state of the turbulence
evolution.
Abbreviations: AA – anomalous absorption; AMP – arithmetic mean peak; BC – broad continuum;
BDE – broad dynamic emission; BDM – broad downshifted maximum; BSS – broad symmetrical
structure; BUM – broad upshifted maximum; BUS – broad upshifted structure; CW – continuous
wave; DM – downshifted maximum; DP – downshifted peak; DSEE – diagnostic stimulated electromagnetic emission; EB – electron Bernstein; ERP – effective radiated power; FAA – fast anomalous
absorption; FAS – field-aligned striation; FNC – fast narrow continuum; HF – high frequency; HFIL
– HF enhanced ion line; HFPL – HF enhanced plasma line; IA – ion acoustic; ISR – incoherent
scatter radar; LH – lower hybrid; LPC – low power continuum; MDR – multiple Doppler radar;
OTSI – oscillating two-stream instability; PIC – particle-in-cell; QCW – quasi-continuous wave;
SAA – slow anomalous absorption; SEE – stimulated electromagnetic emission; SNC – slow narrow
continuum; TOD – trapped oscillation decay; UH – upper hybrid; UM – upshifted maximum; UP –
upshifted peak; UWE – upshifted wideband emission.
1. Introduction
A powerful electromagnetic wave (pump wave) transmitted into the ionosphere
from the ground can excite a wide range of plasma processes. These processes
are studied with different diagnostic techniques, such as ground-based incoherent
scatter radar (ISR), coherent scatter radar, low power diagnostic waves, scintilla∗ On leave from Swedish Institute of Space Physics, Uppsala Division, Box 537, SE-751 21
Uppsala, Sweden.
Space Science Reviews 98: 223–328, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
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T. B. LEYSER
tion measurements with satellites, airglow measurements during nighttime, and in
situ measurements by sounding rockets and satellites. By far, the richest plasma
response to electromagnetic pumping has been seen by simply monitoring on the
ground the weak non-thermal electromagnetic radiation which is stimulated by
the pump wave in the interaction region at a few hundred kilometers altitude and
which has escaped from the ionospheric plasma. This pump-induced radiation has
been termed stimulated electromagnetic emissions (SEE). Commonly used terms
in the Russian literature are artificial ionospheric radiation or radio emission. The
used pump frequency f0 is of high frequency (HF), i.e., typically a few megahertz and near the electron plasma frequency fp in the pump–plasma interaction
region. The SEE commonly covers a spectral width of the order of 100 kHz, or
f/f0 ≈ 0.02, around the ionospherically reflected pump wave, but is also observed at harmonics and sub-harmonics of f0 . SEE has been detected for f0 at
least from 2.8 to 9.4 MHz. The SEE, thus, occurs at much higher frequencies than
the HF pump-excited secondary electromagnetic radiation in the VLF, ELF, and
ULF bands (Barr, 1998), which are not discussed in the present report. On the
other hand, the SEE occurs at much lower frequencies than the electromagnetic
emissions at optical frequencies, which are studied in experiments of HF pumpenhanced airglow (Bernhardt et al., 1989; Brändström et al., 1999). The present
review is focused on the experimental results of the SEE spectral and temporal
features whereas associated theoretical, numerical, and simulation results are, in
general, covered in less detail. This reflects our understanding of the SEE, which
does not yet match the level of richness and complexity of the experimental results
themselves.
SEE was discovered in 1981 in experiments at the HF pump facility Heating
near Tromsø in Norway (Thidé et al., 1982). With this finding it became possible to study HF pumped nonlinear processes in the ionospheric plasma directly
without the need of additional electromagnetic waves for probing, such as used in
radars. Also, the SEE technique resembles the situation when observing naturally
driven plasma turbulence processes, e.g., near planets and the sun, where one cannot use radar or other probe waves for sounding. Another emission phenomenon,
stimulated by short electromagnetic pulses of low average power transmitted into
an electromagnetically pumped region, was discovered earlier (Belikovich et al.,
1981; Terina, 1995), which is now referred to as the diagnostic SEE technique
as described in Section 3.2. The SEE and diagnostic SEE phenomena are related
in the sense that both involve the transformation of an electromagnetic wave into
electrostatic fluctuations and back to electromagnetic radiation. The two techniques
differ in that the direct SEE technique only requires the pump wave while diagnostic SEE utilizes an additional weak electromagnetic wave for probing, in a sense
similar to the radar technique. Since its discovery, SEE was studied extensively
at the Heating facility of the European Incoherent Scatter association (EISCAT),
Tromsø (Stubbe et al., 1982, 1984, 1985, 1994; Stubbe and Kopka, 1983, 1990;
Thide et al., 1983; Leyser et al., 1989, 1990; Rietveld et al., 1993; Stubbe, 1996),
STIMULATED ELECTROMAGNETIC EMISSIONS
225
at the Sura (Boiko et al., 1985; Erukhimov et al., 1987; Leyser et al., 1992, 1993,
1994; Bernhardt et al., 1994; Frolov et al., 1994, 1996, 1997a, b, d, 1998, 1999;
Sergeev et al., 1994, 1997) and Zimenki (Boiko et al., 1985) facilities in Russia,
Gissar facility in Tadzhikistan (Yerukhimov et al., 1987), Arecibo facility in Puerto
Rico (Thidé, 1984; Fejer et al., 1985; Thidé et al., 1989, 1995), and High Power
Auroral Stimulation (HIPAS) facility in Alaska (Armstrong et al., 1990; Wong
et al., 1990; Cheung et al., 1997, 1998).
The sensitivity of the SEE to details in the pump–plasma interaction conditions,
such as f0 , pump duty cycle, pump power, and pumping by additional waves,
makes SEE an important tool for studying nonlinear plasma response to electromagnetic HF pumping. Further, as observed already in the early experiments in
1981 the SEE exhibits a wide range of time scales (Thidé et al., 1983). Temporal
features from less than a millisecond up to tens of seconds for a given pump power
have been reported, thus spanning over four orders of magnitude. On the long time
scale the SEE appears as a complex self-organized response of the magnetized
ionospheric plasma in which the short time scale ponderomotive processes are
slaved by the slow structuring of the plasma due to thermal (involving particle
collisions) instability.
The slow structuring of the plasma into geomagnetic field-aligned striations
(FAS) is one of the most important phenomena in ionospheric HF pump experiments. The structuring of the plasma significantly increases the coupling between
the pump wave, as well as other radio waves propagating through the structured
region, and the plasma, as manifested, e.g., in the anomalous absorption of the
radio waves. The FAS are narrow filaments of mainly density depletions of a few
percents with a wide range (Erukhimov et al., 1987) of transverse dimensions l⊥
ranging from less than 1 m (Minkoff et al., 1974), via sub-kilometer scale (Basu
et al., 1997), to more than 1 km (Duncan and Behnke, 1978; Farley et al., 1983)
and elongated several tens of kilometers along the ambient magnetic field (Hedberg
et al., 1983). They were discovered already in the early experiments investigating
nonlinear properties of the ionospheric plasma by HF pumping (Thome and Blood,
1974; Fialer, 1974). As discussed below, long time scale SEE is closely connected
to the presence of small-scale FAS with l⊥ ≈ 1–10 m. The FAS can be viewed upon
as dissipative structure which is a response of the plasma to the pump energy flux
that results in an ordered behavior characterized by symmetry breaking and correlations over a macroscopic range. The small-scale FAS are a feature of the driven
upper hybrid (UH) turbulence (Gurevich et al., 1995a, b; Istomin and Leyser, 1997)
while the large-scale FAS are attributed to thermal self-focusing instability of the
pump wave (Guzdar et al., 1996, 1998).
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T. B. LEYSER
Figure 1. Schematic diagram of the plasma density profile in the daytime ionosphere, indicating
the plasma and UH resonance heights for a vertically incident electromagnetic pump wave with
f0 < fcrit . Taken from Leyser et al. (1990).
2. Ionospheric F-Region Plasma
The earths ionosphere is the transition region between the non-ionized atmosphere
and the essentially fully ionized magnetosphere. The combined effect of decreasing
density of neutral atoms and molecules with increasing altitude in the atmosphere
together with increasing intensity of ionizing solar ultra-violet radiation gives a
maximum plasma density during daytime at a few hundred kilometers altitude.
The corresponding maximum ionospheric electron plasma frequency, termed the
critical frequency fcrit , may reach 10 MHz or more. Most of the SEE experiments
have been performed in the ionospheric F-region, which contains the maximum
plasma density but is still weakly ionized. During daytime, the ratio of charge
particle to neutral particle concentration varies from approximately 10−8 at 100 km
to 10−4 at 300 km and 10−1 at 1000 km altitude. Near the daytime F-region peak
where the plasma density is the largest, the plasma consists of electrons and mainly
O+ ions.
A schematic diagram of the daytime bottomside ionospheric plasma density
profile is shown in Figure 1. The reflection height of a vertically incident electromagnetic wave in the ordinary mode with f0 < fcrit is indicated and typically
occurs at 200–250 km at the plasma resonance height where fp = f0 . Since the
daytime plasma near the F-region peak is weakly magnetized in the sense that
fp fe ≈ 1–1.5 MHz, the UH resonance height where the local UH frequency
STIMULATED ELECTROMAGNETIC EMISSIONS
227
fuh = (fp2 + fe2 )1/2 = f0 typically occurs only a few kilometers below the reflection height (fe is the electron gyro frequency). A spectral width of the SEE of
f ≈ 100 kHz around f0 , thus, corresponds to f/fe ≈ 7–10%. Since fp fe ,
the SEE frequency typically is f ∼ f0 ∼ fuh ∼ fp .
The scale height of the ambient plasma density profile in the bottomside F
region where the pump wave is reflected is typically 50 km, which is much larger
than the vacuum pump wavelength c/f0 ≈ 30–100 m (c is the vacuum light speed).
The geomagnetic field, which decreases with increasing altitude, has a scale height
of approximately 2200 km, which is significantly larger than that of the plasma
density. The angle θ of the geomagnetic field to the vertical plasma density gradient
varies from near 0◦ near the magnetic poles to near 90◦ at the equator.
With a typical electron temperature of 1500 K ≈ 0.13 eV, the electron Debye
length is about 0.5 cm and the thermal electron gyro radius is 2 cm. Further, the
β-value, which is the ratio of the particle pressure to the magnetic field pressure,
is low. Typically β ≈ 10−5 , which means that the geomagnetic field can be considered unperturbed by the plasma. The effective electron collision frequency is
νe ≈ 500 s−1 at a pump–plasma interaction altitude of 200 km and the thermal electron mean free path is about 1 km. The collision frequencies increase significantly
with decreasing altitude below about 150 km.
3. Experimental Technique
3.1. P UMP WAVE
The ionospheric HF pump facilities are electromagnetic wave transmitters with
large high-gain antenna arrays operating at frequencies between approximately
3–10 MHz, as dictated by the ionospheric fp and fe . In comparison with laboratory plasma experiments, the long lifetime of the ionospheric plasma and slow
transport rate implies that complex plasma processes can be excited at a wide
range of temporal and spatial scales with a moderate transmitted power of approximately 1 MW. A vertically transmitted pump wave with an effective radiated
power (ERP), which is the product of the transmitted power and the antenna gain,
of typically 250 MW corresponds to an energy flux of approximately 0.3 mW m−2
at 250 km altitude assuming free space propagation, which neglects ionospheric
absorption. This corresponds to the ratio of pump to thermal plasma energy of
ε0 |E0 |2 /(NkB Te ) ≈ 10−4 –10−3 , where ε0 , E0 , N, kB , and Te denotes the vacuum
dielectric permittivity constant, pump electric field, electron density, Boltzmann
constant, and electron temperature, respectively. Thus, although the HF pump wave
is commonly referred to as being powerful, the pumping is rather weak when
compared to the plasma thermal energy density. The linear absorption in the lower
ionosphere (D-region) may typically be 6–10 dB during daytime, and much lower
during nighttime. Maximum pump power in the ionosphere is therefore obtained
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T. B. LEYSER
Figure 2. Electric field of a vertically propagating ordinary mode wave in its reflection region. (a)
The magnitude of the electric field strength. (b) The electric field component perpendicular to the
ambient magnetic field. The wave frequency is f0 = 4.04 MHz, θ = 13◦ , fe = 1.35 MHz, the wave
reflection height is 230 km, and the background plasma density scale height is 50 km. The wave
electric field is taken to be 1 V m−1 at 100 km altitude. Taken from Leyser (1989).
for nighttime conditions, although the ionospheric plasma itself typically slowly
decays after sunset because of the lacking ionizing solar radiation.
The electric field distribution of a vertically transmitted ordinary mode electromagnetic wave can be obtained from general analytic formulas, which have been
derived within a uniform approximation and are valid throughout the reflection
region, albeit neglecting nonlinear effects (Lundborg and Thidé, 1985, 1986). Figure 2 displays the pump electric field for the case of f0 = 4.04 MHz, θ = 13◦ ,
STIMULATED ELECTROMAGNETIC EMISSIONS
229
which corresponds to the EISCAT-Heating facility, background plasma density
scale height of 50 km, and a reflection height of 230 km. With fe = 1.35 MHz the
UH resonance region is 5.6 km below the reflection height. Figure 2(a) shows the
magnitude of the electric field and Figure 2(b) shows the electric field component
perpendicular to the ambient magnetic field. It is seen that the parallel electric field
component undergoes swelling in the reflection region, i.e., the amplitude of the
electric field increases significantly at standing wave maxima successively closer
to the reflection height. The turning of the pump electric field from parallel to
the geomagnetic field at the reflection altitude to essentially perpendicular to the
geomagnetic field in the UH resonance region at a few kilometers lower altitude,
facilitates strong excitation in the entire altitude region from the plasma resonance
to well below the UH resonance height in high latitude experiments (Leyser et al.,
1990).
Different pump facilities differ mainly in the availability of diagnostic tools,
which will not be detailed here. Technical parameters for different facilities can be
found in the literature, for EISCAT-Heating (Rietveld et al., 1993), Arecibo (Fejer
et al., 1985), HIPAS (Wong et al., 1990), and Sura (Bernhardt et al., 1994). A factor
which directly influences the excited plasma processes is the different geomagnetic
field angle at the different latitudes of the facilities. The angle of the magnetic
field to the downward vertical at an interaction height of 200 km is θ ≈ 13◦ at
EISCAT, θ ≈ 14◦ at HIPAS, θ ≈ 19◦ at Sura, and θ ≈ 42◦ at the Arecibo facility.
For larger θ, swelling of the parallel electric field component is smaller and the
electric field turns from parallel to the geomagnetic field towards horizontally in
a larger distance below the reflection height (Leyser, 1991), so that excitation of
Langmuir turbulence within small angles to the magnetic field becomes relatively
more important compared to UH turbulence. The long time scale SEE intensity is
typically weaker in experiments at the mid-latitude Arecibo facility, compared to
that at higher latitudes (Thidé et al., 1989), which has been attributed to a higher
background noise level as well as the weaker excitation of FAS and related UH
phenomena at lower latitudes, due to the smaller perpendicular component of the
pump electric field in the UH resonance region.
Langmuir turbulence, excited most strongly in the plasma resonance region
driven essentially parallel to the geomagnetic field, and UH turbulence, excited
most strongly near the UH resonance region driven essentially perpendicularly to
the geomagnetic field, are very different. Langmuir turbulence involves basically
ponderomotive interactions between Langmuir and ion-acoustic (IA) oscillations,
and has been extensively studied experimentally in the ionosphere with primarily
ISR. UH turbulence involves in addition to UH oscillations, e.g., lower hybrid
(LH) waves instead of IA waves. Further, the magnetized electron dynamics perpendicular to the ambient magnetic field makes the threshold for instabilities involving thermal transport type processes lower than that involving ponderomotive
interactions, so that the UH turbulence includes both ponderomotive and transport processes for typical HF pump powers in ionospheric plasma experiments.
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T. B. LEYSER
Thus, whereas Langmuir turbulence includes dynamics at the short time scale of
electron fluctuations and slow time scale of ion fluctuations, UH turbulence generally, in addition, includes the much slower dynamics of the transport processes
so that the turbulence can be more complex. However, whereas aspects of the
transport processes, such as FAS, have been studied experimentally in some detail
by coherent scatter radars, the higher frequency aspects of UH turbulence, as well
as the interplay between processes at the widely different time scales, is largely
unexplored and remains a most important topic for future research. SEE measurements provide one way of accessing information about the UH turbulence, albeit
indirectly.
The experimental results discussed pertain to ordinary mode pumping, which
has been by far the most commonly used pump wave polarization. With ordinary
mode is meant the entire dispersion surface on which the wave electric field is
linearly polarized in the direction along the ambient magnetic field for the wave
vector perpendicular to the ambient magnetic field and left-handed circularly polarized (i.e., opposite to the electron gyro motion) when propagating parallel to the
magnetic field. Thus, at high latitudes where the magnetic field is directed nearly
vertically downwards, an ordinary mode electromagnetic pump wave is approximately right-handed circularly polarized when propagating upwards anti-parallel
to the geomagnetic field and left-handed circularly polarized when propagating
downwards after reflection in the ionosphere. For intermediate propagation angles
the wave field is elliptically polarized.
Unless otherwise stated, all experimental results in this review were obtained for
f0 < fcrit . Also, the SEE itself for ordinary mode pumping is strongly polarized
although the sense of polarization was not determined (Thidé, 1984), but it is likely
to be in the ordinary mode too. This is because of the significantly stronger SEE
excited for ordinary mode pumping than for extraordinary mode pumping (Thidé
et al., 1982, 1983), which suggests that the SEE is excited in the UH and/or the
plasma resonance region that occur at higher fp than that at the reflection height of
an extraordinary mode wave. Since extraordinary mode electromagnetic radiation
is evanescent above its reflection height it cannot escape this region even if it would
have been produced, whereas ordinary mode radiation can escape and be detected
on the ground.
3.2. P UMP MODES
SEE have been investigated by a variety of different pump modes (Sergeev et al.,
1994). The duty cycle of a sufficiently strong single pump wave at a given f0 determines the type of processes excited. For pumping, both slow (thermal) transport
and rapid ponderomotive processes are excited and interact in a quasi stationary
state. At pump-off the decay time of pump-excited small-scale FAS is of the order of seconds. For quasi-continuous wave (QCW) pumping, employing a much
shorter pump-off time than pump-on time (typically cycling the pump 180 ms
STIMULATED ELECTROMAGNETIC EMISSIONS
231
on/20 ms off), the intensity and dynamics of FAS is approximately the same as
for CW pumping, so that the growth and decay of the ponderomotive turbulence
itself can be studied during conditions of developed FAS. For QCW pumping it is
essential that the pump-off period is significantly shorter than the decay time of the
FAS, and on the other hand, is longer than the decay time of the ponderomotive
turbulence. At successively lower duty cycles the influence of different scales of
FAS on the ponderomotive turbulence can be singled out and studied, since FAS
of larger scales across the ambient magnetic field have longer growth and decay
times.
At sufficiently low duty cycles the average pump power does not exceed the
threshold for FAS excitation so that the ponderomotive turbulence can be studied
by itself in a locally homogeneous ambient plasma (Sergeev et al., 1994; Cheung
et al., 1997), although the instability threshold for ponderomotive turbulence in
homogeneous plasma is larger than that for slow transport turbulence. Another
purpose of low duty cycle experiments is to study the evolution of plasma turbulence during conditions which are independent of previous HF pumping, i.e., cold
pump-on. A pump duty cycle of 20 ms on/980 ms off corresponds to a duty cycle
of approximately 2%. Investigations of Langmuir turbulence with the Arecibo ISR
suggest that duty cycles of 0.5% or lower could be regarded as genuine cold starts
at full pump power at Arecibo (Sulzer and Fejer, 1994). Higher duty cycles up to
5% have been referred to as quasi-cold start (Fejer et al., 1991).
Further, when using such short pulses immediately following CW or QCW
pumping, the dynamics of the ponderomotive turbulence at different levels of FAS
can be studied, since the FAS slowly decay after the pump is switched from CW or
QCW mode to the pulsing diagnostic mode and the average power during the low
duty cycle pulsed pumping is sufficiently low to not significantly affect the FAS
intensity. For such a preconditioned plasma state, when pump-excited FAS already
exist, the SEE is excited within a few milliseconds, so that the diagnostic pump
pulses can be made very short (typically 20 ms) compared to the evolution time of
order of seconds for the FAS and associated long time scale SEE when no preconditioning by previous pumping has occurred. The SEE excited during such pulsed
pumping has been termed diagnostic SEE (DSEE) (Frolov et al., 1994; Sergeev
et al., 1994). The DSEE technique is also used in double pump experiments in
which the pulsed pump has a slightly different frequency than the high-duty cycle
principal pump.
With two simultaneously operated high-duty cycle pump waves separated by
typically at most a few tens of kilohertz (corresponding to f0 /f0 of a few percent), i.e., of the order of the bandwidth of the SEE spectrum itself, a different and
presently little understood excitation regime is employed, which may give rise to
new emissions as well as suppress emissions that exist for a single pump (Leyser,
1989).
A final distinct pump regime discussed in the present review is referred to as
additional pumping (Frolov et al., 1996, 1997b). One of the pump waves is used as
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T. B. LEYSER
a diagnostic wave with the frequency f01 and is transmitted in CW, QCW, or low
duty cycle mode. This pump wave excites SEE which is used to study the effect
of, e.g., FAS. The second pump wave with the frequency f02 is used to control
the intensity of FAS in the plasma interaction region of the first pump wave. To
avoid overlapping of the respective plasma and UH resonance regions of the two
pump waves, to reduce to a minimum, e.g., the mutual anomalous absorption of the
waves, the pump frequencies in this regime are separated by at least 200–300 kHz
(corresponding to (f01 − f02 )/fe ≈ 15–25%). With f02 > f01 the additional pump
is present in the interaction region of the first pump, whereas with f02 < f01
the additional pump reflects below the interaction region of the first pump. Often
both pump waves are operated with a high duty cycle. As the perturbations from
the second pump reach the interaction region of the first pump, through transport
processes or super-thermal electrons along the ambient geomagnetic field, the conditions for the FAS development and SEE excitation are changed. By monitoring
the SEE from the diagnostic pump the effects of the FAS or super-thermal electrons
can be studied.
3.3. SEE MEASUREMENTS
The SEE is commonly observed on the ground, e.g., with simple electric dipole
antennas. The SEE frequency spectrum can be monitored directly with a high
resolution spectrum analyzer. Preferably, the SEE signal is captured by means of
a digital sampling system which coupled together with a digital signal processor
also allows for real-time monitoring of the spectrum. The SEE intensity reaches
typically 50–60 dB below that of the received ionospherically reflected pump wave.
The temporal evolution of the SEE discussed in the present treatment is that of an
integrated intensity in a narrow frequency band (typically a few kilohertz which
corresponds to f/f0 ≤ 10−3 ) in selected parts of the SEE spectrum, which has
been measured with sensitive HF receivers (Derblom et al., 1989; Sergeev et al.,
1994, 1997).
The SEE is typically received at a site within at most a few tens of kilometers
from the pump facility, which is a much shorter distance than that to the ionospheric
interaction region at 200–300 km altitude. Although a few measurements have been
performed at larger angles to the vertically transmitted pump wave (Thidé et al.,
1982) no systematic investigations of the dependence of the SEE on the angle of
arrival have been reported.
4. SEE Spectral Features
In this section experimental results concerning different components of the frequency spectrum of the SEE are reviewed. The different SEE features can be associated with either a ponderomotive stage of the pump–plasma interaction, which is
STIMULATED ELECTROMAGNETIC EMISSIONS
233
obtained during the first few tens of milliseconds after pump-on for low pump duty
cycles, or to a mixed stage in which ponderomotive and thermal type processes
interact, which is obtained after several hundreds of milliseconds and for high
pump duty cycles also after shorter times.
The first SEE spectral feature discussed below is observed during low pump
duty cycles while all the following features are excited during high pump duty
cycles. Following Frolov et al. (1997d) a distinction of two different narrow continuum features is made, which exhibit different temporal evolutions. In the present
review these features are referred to as the fast narrow continuum (FNC), which
occurs at low pump duty cycles, and the slow narrow continuum (SNC), which
occurs at high pump duty cycles. In previous literature, the SNC was referred to as
the narrow continuum (Leyser et al., 1993). Although the spectral characteristics
of the FNC and SNC in the quasi-stationary phase on their respective time scales
appear similar, they are discussed separately in Sections 4.1 and 4.2, respectively.
4.1. FAST NARROW CONTINUUM (FNC)
The FNC is a skewed asymmetric spectral feature with more energy in the lower
than in the upper sideband of f0 , with the emission intensity decreasing with larger
shifts from f0 . It is the only spectral feature which occurs before FAS are excited
when pumping at a sufficiently low duty cycle or for a cold start, i.e., without
previous pumping. There exist a number of measurements which relate to the FNC.
It was found that intensive SEE at the downshifted frequency f− = −(f − f0 ) =
15 kHz is excited within a few milliseconds after pump-on for a cold start, with
f0 = 5.105 MHz and f0 = 5.75 MHz (Boiko et al., 1985). The FNC exhibits an
intensity overshoot within approximately 10 ms after pump-on (Boiko et al., 1985;
Sergeev et al., 1994, 1998; Waldenvik, 1994; Frolov et al., 1997d). The overshoot
is a pronounced emission intensity maximum at a finite time after pump-on after
which the intensity decreases. An early overshoot of the SEE was also found in
initial experiments at the Gissar (Erukhimov et al., 1987) and Tromsø facilities
(Thidé, 1985).
4.1.1. Low Pump Duty Cycles
A series of results from low duty cycle experiments at the Sura facility have been
presented (Frolov et al., 1997d; Sergeev et al., 1998). The purpose of such experiments with a duty cycle of the order of 1% or less is to minimize thermal
effects and associated density perturbations and to avoid cumulative heating effects
(preconditioning). When cycling the pump 15 ms on/985 ms off for 4.7856 MHz
with a pump ERP of approximately 75 MW it was found that the SEE at f− ≈ 8–
28 kHz reaches a pronounced maximum in tmax ≈ 1–8 ms, where shorter tmax
correspond to smaller f− (Sergeev et al., 1998). The overshoot of the FNC is
correlated with the occurrence of ponderomotive self-action of the pump wave,
which is typically a more than 10 dB decrease in the reflected pump wave power
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T. B. LEYSER
within 2–3 ms after pump-on. The SEE intensity maximum coincided with the
ending of the ponderomotive self-action of the pump at f0 . The ponderomotive
self-action of the pump wave was initially studied at the Tromsø facility (Fejer and
Kopka, 1981).
Further, the decay phase of the overshoot coincides with the appearance of
quasi-periodic oscillations in the reflected pump wave intensity (Frolov et al.,
1997d). The duration of the overshoot, which corresponds to a minimum in the
SEE intensity, occurs at tmin ≈ 10–20 ms for f− ≈ 8–11 kHz while tmin ≈ 30–
40 ms for f− ≈ 20–30 kHz. It is important to note that the overshoot of the
FNC and the associated significant decrease in the FNC intensity occurs before
the onset of fast anomalous absorption (FAA) of the pump wave, which has an efolding growth time of τFAA ≈ 0.3–0.5 s, and the associated growth of FAS with
l⊥ ≈ 3 m (Frolov et al., 1997d). Therefore, the overshoot of the FNC reflects a
property of the turbulence evolution itself and is not a consequence of FAA of the
pump wave and SEE.
For a longer pump pulse of 0.2 s every 10 s, it was found that a second weaker
overshoot occurred with the maximum intensity approximately 100 ms after pumpon (Sergeev et al., 1998).
With tmin varying between 10–40 ms depending on f− , the decay of the SEE
after a 15-ms pump pulse thus corresponds to different stages of the overshoot
for different f− . For f− = 8–11 kHz, at which pump-off and the SEE decay
occurs after the overshoot, the decay time after pump-off is τd ≈ 0.8–1.2 ms. For
f− = 22–28 kHz, at which pump-off occurs in the initial stage of the overshoot,
τd ≈ 1.8–2.0 ms. Therefore. it is concluded that τd decreases with the development
of the overshoot. It is an interesting result that, as mentioned above, the overshoot
of the FNC is a property of the turbulence itself as well as is associated with a
decrease in the decay time of the turbulence after pump-off. It can therefore not be
excluded that the cause of the overshoot and the decrease in the decay time is the
same (Sergeev et al., 1998). Further, the dependence of τd on f− decreases for
longer pump pulses and is almost absent for 100-ms and 200-ms pulses (repeated
every 5 s and 10 s, respectively, so that the pump duty cycle is 2%).
The dynamics of the FNC feature depends on the pump ERP (Frolov et al.,
1997d; Sergeev et al., 1998). The overshoot is only observed for sufficiently high
pump powers. The overshoot time tmax at a given f− increases with decreasing
−η
pump power Pe as tmax ∝ Pe , where η ≈ 0.4–0.5. Also, τd decreases by up to four
times with increasing Pe for Pe > PA . The value of PA is higher for shorter pump
pulses. For example, for a 200-ms pulse, PA ≈ 4 MW, while for a 15-ms pulse,
PA ≈ 18 MW. The largest τd , obtained at the lowest pump ERP, corresponds to
the damping rate of Langmuir turbulence due to electron–ion collisions. The τd
at higher pump ERP thus indicate non-collisional damping, the strength of which
depends on the intensity of the driven Langmuir turbulence. Further, comparing
results for similar experimental conditions in terms of pump power, interaction
height, and pump duty cycle, it was found that τd increased by 4–6 times in the
STIMULATED ELECTROMAGNETIC EMISSIONS
235
Figure 3. SEE obtained from low duty cycle pumping at the HIPAS facility. (a) Weak type SEE
spectrum for 125-ms pumping and 10-s inter-pulse period; (b) Strong type SEE spectrum for 125-ms
pumping and 10-s inter-pulse period. The ionospherically reflected pump wave at f0 is at 51.5 kHz
in the displayed spectra. Taken from Cheung et al. (1997).
transition from daytime to evening-time conditions, which indicates that the presence of photoelectrons in the daytime has a significant effect on the non-collisional
damping of the plasma turbulence (Sergeev et al., 1998).
Experiments with f0 = 2.85 MHz and ERP = 64 MW have been performed
at HIPAS Observatory using a pump duty cycle of 2.5% or less with 25–125 ms
pump duration and 0.25–10 s inter-pulse periods (Cheung et al., 1997). Two distinct types of SEE spectra were observed, referred to as the weak and strong type,
the latter of which is significantly different from those obtained during high duty
cycle pumping. The weak type of spectrum is characterized by broad, featureless,
asymmetric sidebands on both sides of f0 , as shown in Figure 3(a). Typically, the
SEE spectrum extends 25 kHz below and 15 kHz above f0 .
The strong type of SEE spectrum, shown in Figure 3(b), is characterized by
typically 20 dB higher SEE intensity, wider downshifted (typically 40 kHz) and
upshifted (typically 25 kHz) sidebands, as well as strong frequency modulations of
about 10–20 kHz. This type of SEE spectrum is reported to be excited exclusively
at very low pump duty cycles. Further, the occurrence of these modulated spectra
is intermittent in nature as compared to the non-modulated weak type of spectrum
which is very robust. However, no data on the temporal evolution and/or decay
times at pump-off are reported, which make the measurements difficult to compare
236
T. B. LEYSER
Figure 4. Spectrum of FNC for f0 = 5.828 MHz at 18.6 ms after pump-on from the Sura facility.
In the spectrum the pump at f0 was filtered out with a high pass filter with its -3 dB point at 1 kHz.
Notice that the emission also occurs above f0 . The HF pump was cycled 10 s on/50 s off. Taken from
Waldenvik (1994).
to previous experimental results. One question is during what stage in the overshoot
of the FNC the weak and strong type of spectra were recorded.
4.1.2. Higher Pump Duty Cycles
A detailed study of the early SEE spectral evolution has been presented for cycling
the pump 10 s on/50 s off with f0 = 5.828 MHz (Waldenvik, 1994; Frolov et al.,
1997d). During such a pump duty cycle of approximately 17% the plasma is preconditioned by thermal effects in that FAS from one pump period do not completely
decay away before the next pump period starts, at least not for sufficiently high
pump ERP. The measurements showed that the FNC appears within the first few
milliseconds and with a weak intensity overshoot which peaked at tmax ≈ 30–
50 ms. It was noted that the spectra are very fluctuating particularly the first 50 ms
after pump-on. Further, the time scale of the SEE overshoot is similar (Boiko et al.,
1985; Waldenvik, 1994) to the mini overshoot observed in ISR measurements of the
pump enhanced 430-MHz plasma line at Arecibo (Showen and Kim, 1978; Djuth
et al., 1986). However, it has been noted that a mini overshoot was not observed
at high latitudes, in experiments with the EISCAT-VHF (224 MHz) ISR (Stubbe
et al., 1992).
The SEE spectrum appears more symmetric around f0 approximately the first
10 ms after pump-on, extending less than ±5 kHz around f0 . This is more pronounced at lower pump powers (Pe = 1.2 MW and 5 MW, where Pe is the
effective pump ERP corrected for the linear D-region absorption). At later times
and for higher pump powers the SEE spectrum assumes the typical wedge shaped
STIMULATED ELECTROMAGNETIC EMISSIONS
237
FNC for which the lower sideband is more pronounced than the upper sideband,
although there are significant emissions also at upshifted frequencies as seen in
Figure 4. In Figure 4 f0 was filtered out with a high pass filter with its −3 dB
point at 1 kHz, so that the ionospherically reflected pump wave cannot be seen.
The FNC intensity decreases at approximately a constant slope of 1–3 dB/kHz
with decreasing frequencies below f0 and typically does not extend much below
40 kHz from f0 and somewhat less above f0 in the stationary state. The intensity for
frequencies above f0 is typically 10–20 dB lower than for downshifted frequencies
(Frolov et al., 1997d). In the Russian literature, the FNC has been referred to as the
narrow-band emission (Sergeev et al., 1994).
Following the initial overshoot at tmax ≈ 30–50 ms is a slow intensity decrease
of 6–10 dB during approximately the first 2 s after pump-on. It is interesting to note
that the intensity decrease as well as the time scale of this slow intensity decrease
of the FNC does not, or only very weakly, depend on the pump power, which is
contrary to the case of very low pump duty cycles discussed in Section 4.1.1.
4.2. S LOW NARROW CONTINUUM (SNC)
The SNC feature is excited for high pump duty cycles. Similar to the FNC, a well
developed SNC is associated with emissions both below and above f0 , albeit not
as far above as below f0 . This is contrary to the broad continuum (BC) feature
which is only associated with emissions below f0 , as discussed in Section 4.4. The
SNC can clearly be seen in the steady state SEE spectrum between the downshifted
maximum (DM) feature (see Section 4.5) and f0 when f0 is near or slightly above
sfe for which the BC does not develop (s = 3, 4, 5, . . . ). Here the SNC typically
extends 5–10 kHz below f0 (Leyser et al., 1993, 1994). When f0 approaches sfe
both the intensity and spectral width of the SNC decreases (Leyser et al., 1993;
Frolov et al., 1997d).
For f0 not near sfe , the SNC develops simultaneously in time with the BC and
DM features, indicating that the SNC is excited in the UH resonance region and
that small-scale FAS are involved in the excitation (Frolov et al., 1997d). However,
contrary to the BC and DM, the SNC does not exhibit an overshoot due to the
growth of 10-m FAS. An overshoot has not been observed for FAS with l⊥ ≥ 7 m,
while smaller scale FAS exhibit an overshoot after pump-on (Frolov et al., 1997c).
It may therefore be the case that the SNC is excited by the turbulence associated
with formation of the 10-m FAS.
Experimental results from the HIPAS facility at a low f0 = 3.349 MHz resulted in SEE spectra which, to the present author, appears to be an SNC feature
(Armstrong et al., 1990). The pump was cycled 2 min with the antenna elements
in phase and 2 min out of phase, resulting in a difference in ERP of approximately
7 dB. The SEE spectrum in Figure 5 shows the asymmetric spectrum extending
down to approximately 6 kHz below f0 and up to 2 kHz above f0 . The SEE has a
slow evolution time and exhibits a delay time of approximately 4 s after the pump
238
T. B. LEYSER
Figure 5. SEE spectrum for f0 = 3.349 MHz which resembles an SNC feature from the HIPAS
facility. Notice that the emission also occurs above f0 . Taken from Armstrong et al. (1990).
was switched to in-phase operation. It is interesting to notice the spectral peak at
f− ≈ 1 kHz, which resembles a downshifted peak (DP) feature (discussed in
Section 4.3). Further, it is found that the appearance of the SEE is correlated with
f0 matching the ionospheric critical frequency for the F1 layer and the appearance
of large-scale density irregularities (so called, spread-F).
4.3. D OWNSHIFTED PEAK (DP, nDP) AND UPSHIFTED PEAK (UP)
The DP is a narrow and usually strong emission downshifted by f− = fDP ≈ 1–
3 kHz from f0 and commonly sitting on a SNC extending 10–20 kHz below f0
(Stubbe et al., 1984, 1994; Stubbe and Kopka, 1990). The DP is very sensitive
to small changes in f0 and is only excited for f0 near sfe (Stubbe and Kopka,
1990; Stubbe et al., 1994). The strongest DP emissions have been observed in
experiments at Tromsø for f0 ≈ 4.04 MHz, which is near 3fe , but the DP has also
been seen at s = 4 and 5 (Stubbe and Kopka, 1990). Figure 6 shows the anomalous
absorption (AA) of a weak diagnostic wave at f0 − 70 kHz and SEE spectra for
different f0 near 3fe . For each f0 SEE spectra with two different frequency spans
are shown to highlight different spectral features. The DP can clearly be seen in
the SEE spectra to the left, where a weak DP is seen at fDP ≈ 3 kHz for f0 =
4.00 MHz (a). With the higher f0 in (b) and (c) the DP intensity grows and fDP
decreases. The DP intensity is maximum at the f0 for which the AA is minimum
(Stubbe et al., 1994), which corresponds to (c) in Figure 6. Figure 6 is further
discussed in Section 5.1.
An emission resembling the DP has also been observed in experiments at
Arecibo for f0 = 5.10 MHz (Thidé et al., 1989) and at HIPAS for f0 = 3.349 MHz
(Armstrong et al., 1990). However, it is important to investigate these emissions
by stepping f0 near sfe and compare with the characteristics of the DP discussed
above.
Occasionally the DP emission can be observed to be accompanied by a small
cascade of successively weaker nDP emissions at lower frequencies where for the
STIMULATED ELECTROMAGNETIC EMISSIONS
239
Figure 6. AA of a diagnostic wave with fd = f0 − 70 kHz and SEE spectra for different f0 near
3fe , from the Tromsø Heating facility. Taken from Stubbe et al. (1994).
2DP, f− ≈ 2fDP and for the 3DP, f− ≈ 3fDP (Leyser et al., 1990). Also,
occasionally an UP feature occurs upshifted from f0 at approximately the mirror
frequency of the DP (Stubbe et al., 1984). The UP is weaker than the DP and has
only been reported to be observed for f0 ≈ 4.04 MHz at the Tromsø facility.
In experiments at the Tromsø facility for a fixed pump power and f0 = 4.040
MHz it was observed that either the DP and SNC features appear or the DM (discussed in Section 4.5), with a gradual change between the two which depends on
the ionospheric conditions (Leyser et al., 1990). Commonly a mixture of the two
spectra were seen. It was noted that the DP and SNC occurred when the ionospheric
fcrit was well above f0 (at least a few hundred kilohertz). A similar observation was
made for the DM and DP in Arecibo experiments (Thidé et al., 1989). With our
present understanding that the DP occurs for f0 ≈ sfe (Stubbe and Kopka, 1990)
while the DM is not excited for f0 ≈ sfe (Leyser et al., 1989), and since fe decreases with increasing altitude, it appears reasonable to conclude that the relatively
240
T. B. LEYSER
Figure 7. Dependence of BC on f0 between 4fe and 5fe obtained at the Sura facility. (a)
f0 = 6.74 MHz; (b) f0 = 6.72 MHz; (c) f0 = 6.66 MHz; (d) f0 = 5.78 MHz. The spectral
peak at f− ≈ 10–15 kHz in (b), (c), and (d) is the DM. Taken from Leyser et al. (1993).
high fcrit implied a low pump reflection height for a given f0 , such that the local
fe increased and the condition f0 ≈ 3fe occurred in the interaction region. For
fcrit near f0 the pump reflection height is generally higher, implying that f0 > 3fe
and thereby favoring excitation of the DM. Thus, there is no contradiction (Stubbe
et al., 1994) between the observation that the DP is excited for f0 ≈ 3fe , and, on
the other hand, that the DP is excited for fcrit well above f0 when f0 = 4.040 MHz.
STIMULATED ELECTROMAGNETIC EMISSIONS
241
4.4. B ROAD CONTINUUM (BC)
The BC is a skewed SEE spectral feature which exists only at frequencies downshifted from f0 , as seen, e.g., in Figure 7(b) in which the BC extends down to
f− ≈ 70 kHz. The spectral peak at f− ≈ 12 kHz is the DM, which is discussed
in Section 4.5. The FNC and SNC, on the other hand, are associated with emissions both up- and downshifted from f0 . The BC has been observed for a wide f0
range, at least from 3.515 MHz to 8.00 MHz. In the Russian literature, the BC has
been referred to as the wideband, or broadband, component (Sergeev et al., 1994).
In the early literature, before the temporal evolution of the entire SEE spectral
structure was studied, no distinction was made of the BC, FNC, or SNC, but the
skewed spectral structure was simply referred to as the continuum (Stubbe et al.,
1984; Leyser et al., 1990).
In the stationary state the BC may extend significantly further below f0 than the
SNC, with frequency components down to f− ≈ 170 kHz (Leyser et al., 1993).
A well developed BC may exhibit a plateau which is a frequency range of a few
tens of kilohertz in which the emission intensity is approximately constant (Leyser
et al., 1993), which can be seen in Figure 7(b) for f− ≈ 20–40 kHz. Sometimes
the emission intensity at the plateau, which occurs at larger frequency downshifts
than the DM, is higher than that between the DM and f0 . For lower frequencies
than the plateau the emission intensity decreases exponentially with decreasing
frequency. Further, the evolution time after pump-on towards a steady state is of
the order of seconds for high ERP, with shorter growth time for higher pump power
and plasma preconditioned by previous pumping. The growth of the BC occurs on
the same time scale as that of 1-m scale FAS, whereas the BC intensity decreases
slightly with the slower growth of 10-m scale FAS (the intensity decrease is more
significant at larger downshifts from f0 ) (Sergeev et al., 1994). In high duty cycle
experiments (implying a preconditioned plasma) at the Sura facility the BC was
observed to develop in an initial overshoot which reached its maximum 1–7 ms
after pump-on (Sergeev et al., 1997).
The dependence of the BC on the pump power has been studied for f0 =
5.828 MHz and cycling the pump 10 s on/50 s off (Waldenvik, 1994). The pump
power was increased in steps after two consecutive pump cycles, i.e., every two
minutes. Taking into account the linear D-region absorption, the radiated pump
powers were Pe ≈ 1.2 MW, 5 MW, 10 MW, and 30 MW, which corresponds
to pump electric field strengths of approximately 0.5–2.5 V m−1 at the first Airy
maximum and 0.1–0.5 V m−1 at the UH resonance height. The BC did not appear
during the 10-s pump periods for the lowest pump power of Pe = 1.2 MW but
appeared for Pe = 5 MW with a delay time tgd = 6 s after pump-on. The growth
delay time decreased with increasing pump power and was approximately tgd = 1 s
for the highest used Pe = 30 MW. The delay time is only weakly dependent,
or independent, on f− for a given pump power. Further, at the highest pump
242
T. B. LEYSER
power the BC exhibited a slow overshoot at approximately 3 s after pump-on for
downshifts of about 50 kHz and more.
The long evolution time of the BC is attributed to the formation of FAS (Waldenvik, 1994). The delay time for the appearance of the BC scales approximately as
tgd ∝ Pe−1 which is similar to that observed for the excitation of FAS. With this
scaling the delay time for the appearance of the BC at the lowest power level of
Pe = 1.2 MW is approximately 20 s, which is consistent with that the BC was not
observed in the 10-s pump period although the instability threshold for the BC may
have been exceeded. The threshold for exciting small scale FAS has been found to
be Pe ≈ 0.5 MW.
Further, it has been observed that the evolution time of SEE at f− = 35 kHz
for f0 = 4.785 MHz corresponds to that of AA of the pump wave as well as of AA
of probe waves with frequencies close to f0 (Boiko et al., 1985). Also, the lower
frequencies of the BC develop at a shorter time scale, at least for low ERP. For ERP
= 20 MW the time in which the SEE reaches its maximum (stationary) intensity
decreased from 3 to 0.5 s (from 10 to 2 s) for f− changing from 10 to 80 kHz
(Boiko et al., 1985).
The evolution of the BC after pump-on depends on preconditioning. The temporal evolution at f− = 35 kHz for f0 = 5.105 MHz and ERP=150 MW has
been studied by cycling the pump 2 s on/8 s off (Boiko et al., 1985). Following
a cold pump-on, the SEE intensity increased essentially monotonically during the
entire 2-s pump period, after an initial overshoot within 0.1 s. After a few pump
cycles an additional overshoot developed which peaked at approximately 0.2–0.3 s
and which increased in amplitude during the successive pump periods. For low
ERP≤ 20 MW the initial overshoot occurred at 0.3–0.5 s after pump-on and was
several times smaller than the main overshoot.
For f0 > 5.4 MHz the SEE intensity at f− = 10–60 kHz has been reported to
increase toward a maximum simultaneously with the evolution of AA (Erukhimov
et al., 1987). On the other hand, for f0 < 5.4 MHz the maximum intensity occurs
before or together with the detection of AA of the pump wave, whose evolution
instead coincides in time with the attainment of the steady state SEE spectrum.
The growth and decay times of the BC for a preconditioned plasma have been
studied by QCW pumping (Sergeev et al., 1994, 1995, 1997). The pump was cycled
180 ms on/20 ms off for 2–3 min, which was followed by 7–8 min of pump-off to
assure a cold start for the next pump cycle. This facilitated repeated registrations
of SEE growth and decay times with a 200-ms resolution during quasi-steady
conditions for the FAS intensity and spectrum. The used f0 = 5.752 MHz and
f0 = 4.785 MHz are far from sfe . Following pump-on after the 20-ms pump-off
period, the BC exhibited an overshoot which peaked after 5–10 ms. The characteristic growth time as well as the intensity of the overshoot relative to the stationary
intensity increased with increasing f− . Also, the maximum intensity of the overshoot was approximately constant for different levels of FAS while the overshoot
time decreased slightly for more developed FAS intensity, as observed shortly after
STIMULATED ELECTROMAGNETIC EMISSIONS
243
the cold turn-on of QCW pumping. However, the stationary intensity of the BC
increased significantly with the development of FAS, so that the strength of the
overshoot relative to the stationary intensity increased with the development of the
FAS. This overshoot occurs on a similar time scale as the mini-overshoot observed
in the plasma line from 430-MHz Arecibo ISR measurements (Showen and Kim,
1978).
Further, the onset of decay of the BC at pump-off is delayed by tdd ≈ 0.5–
3.2 ms after pump-off for f− ≈ 20–63 kHz, with increasing tdd for increasing
f− (Sergeev et al., 1994, 1995, 1997). The decay then proceeds with an exponential decay having an approximately constant e-folding time of τd ≈ 1.5–2.0 ms for
all f− . The earlier onset of the decay at smaller downshifts, together with a fast
decay of the DM (discussed in Section 4.5), causes a flattening of the downshifted
spectrum and appearance of a weak spectral maximum at f− = 20–40 kHz.
Further, the decay delay time tdd is slightly smaller just after the cold QCW pumpon, when the FAS spectrum still develops toward its stationary state. Also, τd
increased slightly form 1.2–1.5 ms to 1.8–2.2 ms when going from day to night
(Sergeev et al., 1995, 1998). The values of τd are shorter than the decay times
expected for electron–ion collisions, both for daytime and nighttime.
The BC exhibits an interesting dependence on f0 relative to sfe (s is a positive
integer) as summarized in Figure 7 which shows spectra for successively lower
f0 below 5fe (Leyser et al., 1993). That the frequency range of the BC is highly
variable was observed already in early experiments (Stubbe et al., 1984), before
the gyro harmonic effects were discovered. The BC appears to be well developed
in two f0 ranges between (s − 1)fe and sfe , where s = 5, 6 has been reported.
Figure 7(a) displays a case where no DM is excited indicating that f0 = 6.74 MHz
≈ 5fe , as discussed in Section 4.5. The spectrum exhibits a weak continuum extending approximately 20 kHz below f0 and a weak broad upshifted maximum
(BUM) with its maximum intensity about 15–20 kHz above f0 (discussed in Section 4.7). The BC is then well developed for f0 between 6.69 and 6.73 MHz as
exemplified with the spectrum in Figure 7(b), where the BC extends 70 kHz below
f0 and a DM is seen at f− ≈ 10 kHz. Both the BC and DM features are weaker
for f0 between 6.46 and 6.68 MHz, as in Figure 7(c) for f0 = 6.66 MHz. For
f0 = 5.868 MHz and lower the BC and DM are again well developed, as in
Figure 7(d). Notice also what appears to be the broad upshifted structure (BUS),
discussed in Section 4.8, extending nearly 100 kHz above f0 . It is interesting to
note that the wide BC which is excited for f0 just below sfe and seen in Figure 7(b)
has not been observed for the lower s = 3, 4.
Whereas the BC is well developed for f0 just below sfe , the BC is hardly excited
for f0 between about sfe and (s + 0.1)fe . As further discussed in Sections 4.5
and 4.7, in this f0 range the SEE spectrum is dominated by strong DM features
and a SNC at downshifted frequencies and BUM features at upshifted frequencies
from f0 . Particularly, the BC is excited for those f0 for which the BUM and nDM
(n ≥ 2) features are not excited.
244
T. B. LEYSER
4.5. D OWNSHIFTED MAXIMUM (DM, nDM) AND UPSHIFTED MAXIMUM
(UM)
The DM emission is a prominent and commonly observed SEE spectral feature
for sufficiently long pump duration, noted already in the early experiments as a
pronounced peak at f− ≈ 8–12 kHz (Thidé et al., 1982, 1983). In the Russian
literature the DM has been referred to as the principal spectral maximum (Sergeev
et al., 1994). The DM peak intensity typically occurs at
f− = fDM ≈ 2 × 10−3 f0 ,
(1)
which was initially found in Tromsø experiments for f0 between 2.8–7.1 MHz
(Stubbe et al., 1984; Thidé, 1984), has been verified in experiments at the Gissar
facility (Yerukhimov et al., 1987), and extended up to f0 = 9.44 MHz at the
Sura facility (Leyser et al., 1994). Deviations from the empirical Equation (1) may
depend on the proximity of f0 to sfe and the effective pump power. Further, the
frequency range associated with the DM emission increases with f0 . Figure 8
displays the DM for f0 = 4.04 MHz, f0 = 6.72 MHz, and f0 = 9.31 MHz. It
is seen that the highest frequency components at about f− ≈ 7–8 kHz do not
exhibit a significant dependence on f0 while the downshift of the lowest frequency
components of the DM at f− ≈ 12 kHz in Figure 8(a), f− ≈ 18 kHz in
Figure 8(b), f− ≈ 23 kHz in Figure 8(c) increases with f0 . The DM is the first
spectral feature to be excited when the ionospheric fcrit approaches f0 from below
during sunrise and develops even when fcrit is a few hundred kilohertz below f0
(Leyser et al., 1990). The DM thus appears at lower fcrit than the BC.
The DM and BC features appear to have the same, or very similar, pump power
threshold (Leyser et al., 1994). Investigations of the dependence on pump power
show that the appearance of the DM exhibits a delay time similar to that of the
BC, when cycling the pump 10 s on/50 s off (Waldenvik, 1994). However, whereas
the BC intensity may saturate for a high pump power of Pe = 30 MW the DM
continues to grow throughout the 10-s pump duration.
Following pump-on the DM high frequency components are the first to appear,
which is followed by successively lower frequencies but also a narrow range of
slightly higher frequencies (Waldenvik, 1994; Leyser et al., 1994). This results in
that the downshift of the growing DM peak from f0 as well as the spectral width
increases during the growth period. The DM emission grows on a time scale of the
order of seconds after pump-on (Thidé, 1985; Leyser et al., 1990), similar to but
not the same as the BC. The slow growth has made the DM associated with the
presence of FAS. The DM has also been observed to be strong during conditions of
spread-F (Leyser et al., 1990). Further, although the DM and BC features grow on
similar time scales after pump-on, their temporal evolutions may be very different
(Leyser et al., 1990). As shown in Figure 9, when monitoring the DM and BC
intensities, respectively, in an approximately 1-kHz bandwidth at f− = 9 kHz
and f− = 23 kHz for f0 = 4.544 MHz, it was observed that the DM was
STIMULATED ELECTROMAGNETIC EMISSIONS
245
Figure 8. The DM feature for different f0 . (a) f0 = 4.04 MHz, from the Tromsø Heating facility. (b)
f0 = 6.72 MHz, from the Sura facility. (b) f0 = 9.31 MHz, from the Sura facility. (a) is taken from
Leyser et al. (1990) while (b) and (c) are taken from Leyser et al. (1993).
repeatedly ‘turned’ off to a lower intensity and on during a 30-s pump period,
whereas the BC intensity decreased slightly after an overshoot which peaked 2–3 s
after pump-on. It is presently not possible to tell whether the remaining SEE at
‘DM off’ at the frequency downshift of the DM actually should be attributed to the
DM or to a superposed BC. This dynamic behavior appears to depend on the pump
power and on ionospheric conditions.
The evolution time depends significantly on preconditioning of the plasma.
Similar to the BC, for a cold pump-on, when the pump has been off for approximately 5 min or longer, the growth times are significantly longer than for a preconditioned plasma, but with the high frequency components of the DM still having
the shortest growth time (Leyser et al., 1994). For a cold pump-on, a significant
overshoot at f− = 15 kHz for f0 = 5.105 MHz within 10 ms has been found
246
T. B. LEYSER
Figure 9. SEE intensity versus time at 9 and 23 kHz below f0 = 4.544 MHz, from the Tromsø
Heating facility. Taken from Leyser et al. (1990).
in experiments at the Sura facility (ERP ≈ 150 MW), which was suppressed with
increasing preconditioning (Boiko et al., 1985). The strong SEE on such short time
scales for a cold pump-on are attributed to the FNC feature discussed in Section 4.1.
Intensity overshoots on different time scales in the frequency ranges of the DM and
BC were also reported from experiments at the Gissar facility (Yerukhimov et al.,
1987).
As a result of the dependence on preconditioning both the BC and DM exhibit an hysteresis effect during pump power stepping (Leyser et al., 1994). When
starting at a low pump power and cycling the pump 1 min on/1 min off (f0 =
6.70 MHz), doubling the power each new pump period up to a maximum level,
and then successively decreasing the pump power, the BC and DM emission levels
are higher on the down-leg than on the up-leg of the power stepping. The hysteresis effect was much weaker at the lower pump duty cycle of 10 s on/50 s off
(f0 = 5.828 MHz), however, for which the SEE intensity did not saturate during
the short pump periods, except at the highest pump powers. Also, the hysteresis
effect occurs only above a certain threshold pump power (Boiko et al., 1985). An
interesting observation concerning the DM spectrum is that for the lower pump
powers, when the DM peak intensity was smaller than its maximum value for full
pump power, fDM was approximately equal to and sometimes larger than for the
full power DM. This is different from, e.g., the temporal evolution of the DM after
pump-on in which successively lower frequency components appear with time and
thereby fDM increases as the DM peak intensity grows (Leyser et al., 1994). Hysteresis effects have previously been discussed for FAS themselves in experimental
STIMULATED ELECTROMAGNETIC EMISSIONS
247
Figure 10. SEE spectra for low-power (about 80 kW ERP) pulses at the HIPAS facility. (a) Before
preconditioning by powerful pumping. Only the ionospherically reflected low-power pump which is
at f0 = 4.56 MHz can be seen. (b) After preconditioning by powerful pumping. A well developed
and discrete DM is seen at f− ≈ 10 kHz. Taken from Cheung et al. (1998).
results (Jones et al., 1983) and theoretical models (Grach et al., 1981; Inhester
et al., 1981; Dysthe et al., 1982).
Hysteresis effects of the DM have been further studied at the HIPAS facility
by pumping three consecutive 10-s pulses at low power (about 80 kW ERP), at
f1 = 4.50, 4.53, and 4.56 MHz, respectively; 136-s of pumping at high power
(∼ 20 MW ERP) and f0 = 4.53 MHz; a second sequence of low-power pulses; a
second high power pulse at f0 , starting 44 s after the first high-power pulse is off;
and a third sequence of low-power pulses (Cheung et al., 1998). The low-power
pulsing thus constitutes the DSEE technique. Preconditioning by the first powerful
pump pulse makes a significant influence on the subsequent low power pulses.
As shown in Figure 10(a), the first series of low power pulses do not excite SEE.
However, as seen in Figure 10(b), the second series of low-power pulses give a
strong and discrete DM feature. Notice also the steep HF flank of the DM. This is in
contrast to the DM observed during the high-power pumping, which is broader and
frequently superposed on a continuum background (presumably the BC). Further,
the temporal evolution of the SEE intensity at f− = 9 kHz (corresponding to the
DM), exhibited a slow growth during about 10 s following turn-on of the first highpower pulse, while the SEE intensity has a strong overshoot within the first few
seconds after turn-on of the second high-power pulse, during which the ionospheric
plasma is preconditioned by the first high-power pulse. The overshoot is associated
with a significant broadening of the SEE spectrum.
The DM is not detectable in a narrow f0 range when f0 ≈ sfe , for stable
ionospheric conditions (Leyser et al., 1989, 1990, 1992, 1994). This has been studied in a number of f0 -stepping experiments, in which the pump wave is transmitted
in CW mode at a given f0 for typically a few minutes before the pump is turned
off and f0 is changed to a new value. In Figure 11(a) where f0 > 4fe , a DM is
seen at f− ≈ 9 kHz and a BUM at f+ = f − f0 ≈ 35 kHz. The DM is absent
248
T. B. LEYSER
Figure 11. Dependence of SEE spectrum on f0 near 4fe from the Tromsø Heating facility. The spectra are 200 kHz wide. (a) f0 = 5.443 MHz > 4fe , (b) f0 = 5.403 MHz ≈ 4fe , (c) f0 = 5.383 MHz
< 4fe . Taken from Leyser et al. (1989).
STIMULATED ELECTROMAGNETIC EMISSIONS
249
Figure 12. Pump frequency range in which the DM feature is not detected at different electron gyro
harmonics, from the Sura facility Taken from Leyser et al. (1994).
in Figure 11(b) where f0 ≈ 4fe , whereas in Figure 11(c) where f0 < 4fe the DM
is again present but the BUM is absent. As depicted in Figure 12, the f0 range
in which the DM is absent is approximately 10 kHz at s = 4 (corresponding to
f0 /f0 ≈ 2 × 10−3 ) and decreases to 0.2 kHz or less at s = 7 (corresponding
to f0 /f0 ≈ 2 × 10−5 ) (Leyser et al., 1992, 1994). The spread in the data for
a given s in Figure 12 is attributed to varying ionospheric conditions during the
measurements and the larger values occur during more quiet conditions. Thus, only
the largest values for each s should be considered in studying the dependence of
the forbidden f0 range on s. The f0 range at s = 7 is of the order of the collisional
damping rate of plasma waves. The absence of the DM for f0 ≈ sfe can be used
to determine the magnitude of the local magnetic field in the pump–plasma interaction region, where the local fuh matches the harmonics of the local fe (Leyser
et al., 1989, 1990, 1992). Particularly, the higher s give an higher resolution in the
determination of the local magnetic field strength.
The successive weakening of the DM as f0 approaches sfe consists of the disappearance of successively higher frequency components, beginning with the lowest
frequencies (Leyser et al., 1994). Thus, the main part of the DM is suppressed in
larger f0 ranges than those plotted in Figure 12 which concerns the part of the
DM that exists for f0 closest to sfe and which is downshifted the least from f0 ,
namely by approximately the LH resonance frequency flh . For f0 near 3fe the DM
feature has been observed to be split into two or more narrow spectral lines (Leyser
et al., 1990; Stubbe et al., 1994). Further, there is an interesting similarity in the
spectral evolution of the DM after pump-on compared to when f0 is increasingly
detuned from sfe (Leyser et al., 1994). Starting from f0 ≈ sfe where the DM
is absent and then increasing the separation of f0 from sfe , successively lower
frequency components of the DM appear so that fDM successively increases toward fDM ≈ 2 × 10−3 f0 , as given by Equation (1). Similarly, after pump-on
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T. B. LEYSER
for an f0 not near sfe , first the high frequency components of the DM appear and
then successively lower frequency components so that fDM increases with time.
This led to the conjecture that successively lower frequency components of the
DM are due to UH oscillations with successively larger wave number component
k⊥ perpendicular to the magnetic field (Leyser et al., 1994). The linear dispersion
properties of UH waves gives that the available k⊥ range increases as f0 is increasingly detuned from sfe . On the other hand, the slow temporal development
of small-scale FAS provides the possibility of larger and larger k⊥ in the central
region of the FAS, either by expansion downwards of the interaction region toward
lower plasma densities or locally by trapped UH oscillations inside the FAS with
the decreasing plasma density of the growing FAS amplitude.
For frequencies downshifted from f0 the most intense components of the SEE
spectrum have the shortest decay time after pump-off. In high duty cycle QCW
pumping the DM exhibited a rapid decay of up to 10–12 dB within less than 1 ms
after pump-off (Sergeev et al., 1994, 1995, 1997, 1998). The decay then continued
at a similar rate as that of the BC at larger frequency downshifts, i.e., with an efolding time of τd ≈ 1.5–2 ms, for f0 = 5.75 MHz which is not near sfe . Similarly,
for the smaller f− ≈ 8 kHz, at which the DM intensity is lower than that of the
DM peak, τd ≈ 1.25 ms (Sergeev et al., 1998). The fast early decay stage of
the DM becomes less pronounced at lower pump power. For f0 = 9.31 MHz,
which is near 7fe , the DM has been observed to have an even shorter decay time
of τd ≈ 0.5 ms (Sergeev et al., 1995). As discussed in Section 4.7 for the BUM
feature, for frequencies upshifted from f0 it is generally not the case that the most
intense frequency components have the shortest decay time.
The DM may be accompanied by a small cascade of successively weaker nDM
emissions (n = 2, 3,4) which appear at successively lower frequencies (Stubbe
et al., 1984; Erukhimov et al., 1987; Leyser et al., 1990, 1994), as in Figure 13(a)
which shows a DM, 2DM, and a 3DM. These nDM features are identified in a
relatively narrow f0 range slightly above sfe , where the BC is not excited (Leyser
et al., 1992). It should be noted that the 2DM has also been observed for f0 slightly
below 4fe (Figure 4 in Leyser et al. (1990)). Also, here the BC is not well developed, which is contrary to the case at higher gyro harmonics for which the 2DM
has only been observed for f0 slightly above sfe . The downshift of the 2DM peak
from f0 is typically 1–2 kHz less than 2fDM (Leyser et al., 1994). The downshift
of the highest frequency components of the 2DM is approximately 8–9 kHz from
the DM peak, independent of f0 and similar to the downshift of the highest DM
frequencies from f0 . Further, the weakening of the 2DM as f0 approaches sfe is
correlated with a decrease in the DM low frequency components, and not primarily
with a decrease in the DM peak intensity. This indicates that the 2DM is excited
through the DM in a cascade type process, from the lowest frequency components
of the DM up to the DM peak intensity, i.e., the lower half of the DM spectrum.
The DM and nDM features sometimes exhibit different temporal evolutions
after pump-on, as in Figure 13 which displays two SEE spectra recorded 8 s apart
STIMULATED ELECTROMAGNETIC EMISSIONS
251
Figure 13. Two SEE spectra at f0 ± 50 kHz for f0 = 8.10 MHz, which is slightly above 6fe ,
recorded within the same pump period at the Sura facility. (a) A DM, 2DM, and 3DM emission with
peak intensities at approximately 13, 29, and 42 kHz below f0 (12:04:02 UT). (b) A DM and 2DM
emission at approximately 15 and 29 kHz below f0 , respectively (12:04:10 UT). The arrows labelled
A, B, and C are discussed in the text. Taken from Leyser et al. (1994).
during the same pump period (Leyser et al., 1994). The 3DM can only be seen in
the first seconds after pump-on (Figure 13(a)) but not in the later stationary part
of the pump period (Figure 13(b)). This temporal development of the 3DM can
sometimes also be seen for the 2DM when initially there are only a DM and a
2DM present.
During additional pumping at the Sura facility with two pump waves at different
frequencies the DM and 2DM for the pump at f01 and 26 MW ERP was enhanced
by the second pump at f02 ≈ f01 + 0.4 MHz and 105 MW ERP, so that the second
pump was present in the interaction region of the first pump (Bernhardt et al.,
1994). For CW pumping during 4 min at f01 = 7.8164 MHz the SEE spectra
exhibited a DM and 2DM. After 2 min of pumping at f01 , the second pump at
f02 = 8.2400 MHz was turned on for the remaining 2 min of the period, which
resulted in a 2-dB enhancement of the DM and 2DM of the pump at f01 .
It is interesting to note that the opposite result also has been obtained for the
DM and also at the Sura facility. In the experiment the first pump at f01 was again
used as a diagnostic wave to excite SEE and was transmitted CW at approximately
24 MW ERP and the second pump at f02 = f01 + 350 kHz, cycled 10 s on/10 s off
at approximately 100 MW ERP, was used to excite additional ionospheric perturbations (Frolov et al., 1996, 1997b). In these evening time experiments the intensity
252
T. B. LEYSER
of the DM from the first pump decreased by 3 dB as the second pump was turned
on. The quenching and recovery times of the DM were both 2–3 s as the second
pump was cycled on/off. The seemingly contradicting results (Bernhardt et al.,
1994; Frolov et al., 1996; Frolov et al., 1997b) for the DM is likely to be a result
of the double role that the FAS play for the DM, since the FAS, on the one hand,
cause the excitation of the DM and, on the other hand, cause AA of the excited
electromagnetic radiation as it propagates out from the interaction region.
Associated with the DM is a UM occurring upshifted from f0 approximately
at the mirror frequency of the DM and typically 10–20 dB weaker than the DM
(Stubbe et al., 1984; Thidé et al., 1989). However, the frequency shift of the UM
peak from f0 is smaller than that of the DM (Stubbe et al., 1984; Frolov et al.,
1999). A weak UM feature can be seen at f+ ≈ 7 kHz in Figure 8(a) and at
f+ ≈ 10 kHz in Figure 8(b). Similar to the DM, the UM frequency downshift
from f0 depends linearly on f0 , and the difference in the frequency shift of DM
and UM peak is about 2 kHz, independent of f0 (Frolov et al., 1999). Also, it has
been observed that the UM only appeared for the highest pump power used (Pe =
30 MW) whereas the DM was present also for lower pump powers (Waldenvik,
1994). The UM and DM features exhibited strikingly similar temporal evolution
after pump-on, with the same delay time and break point in a two-stage growth
dynamics. In high pump duty cycle experiments at f0 = 5.75 MHz, the UM was
found to have a decay time of approximately 0.7 ms after pump-off, similar to that
of the rapid early decay stage of the DM (Sergeev et al., 1997). This decay time is
shorter than that expected for collisional damping of plasma waves.
The DM is always present when the UM is seen. However, there does not appear any simple correlation between the UM and DM intensities. For similar DM
intensities and f0 , the UM has been observed to be present some times and not at
all identifiable at other times, suggesting that the UM excitation depends on special
conditions in the ionospheric plasma (Leyser et al., 1994).
4.6. B ROAD SYMMETRICAL STRUCTURE (BSS)
The BSS consists of two broad maxima occurring symmetrically around f0 at f −
f0 ≈ ±(15–30) kHz and has only been observed to be excited in a narrow f0 range
of about 40 kHz, or f0 /f0 ≈ 1%, just above 3fe (Stubbe and Kopka, 1990; Stubbe
et al., 1994). A well developed BSS is seen in Figure 14(c) with spectral maxima
at f− ≈ 21 kHz and f+ ≈ 24 kHz. See also the BSS in Figure 6(d). The
BSS is the feature which is most symmetric around f0 of all the identified SEE
spectral structures. Although both the BSS and DP are excited for f0 near 3fe the
two spectral features have not been observed to coexist. The BSS appears to occur
at slightly larger f0 than the DP.
The BSS feature exhibits a characteristic dependence on f0 near 3fe , as illustrated in Figure 14. At the lowest f0 (Figure 14(a)) the BSS is not identifiable, but
only a prominent DM and a weaker UM feature can be seen. With increasing f0 the
STIMULATED ELECTROMAGNETIC EMISSIONS
253
Figure 14. Dependence of the BSS on f0 near 3fe obtained at the Tromsø facility. All spectra exhibit
the DM and UM whereas spectra (b)–(e) also show the BSS. Taken from Stubbe and Kopka (1990).
BSS develops and moves toward larger frequency offsets from f0 . The BSS settles
at f± ≈ 30 kHz (Figure 14(d) and 14(e)) where it gradually disappears in place
for higher f0 . This f0 dependence is different from that of the asymmetric BUM
feature discussed in Section 4.7. However, it is interesting that both the BSS and
the BUM do not appear closer to f0 than about 15 kHz. Further, it has been noted
that the BSS appears within offset frequencies from f0 between flh and f0 − 3fe
(Huang et al., 1995).
4.7. B ROAD UPSHIFTED MAXIMUM (BUM) AND BROAD DOWNSHIFTED
MAXIMUM (BDM)
The BUM feature is a relatively broad emission which exists above f0 , at f+ ≈
15–200 kHz, and was already observed in the early experiments at Tromsø in 1981
for f0 = 5.423 MHz (Thidé et al., 1983; Stubbe et al., 1984, 1985). The BUM is
excited approximately for (s − 0.01)fe < f0 < (s + 0.1)fe and has been observed
for s = 3–7 (Leyser et al., 1990, 1993). Figure 15 shows an example of the BUM
for f0 = 4.04 MHz (s = 3) at f+ ≈ 25 kHz, in addition to a DM, 2DM, 3DM,
and UM. At the time of writing it is not known what conditions favor excitation of
254
T. B. LEYSER
Figure 15. BUM feature at f+ ≈ 25 kHz for f0 = 4.04 MHz near 3fe from the Tromsø facility. In
the lower sideband a DM, 2DM, and 3DM can clearly be seen and in the upper sideband also a UM
feature. Taken from Leyser (1989).
the BSS (Figure 14(c)) or the BUM (Figure 15) at s = 3. However, for f0 ≥ sfe
and s > 3 only the asymmetric BUM is excited and not the BSS. Possibly the BSS
and BUM are related features with a common underlying instability, which depending on ionospheric conditions gives symmetrical or asymmetrical electromagnetic
radiation around f0 .
The BUM frequency upshift, fBUM = fBUM − f0 where fBUM is the frequency of the peak intensity of the BUM, exhibits a characteristic dependence on
f0 (Leyser et al., 1989). In Figure 11(a), fBUM ≈ 40 kHz for f0 = 5.443 MHz
while in Figure 11(b) fBUM ≈ 18 kHz for f0 = 5.403 MHz. For f0 not too
close to sfe , such that the BUM cutoff (discussed below) is not developed, this
dependence is given by the empirical relation
fBUM − f0 = f0 − sfe ,
(2)
where f0 > sfe and s = 4, 5, 7 have been studied. For f0 ≈ sfe when the DM
is suppressed, the BUM is slightly weaker but usually still well developed, at least
for s > 3 (Leyser et al., 1989, 1990). It may be noted that the BUM is excited
also for f0 slightly below sfe , down to f0 ≈ sfe − 10 kHz (s = 5) (Frolov et al.,
1996). The BUM intensity is maximum usually for fBUM ≈ 30–40 kHz. Further,
the f0 range for which the BUM is excited and the spectral width of the BUM
increase when stepping from s = 4 via s = 5 to s = 6 (Stubbe et al., 1994). Using
Equation (2), the value of fe and, thus, the geomagnetic field strength in the pump–
plasma interaction region can be estimated, which was found to be consistent with
that obtained independently from the quenching of the DM. Also, by monitoring
fBUM , the local fe in the interaction region can be followed as a function of time
without needing to keep track of the resonance f0 ≈ sfe as is the case when using
the quenching of the DM.
STIMULATED ELECTROMAGNETIC EMISSIONS
255
The BUM exhibits a cutoff frequency which is a frequency upshift from f0
below which the BUM cannot be identified, so that when f0 is successively lowered
toward sfe the BUM frequency upshift does not decrease further but the BUM
intensity decreases successively (Leyser et al., 1990, 1993; Stubbe et al., 1994;
Frolov et al., 1996). Thus, the BUM frequency upshift depends much weaker on
f0 when f0 is close to sfe compared to when f0 is further away from sfe for which
Equation (2) holds. The cutoff appears as a steep edge in the spectrum, for f0
such that the BUM appears sufficiently close to f0 . Further, the cutoff frequency
increases with increasing gyro harmonic s, such that at −6 dB below the BUM
peak it is (Leyser et al., 1993)
−6 dB
= (6s − 16) kHz,
fcutoff
(3)
where s = 4–7. Closer to the background noise level the cutoff exhibits a slightly
weaker dependence on s, implying that the constant slope of the cutoff in the
spectrum (on a logarithmic intensity scale) is smaller for larger s.
At cold pump-on the BUM evolves on a time scale of several seconds, which
is longer than that of the BC and DM features and which has been associated with
the formation of FAS (Leyser et al., 1990; Frolov et al., 1996). The characteristic
time of the BUM evolution corresponds to the growth of FAS with l⊥ ≈ 6–10 m
(Frolov et al., 1996). On the other hand, in high pump duty cycle measurements and
resulting preconditioned ionospheric plasma, the SEE intensity has been observed
to increase to a maximum within only 1 ms after pump-on (Sergeev et al., 1997).
However, as discussed below, the role of the FAS for the BUM excitation is not
clear.
The BUM exhibits a decay after pump-off with a well-defined exponential time
dependence (Sergeev et al., 1995, 1997) and a decay time which is shorter than
that expected from collisional damping of plasma waves (Sergeev et al., 1994). For
f0 = 6.72 MHz ≈ 5fe + 45 kHz, the decay time increased from about 0.8 ms at
f+ = 20 kHz to 1.0 ms at f+ = 100 kHz (Sergeev et al., 1997). The decay time
decreased further for all f+ as f0 approached 5fe . A summary of the decay times
for different f0 is shown in Figure 16, both for down- and upshifted frequencies.
The decay time typically increases with increasing frequency shift from f0 . Thus,
whereas the SEE components downshifted from f0 having a higher intensity also
have a shorter decay time, this is not the case for the BUM.
The BUM emission is associated with nBUM features (n = 2, 3), which are
successively weaker spectral maxima appearing upshifted from f0 by successively
higher multiples of the BUM upshift (Stubbe et al., 1984, 1985; Leyser et al.,
1990). The 2BUM is upshifted from f0 approximately twice as much as the stronger
principal BUM, as seen in Figure 11 where the BUM peaks at f+ ≈ 35 kHz and
the 2BUM peaks at f+ ≈ 70 kHz.
Experiments near 4fe and 5fe at the Sura facility suggest the distinction of a fast
and a slow BUM (Frolov et al., 1996). The fast BUM refers to the overshoot on a
few seconds time scale following pump-on while the slow BUM refers to the post-
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T. B. LEYSER
Figure 16. Typical decay times after pump-off of SEE at different frequencies obtained at the Sura
facility, for f0 = 5.750 MHz (curve 1), 6.720 MHz (curve 2), 6.700 MHz (curve 3), and 6.665 MHz
(curve 4), where 5fe ≈ 6.670 MHz. Taken from Sergeev et al. (1997).
Figure 17. Spectra of the BUM for f0 = 5.456 MHz near 4fe for different pump powers, when the
power was increased in 3-dB steps at the Sura facility. Taken from Wagner et al. (1999).
overshoot quasi-steady state spectrum. The fast BUM is excited predominantly
near sfe , for f0 − sfe < 30 kHz. As f0 − sfe increases the intensity of the fast
BUM decreases whereas the growth rate and intensity of the slow BUM increases.
Also, the initial decay rate of the BUM overshoot is observed to be largest for a
preconditioned plasma (Wagner et al., 1999). The decay rate was larger for an inter
pump pulse interval of 15 and 30 s and smaller for an inter pulse interval of 60 s
and longer.
The spectral shape of the BUM feature depends on the pump power (Wagner
et al., 1999). Figure 17 shows the BUM spectrum for different pump ERP levels
and f0 = 5.456 MHz (0 dB corresponds to approximately 150 MW ERP). The
pump power was changed in 3-dB steps in a symmetric staircase pattern during
CW pumping, from −15 dB to 0 dB and was then stepped back down to −15 dB.
The dwell time at each power level was 2 min. After 2 min pump-off the power
stepping cycle was repeated once. As seen in Figure 17, at the lowest power level
the BUM spectrum is bell-shaped. At increasing pump power the BUM intensity
STIMULATED ELECTROMAGNETIC EMISSIONS
257
Figure 18. Temporal evolution of the integrated BUM power and DM power at f− = 9 kHz for
f0 = 5.456 MHz near 4fe during two cycles of stepping the pump power in a staircase pattern at
the Sura facility. There was an interval of 2 min pump-off between cycle 1 and 2. The data for the
DM was smoothed using a three-point moving average and the power was depressed by 6 dB for
convenience of presentation. Taken from Wagner et al. (1999).
increases and a high-frequency ramp develops, which extends more than 200 kHz
above f0 at the highest pump power.
The role of thermal effects and the associated formation of FAS for the BUM
excitation is an open question. Whereas the BC, DM, and BUM features all have
a relatively long evolution time following a cold pump-on, similar to that of the
nonlinear evolution of thermal effects and the self-structuring of the plasma density into small-scale FAS, the BUM is the only feature which does not exhibit
an hysteresis effect during pump power stepping (Frolov et al., 1996; Wagner
et al., 1999). Figure 18 shows the integrated BUM power and the DM power at
f− ≈ 9 kHz during the experiment in which the pump power was stepped in the
two cycles of staircase pattern described above (Wagner et al., 1999). It is seen
that whereas the BUM power abruptly adjusts to the successive pump power steps,
the DM power varies relatively smoothly. The abrupt change in the pump power
is consistent with the 1-ms growth time in high pump duty cycle measurements
mentioned above (Sergeev et al., 1997). Further, the BUM power is similar on the
power-up and power-down side in both power stepping cycles, with only slightly
smaller BUM power on the power-down side. Assuming a power-law dependence
of the BUM power on the pump power, the data indicate a power law exponent
between 1.9 and 1.5 for the power-up phases and between 2.0 and 1.8 for the powerdown phases of cycles 1 and 2, respectively. On the other hand, the DM exhibits
a pronounced hysteresis effect (Leyser et al., 1994) with higher DM powers on
258
T. B. LEYSER
the power-down side of cycle 1 than during power-up phase. The DM intensity
depends on f0 − sfe when f0 is near sfe . The lesser hysteresis effect of the DM in
cycle 2 can be attributed to a decrease of f0 − 4fe due to changes in the interaction
altitude leading to changes in the local fe during cycle 2 (Wagner et al., 1999).
The BUM power exhibits interesting transients at the pump power transitions
during the power stepping (Frolov et al., 1996; Wagner et al., 1999). In Figure 18
it is seen that the BUM power exhibits overshoots on the power-up side and undershoots at the power-down side. The observed variability in the transients was
tentatively attributed to the poor time resolution of the spectral measurements and
synchronization between the measurements and the pump power stepping (Wagner
et al., 1999). Nevertheless, the overshoots and undershoots are relatively larger and
steeper for lower pump powers. The opposite result has also been found, that the
undershoot of the BUM intensity was most pronounced for a pump ERP exceeding
40 MW albeit for the BUM power in a narrow frequency range instead of the total
integrated BUM power as in Figure 18 (Frolov et al., 1996). This may be a result of
that different parts of the BUM spectrum could exhibit differences in the transients
at the pump power transitions.
The effects of preconditioning for the BUM has been further studied with a
single pump as well as two pump waves at different frequencies (so called, additional pumping) at the Sura facility (Wagner et al., 1999). The BUM intensity
for a single pump is substantially reduced for inter pump pulse intervals of 15 and
30 s relative to longer inter pump pulse intervals, which is attributed to residual
conditioning from the prior pump pulse. The residual conditioning is greater for a
30-s pump pulse than for a 15-s pulse, implying that at least for pump pulses up
to 30 s, the conditioning is enhanced with increasing duration of the pumping. The
preconditioning is negligible for an inter pulse interval of 60 s, indicating that the
life time of the residual conditioning is between 30 and 60 s. Also, the integrated
BUM power was several decibels lower for a pump pulse which was preceded by an
inter pulse interval of 15 and 30 s than for a pump pulse preceded by a longer inter
pulse interval. Such a preconditioning effect was not observed for the integrated
DM power.
In the additional pumping experiments, the first pump was at a frequency f01
close to but greater than 4fe and was used to excite the BUM, while the second
pump was at a frequency f02 ≈ f01 + 370 kHz which was sufficiently far above
4fe so as to not excite a BUM but only BC and DM features (Wagner et al., 1999).
Although the BUM was generally weak, a significantly larger BUM at all frequencies was excited by the first pump when the additional pump was off (3 dB at the
BUM peak), which clearly shows that the BUM is suppressed by the additional
pumping. The decay time of the conditioning effect by the second pump was found
to be approximately 12 s.
In other experiments with two pump waves and f02 − f01 = 350 kHz, the
BUM exhibited again a markedly different behavior compared to the DM (Frolov
et al., 1996, 1997b). In these evening time experiments the first pump (f01 ) was
STIMULATED ELECTROMAGNETIC EMISSIONS
259
used as a diagnostic wave to excite the SEE and was transmitted CW at an ERP of
approximately 24 MW. The second pump (f02 ) was cycled 10 s on/10 s off with an
ERP of approximately 100 MW to excite additional ionospheric perturbations. The
intensity of the DM from the first pump decreased by 3 dB as the second pump
was turned on. The quenching and recovery times of the DM were both 2–3 s
as the second pump was cycled on/off. However, the BUM intensity increased as
the second pump was turned on. The growth time of the BUM was 2–3 s, similar
to the quench time of the DM, whereas the decay time as the additional pump
was turned off was 0.1–0.3 s. The BUM enhancement was only observed for a
sufficiently powerful additional pump, above an ERP of approximately 25 MW. In
another experiment in which the diagnostic pump (f01 = 5.40 MHz) was cycled
1.5 s on/0.5 s off and the additional pump (f02 = 5.75 MHz) was cycled 10 s
on/10 s off the BUM enhancement from the additional pumping exhibited a decay
time of τed ≈ 60 ms as the additional pump was turned off. The 2–3 s quench
(relaxation) time for the DM, as the second pump is turned on (off), is associated
with an increase (decrease) in the FAA due to increasing (decreasing) intensity of
meter-scale FAS, i.e., with the scales transverse to the geomagnetic field of l⊥ ≈ 3–
5 m. From the similarity of the BUM growth time with the DM quench time it is
concluded that the BUM enhancement is also due to the increasing intensity of
meter-scale FAS. However, the BUM excitation process is not simply related to
the FAS intensity since the BUM decays much faster than the FAS decay as the
second pump is turned off. Also, the delay time after the second pump is turned
off before changes in the DM and BUM intensities occur does not exceed 10 ms,
which is significantly shorter than the expected plasma transport time along the
ambient magnetic field for perturbations from the second pump–plasma interaction
region to reach down to the first pump–plasma interaction region. The enhancement
of the BUM by additional pumping is contrary to the suppression of the BUM
by additional pumping discussed above, which may, e.g., be due to differences in
the time of day for the experiments, leading to different ionospheric background
parameters, or due to differences in the proximity of f01 to 4fe (Wagner et al.,
1999).
The enhancement of the BUM by additional pumping 3 s on/7 s off is also
observed with f02 < f01 which excludes a local effect of the second pump in
the interaction region of the first CW pump that excites the BUM (Frolov et al.,
1997b). However, in this case the BUM intensity decreased again slowly with the
development of FAA so that at the end of the 3-s additional pumping, the BUM
was weaker than before turn-on of the additional pump. Such an intensity decrease
was not observed in the experiment discussed above in which f02 > f01 .
Additional pumping leads to an increase in the decay time of the BUM at turnoff of the first pump. Without additional pumping the decay time of the BUM
is τd ≈ 0.5–0.8 ms whereas during additional pumping the BUM decay time
increases to τd ≈ 1–2 ms (Frolov et al., 1997b).
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T. B. LEYSER
A final indication of the different roles of FAS for the DM and BUM has been
obtained during single frequency experiments when switching from CW to pulsed
pumping. For a 20-ms pump pulse, the BUM was not excited anymore if the pumpoff time was 80 ms or more (Frolov et al., 1996). The DM, on the other hand,
continued to be excited also for the short pump pulses at low duty cycle and its
intensity decreased slowly together with the decay of the small-scale FAS which
follows the switch from CW to pulsed pumping.
It was found that whereas the variations of the DM intensity during additional
pumping appear to be explained by the small-scale FAS intensity, the BUM intensity is not related only to the FAS intensity. The BUM is enhanced only for a
sufficiently strong additional pump (Pe2 20 MW) and is most enhanced when the
diagnostic pump frequency f01 − sfe 40–50 kHz. When the additional pumping
is switched off, the BUM intensity decreases below that of single frequency pumping and then slowly recovers during the following 3–10 s. This recovery is a result
of the decreasing AA of the diagnostic pump wave as the FAS that were excited by
the additional pump decay.
Experimental results from the Sura facility for f0 near 4fe suggest that the
steady state BUM feature consists of two different components (Frolov et al.,
1998). The first component, labelled BUM1 , exists for f0 − 4fe ≤ 10 kHz and
has maximum intensity when f0 ≈ 4fe . The BUM1 component is the spectral
maximum which has the cutoff discussed above as its low frequency flank. The
frequency upshift of the BUM1 is only weakly dependent on f0 and is slightly
larger than that of the cutoff given in Equation (3) with s = 4, since the cutoff is
measured at −6 dB below the BUM peak intensity.
The second component, BUM2 , exists for f0 > 4fe and has maximum intensity
for f0 − 4fe ≈ 20–40 kHz. The BUM2 component exhibits the well known dependence on f0 in Equation (2). The dependence of the BUM on sfe suggests that
the emission is due to electrostatic dynamics perpendicular to the magnetic field.
This is consistent with that the BUM2 is excited even when the ionospheric fcrit is
down to 150 kHz below f0 , which indicates that the emission is excited in the UH
resonance region and not at the plasma resonance where fp ≈ f0 . It is interesting
to note that whereas the BUM1 component has maximum intensity for f0 ≈ 4fe
when the DM is absent and when the small-scale FAS have a minimum intensity
(discussed in Section 5.3), the BUM2 occurs during conditions of well developed
FAS.
A strongly developed BUM is often accompanied by a much weaker broad
downshifted maximum (BDM), which occurs approximately at the mirror frequency of the BUM (Stubbe et al., 1984). Unlike the BUM, the BDM has exhibited
a strong temporal variability. However, detailed experimental results for the BDM
are still lacking, but are needed to clarify the possible relation to the BUM and, on
the other hand, a possible relation of the BDM and BUM to the BSS feature for f0
near 3fe .
STIMULATED ELECTROMAGNETIC EMISSIONS
261
Figure 19. Well developed BUS feature in the upper sideband of the pump for f0 = 4.50 MHz at
the Sura facility. In the lower sideband are the commonly observed BC and DM features indicated.
The numerous sharp spikes in the spectrum are due to interfering broadcasting stations. Taken from
Frolov et al. (1997a).
4.8. B ROAD UPSHIFTED STRUCTURE (BUS)
The BUS is a broad emission which occurs at frequencies above f0 and for f0
far from sfe (Frolov et al., 1997a). Figure 19 shows a well developed BUS in the
frequency range f+ ≈ 10–170 kHz for f0 = 4.50 MHz. In this case, the BUS
exhibits a maximum at f+ ≈ 25 kHz which is approximately 10 dB lower than
that of the DM in the lower sideband. Whereas the BUS and the BUM have a
number of common properties they have a different dependence on f0 . Notable is
that the BUM is only excited in a relatively narrow f0 range near and above sfe of
f0 ≈ 0.1fe and that the frequency upshift of the BUM from f0 depends linearly
on f0 as described by Equation (2). The excitation of the BUS depends on the
gyroharmonic number s (Frolov et al., 1999). The f0 range in which the BUS is
excited decreases with increasing s, in that the maximum f0 in each gyroharmonic
band at which the BUS is excited decreases from f0 − sfe ≈ 700 kHz ≈ 0.5fe
for s = 3, f0 − sfe ≈ 520 kHz ≈ 0.4fe for s = 4, f0 − sfe ≈ 260 kHz ≈ 0.2fe
for s = 5, in the gyroharmonic bands 3fe –4fe , 4fe –5fe , 5fe –6fe , respectively.
Further, the spectral maximum of the BUS, seen at f+ ≈ 20–30 kHz in Figure 19, develops for the lower f0 in these ranges. Experiments with f0 above the
ionospheric fcrit indicate that the BUS, like the DM and BUM, is excited in the UH
resonance region. Note that a BUS also appears to be present in Figure 7(d) for
f0 = 5.868 MHz.
The growth time of the BUS extends to 15–30 s after pump-on, which is longer
than that of the BC and DM. The BUS, like the BC and DM, depends on preconditioning, in that the evolution time following pump-on is shorter for shorter
preceding pump-off times and for higher power levels in the preceding pumping.
However, the BUS does not exhibit a hysteresis effect in the dependence of the
BUS intensity on pump power, which is similar to the BUM feature but contrary
to the DM (Frolov et al., 1997a). The decay time following pump-off is about 0.4–
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T. B. LEYSER
Figure 20. The BDE for with more energy in the upper than in the lower sideband, from the Sura
facility. Taken from Leyser et al. (1993).
0.6 ms which, again, is similar to that of the BUM but shorter than that of the DM
(typically 1.2 ms). Thus, the BUS and BUM have a number of common features,
such as the limited influence of FAS on their evolution, the absence of hysteresis
effects during pump power stepping, and a shorter decay time after pump-off than
the DM (Frolov et al., 1999).
The present author notes that the upshifted BUS together with the downshifted
BC feature resembles the source spectrum from cavitating UH turbulence trapped
in a pre-formed density depletion which has been predicted in numerical computations (Mjolhus, 1997, 1998). However, the BC and BUS spectral features have
different evolution times after pump-on and different decay characteristics after
pump-off, which should be studied in the numerical computations.
STIMULATED ELECTROMAGNETIC EMISSIONS
263
Figure 21. Temporal evolution of the UWE at fixed frequencies above f0 = 4.785 MHz for different
times after pump-on at the Sura facility. Taken from Frolov (1990).
4.9. B ROAD DYNAMIC EMISSION (BDE) AND UPSHIFTED WIDEBAND
EMISSION (UWE)
The BDE is an SEE feature that is not commonly observed and appears to require
specific ionospheric conditions to be excited. The BDE is approximately 200 kHz
wide and appears with more energy at upshifted than downshifted frequencies, as
observed in experiments at the Sura facility and shown in Figure 20 (Leyser et al.,
1993). The BDE feature appeared suddenly as f0 was lowered by 10 kHz from
f0 = 6.60 MHz (Figure 20(a) where no BDE can be seen) to f0 = 6.59 MHz
(Figures 20(b) and 20(c)) where it was observed during 4 min. As f0 was increased back to 6.60 MHz the BDE disappeared and on that particular occasion
the BDE did not reappear as f0 again was successively lowered. In Figure 20 the
spectral structure varies between successive sweeps of the spectrum analyzer with
which the SEE was recorded (6.8-s sweep time). The BDE feature has only been
observed sporadically which is the reason why it is relatively little studied and its
excitation must depend on f0 as well as the conditions in the ionospheric plasma.
The BDE has been observed for f0 = 6.59 MHz, 7.65 MHz, and 7.71 MHz, which
corresponds to f0 ≈ 4.9fe and 5.7fe , respectively, in experiments at the Sura
facility.
Further, an SEE feature occurring at f+ ≈ 50–400 kHz for f0 = 4.785 MHz
with a spectral intensity maximum in the range f+ ≈ 100–200 kHz has been reported from experiments at the Sura facility and is referred to as the UWE (Frolov,
1990; Frolov et al., 1999). The upshifted SEE exhibits a first overshoot within
0.2 s and a second overshoot reaching the peak intensity in approximately 1 s
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T. B. LEYSER
after pump-on. As illustrated in Figure 21 the first overshoot completely dominates the upshifted SEE at the displayed times 0.2 s and 0.4 s after pump-on
and the intensity falls of rapidly with increasing f+ . The time scale of the second overshoot coincides approximately with that of the downshifted component
at f− = 35 kHz as well as that of AA and, thus, the excitation of small-scale
FAS. Also, the time scale of the second overshoot increases slightly with increasing
f+ . After the second overshoot the emission intensity is largely concentrated to
the range f+ ≈ 100–200 kHz as in Figure 21.
A high variability from pump cycle to cycle is observed in the UWE, which in
this respect appears similar to that of the BDE feature. However, further investigations on the spectral structure, evolution times, and decay times after pump-off are
necessary in order to determine a possible relation between the BDE and UWE.
It would be useful to make simultaneous measurements of the SEE and electrostatic oscillations by, e.g., ISR. Using the EISCAT-UHF (933 MHz) radar, Langmuir oscillations which are upshifted by 100–300 kHz from f0 and have a line
width of approximately 100 kHz have been observed to be enhanced by the Tromsø
facility (Isham et al., 1990, 1996). The excitations, referred to as the HF-enhanced
outshifted plasma line (HFOL) (Isham et al., 1996), where observed for cycling
the pump 20 s on/40 s off with f0 = 4.04 MHz (Isham et al., 1990) as well as 1 s
on/9 s off and 1 s on/19 s off with f0 = 6.77 MHz (Isham et al., 1996). Whereas
the HFOL is tentatively suggested (Isham et al., 1996) to be related to the free
mode which results from cavitating Langmuir turbulence (DuBois et al., 1990) it
is unlikely that the same can be said about the BDE and UWE features. The UWE
was found to depend on the excitation of small-scale FAS (Frolov, 1990). Also, for
the high pump duty cycles used, AA of the pump wave is expected to be significant
so that Langmuir turbulence in the pump reflection region would only be weakly
excited. The sporadic appearance of the BDE and UWE may suggest an interaction
with a free energy source in the background ionospheric plasma.
4.10. L OW POWER CONTINUUM (LPC)
SEE can be been observed at very low pump powers for a preconditioned ionosphere.
In experiments at Tromsø, the pump was cycled 30 s at 270 MW ERP and 30 s at
86 kW ERP for f0 = 4.544 MHz (Leyser et al., 1990). For the high ERP, a BC
and DM developed in the lower sideband as well as what appears to be a BUS
above f0 as shown in Figure 22(a). For the low ERP, all above mentioned features
disappeared and a triangular shaped continuum here referred as a low power continuum (LPC), only downshifted from f0 was excited as shown in Figures 22(b)
and 22(c). The LPC exhibited an interesting temporal evolution during the 30s low pump power period. The maximum intensity of the LPC occurring at the
smallest f− decreased with time and, simultaneously, the LPC while maintaining
the triangular shaped spectrum extended toward lower frequencies below f0 . As
seen in Figure 22(b), in the early part of the 30-s low power period the LPC extends
STIMULATED ELECTROMAGNETIC EMISSIONS
265
Figure 22. SEE spectra for f0 = 4.544 MHz excited at very low pump power for a preconditioned
plasma from the Tromsø facility. The pump was cycled 30 s at 270 MW ERP (a) and 30 s at 86 kW
ERP (b) and (c). (b) is from the early part of the 30-s low power period and (c) is from the late part
of the low power period. The triangular shaped LPC feature is seen downshifted from f0 in (b) and
(c). Taken from Leyser et al. (1990).
approximately 35 kHz below f0 , while in the later part shown in Figure 22(c) it
extends 50 kHz below f0 .
As discussed in Section 4.5 and shown in Figure 10, the DM has also been
observed to be excited at very low pump power of approximately 80 kW ERP for
a preconditioned ionosphere at the HIPAS facility (Cheung et al., 1998). In the
spectrum displayed in Figure 10(b) the strong DM is accompanied by a weak continuum which could be the LPC feature. Apparently, the LPC has a lower excitation
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T. B. LEYSER
threshold than the DM, since the LPC has been observed without the DM in the
low power mode, as in Figure 22.
4.11. P UMP HARMONIC EMISSIONS
Several authors have reported weak SEE near 2f0 . Different spectral features have
been observed, but the temporal evolution is similar in most cases. A spectral
maximum downshifted from 2f0 has been found at f2− = 2f0 − f ≈ 2 kHz for
f0 = 5.75 MHz and cycling the pump 1 s on/4 s off during 1 min, in experiments at
the Zimenki facility (Karashtin et al., 1986; Erukhimov et al., 1987). No emissions
upshifted from 2f0 were observed. The maximum SEE power flux was approximately (0.3–4) × 10−20 W m−2 Hz−1 . The decay time of the SEE at pump-off was
6–8 ms which corresponds to collisional damping of plasma waves. Further, the
emission intensity decreased at successive pump cycles which was correlated with
the development of AA of the pump wave.
Different spectra in which the SEE appears roughly symmetrically around 2f0 =
2×3.324 MHz have been observed in experiments at the Tromsø facility (Derblom
et al., 1989). The spectrum depicted in the top panel in Figure 23 exhibits peaks
at ±(40–50) Hz around 2f0 , in the middle panel is an additional central peak at
2f0 , and in the bottom panel is only the central peak present. Often the temporal evolution of these second harmonic emissions is correlated with that of the
ionospherically reflected pump wave and anti-correlated with the SEE intensity at
frequencies downshifted from f0 (such as that of the BC and DM). However, at
times the emission near 2f0 may instead be correlated with the DM intensity in
temporal variations on a few seconds time scale. Also, the SEE near 2f0 exhibits
an overshoot after pump-on which in many events decays below the detection
threshold in a few seconds after pump-on (f0 = 3.324 MHz and 5.423 MHz),
which, thus, occurs on a similar time scale as the development of AA.
An up to 30-kHz wide emission with its maximum intensity downshifted by
16–20 kHz from 2f0 , where f0 = 4.04 MHz, has been observed in experiments at
the Tromsø facility during magnetically disturbed conditions (Blagoveshchenskaya
et al., 1998b). The SEE was detected at St. Petersburg, Russia, which is about
1200 km from Tromsø. Contrary to the spectral features near 2f0 discussed above,
the SEE was observed throughout a 3-min pump duration at a given pump power
and the emission intensity was correlated with the pump power during power stepping. It is suggested that the general spectral characteristics of the feature indicate
that it is the second harmonic of the commonly observed DM feature, although
an explicit comparison with the DM during the experiment is not reported. It may
be noted that the emission downshifted from 2f0 has frequency components all the
way up to 2f0 , whereas the DM has a minimum downshift of 6–8 kHz. An emission
near 2f0 which is associated with the second harmonic of the DM would therefore
be expected to also exhibit a minimum downshift of at least 6–8 kHz from 2f0 .
STIMULATED ELECTROMAGNETIC EMISSIONS
267
Figure 23. SEE spectra near the second harmonic of f0 = 3.324 MHz from the Tromsø Heating
facility. The spectra were recorded within 4 min. Taken from Derblom et al. (1989).
SEE at the third harmonic of the DM frequency has been claimed to be observed
in experiments at the Tromsø facility during conditions of a substorm expansion
(Blagoveshchenskaya et al., 1998a). The pump wave at f0 = 4.04 MHz was
cycled 4 min on/6 min off and reflected from a sporadic-E layer in the lower
ionosphere having a maximum fp above f0 . The SEE was observed in a 33-Hz
bandwidth of a receiver tuned to 12.095 MHz ≈ 3f0 − 25 kHz, which was simultaneously used to observe Doppler shifts of diagnostic waves at 12.095 MHz
scattered off pump-enhanced FAS. However, it appears difficult to distinguish SEE
in the narrow 33-Hz band from a possible spectral broadening of the diagnostic
wave scattered from the density irregularities, such as has been measured in HF
scatter from pump-enhanced FAS at the Sura facility, albeit from the ionospheric F
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T. B. LEYSER
region (Ponomarenko et al., 1999). Also, simultaneous observations of SEE spectra
near f0 were not reported. Such measurements are particularly important for f0
near 4.04 MHz ∼ 3fe , since here the SEE spectrum is very sensitive to f0 and the
DM is not excited for f0 sufficiently near 3fe .
4.12. f0 /2 EMISSION
An emission at f0 /2 for f0 = 5.423 MHz and cycling the pump 30 s on/30 s off has
been detected in experiments at the Tromsø facility and found to be of ionospheric
origin (Derblom et al., 1989). The emission has a narrow spectral width that may
be less than the 10-Hz frequency resolution of the instruments used. Further, the
emission could appear and disappear sporadically during the 30-s pump period.
The emission intensity is correlated with that at 2f0 and, frequently, also with
that of the ionospherically reflected pump wave and the DM feature. However,
no emissions have been found at (3/2)f0 , which is suggested to speak in favor of
an absolute stimulated Raman scattering process instead of a two plasmon decay
process.
4.13. A RITHMETIC MEAN PEAK (AMP)
Already in the early experiments with the Tromsø facility it was found that pumping with two frequencies separated by a few kilohertz produces an extremely narrow SEE peak at the arithmetic mean of the pump frequencies, in addition to the
usual spectral features excited during single-frequency pumping (Stubbe et al.,
1985).
The temporal evolution of the AMP has been studied at the Sura facility for an
ERP of 70 MW at f01 = 9.310 MHz and an ERP of 20 MW at f02 = 9.316 MHz =
f01 + 6 kHz and cycling both pumps 10 s on/50 s off (Waldenvik, 1994). The AMP
was observed to appear with a delay time of about 3 s after pump-on, exhibiting
a slow growth similar to that of the BC and DM features. Whereas the spectral
width of the AMP in the Tromsø experiments (Stubbe et al., 1985) were observed
to be about 3 Hz, it was close to 200 Hz in the Sura experiments (Waldenvik,
1994), which is of the order of the ionospheric F-region effective electron collision
frequency.
5. SEE and Other Diagnostics
In this section simultaneous measurements of SEE and other diagnostics are reviewed. The diagnostic tools include weak HF radio waves to study AA and Doppler
shifts, ISR to measure background plasma parameter values as well as electrostatic
fluctuations, and coherent scatter radars to probe FAS.
STIMULATED ELECTROMAGNETIC EMISSIONS
269
5.1. A NOMALOUS ABSORPTION (AA) OF HF RADIO WAVES
Early correlations between SEE and AA measurements were found in experiments
at the Zimenki and Sura facilities (Boiko et al., 1985), which showed the growth
of SEE at f− = 35 kHz (f0 = 4.785 MHz) occurring together with AA of weak
diagnostic waves of frequencies from fd = f0 + 100 kHz to fd = f0 + 200 kHz
during the first 1–2 s after pump-on. It was later found that the entire SEE band
f− ≈ 10–60 kHz develops together with the growth of AA (Erukhimov et al.,
1987; Yerukhimov et al., 1987). Further, the minimum pump power at which SEE
can be observed coincides with the threshold power for AA of the pump wave
(Yerukhimov et al., 1987).
AA of the pump wave is separated in fast (FAA) and slow (SAA) type (Frolov,
1991; Frolov et al., 1997c). Figure 24 displays in the top panel (a) the SAA of the
pump wave and the generation of FAS at l⊥ = 3 m and 13 m, and in the bottom
panel (c) the SEE intensity after pump-on at f− = 5.6, 10, and 43 kHz. The FAA
had a development time scale of approximately 0.4 s, which is associated with
the formation of FAS with l⊥ ≈ 3 m and correlates with the development SEE at
downshifted frequencies. The SAA growth has a time scale of approximately 5 s,
which is associated with the formation of 10-m scale FAS and is correlated with a
decrease in the lower sideband SEE intensity after an overshoot at approximately
2 s after pump-on. The correlation between the SEE and FAS dynamics displayed
in Figure 24 is further discussed in Section 5.3.
A detailed comparison between the AA of the pump wave and four obliquely
radiated low-power diagnostic waves, and a number of SEE spectral features, for
f0 near sfe (s = 3, 4, 5) has been performed in experiments at the Tromsø facility
(Stubbe et al., 1994). For s = 3 the SEE spectra are characterized by the BSS,
BC, DM, and DP features, whereas for s = 4, 5 the spectra exhibited the BUM,
BC, and DM features. The pump operation lasted typically 6 min at a given f0
including some off/on switching. A typical tuning time to a new f0 was 2 min.
At all harmonics s the DM emission intensity and discreteness, the width of the
downshifted SEE spectrum, and the AA of the diagnostic waves were minimum
at the same f0 ≈ sfe . Further, the changes in AA near the AA minimum occur
together with pronounced changes in the SEE spectra. However, differences in
the SEE and AA were found when comparing different harmonics s, as discussed
below.
Figure 25 exhibits the AA of a low power diagnostic wave with the frequency
fd = f0 − 70 kHz and SEE spectra for different f0 near 5fe . For each f0 are shown
two SEE spectra with different frequency spans, in order to highlight different
spectral features. Comparing the lowest f0 in Figure 25, f0 = 6.60 MHz (a) with
f0 = 6.74 MHz (b), the AA was approximately the same with no reproducible
features in the intermediate f0 range while the SEE BC became slightly weaker
and the DM at f− ≈ 10 kHz slightly sharper. Within the narrow f0 range from
f0 = 6.74 MHz (b) to f0 = 6.76 MHz (c) the AA decreased to a narrow minimum
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T. B. LEYSER
Figure 24. Temporal development of (a) SAA of the pump wave (PW), small-scale FAS with
l⊥ = 3 m and 13 m, and (c) SEE at f− = 5.6 kHz, 10 kHz, and 43 kHz after pump-on (t = 0 s)
for f0 = 4.785 MHz. Panel (b) shows the calculated power index p of the spatial spectrum of FAS
for the experimental results in panel (a). Taken from Frolov et al. (1997b).
while in the SEE spectra the DM disappeared and the BUM appeared. Inside the
AA minimum, the frequency upshift of the BUM from f0 depended only weakly on
f0 , so that it did not follow Equation (2). At only 20 kHz higher f0 (d) the AA was a
few dB higher, the DM emission has became very strong, and the BUM had grown
and was shifted further above f0 . At the highest f0 (e), the AA has reached the level
of points (a) and (b) below 5fe , the BC was well developed, and the BUM weakened somewhat and moved to higher frequencies. Another characteristic feature of
s = 5 was that the AA minimum consisted of a narrow and a wide minimum. The
narrow minimum, which formed the low frequency edge of the wide minimum,
STIMULATED ELECTROMAGNETIC EMISSIONS
271
Figure 25. AA of a diagnostic wave with fd = f0 − 70 kHz and SEE spectra for different f0 near
5fe at the Tromsø Heating facility. Taken from Stubbe et al. (1994).
coincided in f0 range with that of the disappearance of the DM. The wide AA
minimum coincided with the minimum of the entire downshifted SEE spectrum.
For f0 near 4fe the AA and SEE spectra exhibited similar features as for f0
near 5fe , although the BUM existed in a narrower f0 range at s = 4 compared to
that at s = 5.
For f0 near 3fe the situation was quite different, mainly what concerned the
SEE spectra. Figure 6 shows the AA of a diagnostic wave with fd = f0 − 70 kHz
and SEE spectra for different f0 around 3fe . At f0 = 4.00 MHz (a) a weak DP
appeared at f− = fDP ≈ 3 kHz and the DM appeared split, with a separation
of the two DM peaks of approximately 2.5 kHz. At the successively higher f0 in
points (b) and (c) the intensity of the DP emission increased and fDP decreased
while the DM disappeared. At f0 = 4.10 MHz (d), just above the AA minimum
the DP disappeared into the continuum while the DM reappeared and reached a
very high intensity and the BSS with intensity maxima at f± ≈ 23 kHz appeared
as a new feature. Thus the DP emission, which existed mainly for f0 below the
AA minimum, was the strongest when the DM was the weakest, which occurred
simultaneously with the AA minimum. At f0 = 4.12 MHz (e) the BSS attained its
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T. B. LEYSER
maximum frequency shift from f0 and the DM became less distinct. At still higher
f0 the BSS disappeared and the AA reached a maximum at f0 = 4.15 MHz. This
AA maximum for f0 > 3fe was found in all experiments at s = 3 (unlike the AA
maximum at s = 5 in Figure 25 which was not systematically occurring). Very
high values of the AA were found at the maximum (up to 27 dB). At the highest f0
(f) the DM vanished into the BC.
5.2. E LECTRON TEMPERATURE ENHANCEMENTS
Anomalous heating of the ionospheric F-region plasma during daytime HF pumping is closely related to the AA of electromagnetic waves discussed in Section 5.1.
It has been shown that the anomalous heating dominates over collisional heating in
F-region experiments at high latitudes (Honary et al., 1995; Robinson et al., 1996).
Figure 26 shows experimental results for the f0 -dependence of the pump–plasma
interaction near 3fe as studied by measurements of SEE, AA of weak ordinary
mode diagnostic waves, as well as electron temperature (Te ) and density (Ne )
with the EISCAT-UHF ISR near Tromsø (Honary et al., 1995). The pump was
operated in CW mode for 4–5 min on each f0 . The displayed relative changes in
Te and Ne were measured in the range gate of 22.5 km extent nearest to the pump
reflection height. Large-scale Te enhancements by up to 30% of the ambient Te are
observed. In the top panel it is seen that the AA of a diagnostic wave of frequency
fd = 3.5 MHz has a broad minimum centered at f0 = fmin ≈ 4.08 MHz and
that it is asymmetric with lower AA for f0 < fmin and higher AA for f0 > fmin ,
where fmin ≈ 3fe . Notable is also the maximum in AA for f0 slightly above fmin .
Also shown in the top panel is the signal strength of the diagnostic wave averaged
during periods of pump-off, excluding the first 10 s after pump-off in order to allow
the diagnostic wave to recover from AA, which exhibits an unexpected minimum
at f0 = 4.10 MHz ≈ fmin + 20 kHz. The DM intensity plotted in the second
panel is measured relative to the right-sided continuum, thus being a measure of the
discreteness of the DM (Stubbe et al., 1994). Consistent with the results discussed
in Section 5.1, the minimum of the DM emission intensity coincides well with
the minimum of the AA. On the other hand, the DP intensity, measured from
the background noise level, increases approximately linearly with f0 , starting at
f0 ≈ fmin − 100 kHz up to f0 ≈ fmin above which it ceases to exist. The third
panel displays the pump-induced Te and Ne enhancements. It is seen that a broad
minimum where Te /Te is less than 10% occurs when f0 ≈ fmin . The changes in
Ne as f0 varies are much smaller and are of similar size as the experimental errors.
The ambient values of Te and Ne are relatively constant during this experiment as
seen in the bottom panel.
5.3. F IELD - ALIGNED STRIATIONS (FAS)
AA is due to scattering of the electromagnetic waves on a wide spectrum of FAS.
However, by radar scatter measurements instead the dynamics of FAS at a given
STIMULATED ELECTROMAGNETIC EMISSIONS
273
Figure 26. Simultaneous measurements of AA, SEE, ionospheric electron temperature, and density
for different f0 near 3fe obtained at the Tromsø facility. The top panel shows the AA of a diagnostic wave (fd = 3.5 MHz) for different f0 between 3.96 MHz and 4.18 MHz, together with the
relative signal strength of the diagnostic wave as function of time, measured during periods of no
pumping. The second panel shows the intensity of the DM and DP emissions. The third panel shows
the pump-induced relative electron temperature and density changes. The bottom panel shows the
ambient electron temperature and density. Taken from Honary et al. (1995).
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T. B. LEYSER
spatial scale l⊥ transverse to the geomagnetic field can be studied. The long time
SEE features are associated with the formation of small scale FAS. Already in
the early literature it was observed that strong SEE occurs whenever HF radar
backscatter echoes from FAS are strong (Thidé et al., 1983; Boiko et al., 1985).
FAS scales which determine excitation and suppression of SEE are distinguished
by using different pump duty cycles and preconditioning (Erukhimov et al., 1987).
From data on the decay times of FAS with different l⊥ it is found that the excitation
of SEE in the range f− = 10–70 kHz is determined by FAS having l⊥ decreasing
from a few tens of meters to 5 m for increasing f− . The FAS determining the
suppression of the SEE have l⊥ increasing from 3 m to 10 m for increasing f− .
Simultaneous measurements of the temporal evolution of AA of the pump wave
at f0 = 4.785 MHz, of small-scale FAS, and of SEE have been reported (Frolov,
1991; Frolov et al., 1997c). As seen in Figure 24, the SEE intensity near f0 at
f− = 5.6 kHz is stabilized with the growth of FAS with l⊥ = 3 m, while the
SEE intensity at the larger downshifts of f− = 10 kHz and 43 kHz grows and
reaches a maximum at about t ≈ 2 s simultaneously with the intensity of the FAS
at l⊥ = 3 m (Frolov, 1991; Frolov et al., 1997c). The subsequent decrease in the
SEE at f− = 10 kHz and 43 kHz as well as that of FAS with l⊥ = 3 m coincides
with the growth of FAS with l⊥ = 13 m. Further, the overshoot in the SEE intensity
for f− = 10–60 kHz, which coincides with the AA development, increases with
decreasing f0 . For f0 ≥ 6 MHz the overshoot is practically undetectable and the
emission intensity increases monotonically to its steady state level.
For conditions of developed pump-excited FAS, SEE is excited within a few
milliseconds by the pulsed pumping, which is required to maintain the FAS. The
SEE can therefore be used to study dynamics of FAS which occurs on a much
longer time scale, as is exploited in the DSEE technique (Frolov et al., 1994).
Measurements have shown that even for a high pump ERP of 150 MW the formation time for the DSEE for conditions of well developed FAS is 5–30 ms, where the
smaller (larger) values correspond to daytime (evening) experiments. To elucidate
such diurnal variations in the results it has been suggested that attention should be
payed to the possible influence of naturally excited FAS on the SEE and DSEE
dynamics (Frolov et al., 1994).
Further, it was found that although the level of FAS development influences
only weakly the stationary SEE intensity, it has a major influence on the DSEE
dynamics. The typical DSEE dynamics following pump-off consists of an initial
intensity increase followed by a slower intensity decay. The rise time of this overshoot is suggested to be controlled by the decay of the smallest scale FAS with
l⊥ ≤ 5 m at pump-off whereas the subsequent slower intensity decay is suggested
to be determined by the decay of 10-m scale FAS.
An interesting result is that an hysteresis effect is exhibited in the dependence of
the DSEE intensity on the pulsed pump power. For a given pump ERP of 17 MW
and pulsed pump ERP of 1.7–17 MW the maximum intensity of the DSEE at
f− = 15 kHz from the pulsed f0 increased only weakly with increasing pulsed
STIMULATED ELECTROMAGNETIC EMISSIONS
275
pump ERP (12 ms pulse duration and 200 ms inter-pulse period). However, for a
pulsed pump ERP of less than 1.7 MW the DSEE intensity decreased significantly
with decreasing pulsed ERP.
By considering the linear dispersion characteristics of UH waves it has been
conjectured (Leyser, 1989; Leyser et al., 1990) and theoretically predicted (Mjølhus, 1993; Gurevich et al., 1995b) that the excitation of small-scale FAS should
be suppressed for f0 near sfe , as is the case for the DM emission. This has been
experimentally verified by observing a minimum in the intensity of resonant HF
scattering off FAS together with the absence of a DM feature in SEE spectra excited
by the Sura facility for f0 ≈ 4fe (Ponomarenko et al., 1999). The HF diagnostic
wave transmitted from a broadcasting station near Moscow and received at the
UTR-2 radio telescope near Kharkov had a frequency of about 15 MHz implying
l⊥ ≈ 10 m for the resonant FAS in the used scattering geometry. The pump wave
was tuned to a given f0 for 1 min at the time before f0 was increased in steps of
15 kHz during CW pumping. Further, an interesting result is that the spectral width
of the scattered signal increased by a factor of five as f0 was increased from below
to above 4fe . The decay time at pump-off of the flanks of the broad spectrum of the
scattered signal was at least one order of magnitude shorter than that of the central
part. Thus both the spectral width of the HF scatter from the FAS and the SEE
spectrum are asymmetric with respect to f0 around 4fe . Whether this asymmetry is
related or not is presently an open question. It is interesting to note that it is only the
HF electrostatic turbulence, but not the slow transport processes which is directly
sensitive to electron gyro harmonic effects and it is this turbulence which drives the
slow self structuring of the plasma into FAS. At present, it can only be speculated
that electron Bernstein (EB) waves could be involved in both the FAS dynamics and
SEE for f0 slightly above 4fe . The observation of gyro harmonic effects in pumpexcited FAS link together several phenomena believed to be directly or indirectly
related to the existence of FAS, such as SEE, AA of low power diagnostic waves
as well as of the pump wave, electron temperature enhancements, and negative
Doppler shifts of vertical HF diagnostic waves (discussed in Section 5.4).
A reduction in the HF scattering from FAS was also observed in experiments
with the Tromsø facility for f0 ≈ 3fe (Honary et al., 1999). The pump was cycled
2 min on/2 min off at f0 = 4.10 MHz for a few periods, followed by a few cycles
at f0 = 4.05 MHz, and finally the pump was switched back to f0 = 4.10 MHz for
the last few cycles of the experiment. The CUTLASS radar operating between 8–
20 MHz indicated a reduced intensity of FAS as f0 was changed from 4.10 MHz to
4.05 MHz. In the later part of the pump cycling at f0 = 4.05 MHz the ionospheric
conditions changed so that HF scattering intensity increased again and f0 no longer
matched 3fe . The changing of f0 from 4.10 MHz to 4.05 MHz was correlated
with the disappearance of the DM feature in the SEE spectrum. Simultaneous measurements with EISCAT-UHF ISR showed that the HF enhanced ion line (HFIL)
persisted at varying strength only when f0 = 4.05 MHz ≈ 3fe , as discussed in
Section 5.5.
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T. B. LEYSER
HF pump experiments and the DSEE technique have been proposed to be useful
for the study of the formation of naturally driven mid-latitude F-region FAS (Kagan
and Frolov, 1996). Particularly, the excitation mechanism for natural meter-scale
FAS remains an open question. When switching from CW pumping at a given f0
to the diagnostic mode consisting of a 20-ms pulse every 1 s, the DSEE intensity
(measured at f0 − 22 kHz) increased toward a maximum in a time tmax ≈ 4 s and
then subsequently decayed with an e-folding decay constant τd ≈ 3 s. By selecting
the inter-pulse period in the diagnostic mode to 1 s, the dynamics of FAS with
l⊥ ≈ 3 m are studied, which is known to be an important scale for the SEE and at
the same time represents a poorly understood scale for the naturally excited FAS.
A significant increase both of tmax (by factor of 4–7) and τd (by a factor of 10–20)
was observed to be correlated with sunset in the ionospheric F region at the latitude
of the experiments, which was attributed to the changing of the geomagnetic fieldaligned Sq currents from upward to downward at sunset. It is thereby noted that
field-aligned currents can be important for the excitation of natural small-scale
FAS.
5.4. M ULTI - FREQUENCY D OPPLER RADAR (MDR)
By measuring the Doppler shifts of several low-power diagnostic electromagnetic
waves with slightly different frequencies typically near f0 , pump-induced density
profile perturbations of ∼ 0.1% can be reconstructed, with a spatial resolution of
c/fd ∼ 0.1 km and temporal resolution of 1 s (Lobachevsky et al., 1992). Negative
Doppler shifts of the diagnostic wave frequency have been attributed to plasma
density decreases along the diagnostic wave path and positive Doppler shifts have
been attributed to density increases.
Simultaneous SEE and MDR measurements, which employ eight different ordinary mode diagnostic waves at frequencies fi (i = 1, 2, ..., 8), have by direct
measurements shown a connection of SEE with perturbations in the UH resonance region (Lobachevsky et al., 1992). Figure 27 displays the SEE spectrum
and the Doppler recordings for f0 = 6.96 MHz at the Tromsø facility. As seen in
Figure 27(a), the SEE spectrum is wide and intense. The curves in Figure 27(b),
labeled by fi , were obtained by coherently mixing fi down to approximately 3 Hz,
so that in the absence of pump-induced ionospheric perturbations on the diagnostic
wave each channel would display a 3-Hz oscillation with constant amplitude. The
vertical bar at t = 0 denotes pump-on. As seen, the pump–ionosphere interaction
leads to frequency changes of the diagnostic waves and strong Doppler shifts occur
for diagnostic waves which reflect near the UH resonance of the pump wave (where
fuh = f0 ), indicating the excitation of ionospheric irregularities.
For f0 = 4.04 MHz, which is near 3fe , considerable changes in the SEE intensity and spectral width are observed, depending on the ionospheric conditions.
At times the SEE is weak and the Doppler recordings show perturbations only
near the pump reflection height but not near the UH resonance. When the SEE
STIMULATED ELECTROMAGNETIC EMISSIONS
277
Figure 27. SEE spectrum (a) and Doppler recordings (b) for f0 = 6.96 MHz at the Tromsø Heating
facility. Each curve in (b) is labelled by the corresponding diagnostic wave frequency fi . Taken from
Lobachevsky et al. (1992).
278
T. B. LEYSER
instead is well developed, the Doppler records are diffuse, such that pump-induced
effects can not be safely identified, indicating the presence of pump-induced or
natural ionospheric irregularities. The present author notes that the absence of
SEE together with the suppression of density perturbations at the UH resonance
height, is consistent with f0 ≈ 3fe for which pump-excited UH phenomena are
suppressed. These experimental results appear to constitute the first more direct
measurements of a correlation between SEE and UH phenomena.
Simultaneous MDR and SEE results from systematic stepping of f0 around sfe
(s = 4, 6) as well as for different pump reflection heights (hr ) have been presented
(Grach et al., 1997). In addition to measuring the Doppler shifts (fDi ) of eight
different diagnostic waves of frequencies fi (i = 1, 2, ..., 8), also the AA and the
evolution time (τi ) of the AA was studied. To ensure meaningful measurement of
these quantities, the pump-on periods (30–60 s) were separated by long periods
of pump-off (240–390 s), in order to allow pump-enhanced density structures to
decay and/or drift away before the next pump-on.
For f0 > sfe and all hr as well as for f0 < sfe and hr < 220 km, the Doppler
shifts were negative. For f0 ≈ sfe the magnitude of the Doppler shifts, AA, and τi−1
were minimum. For f0 < sfe , hr > 220 km and fi near fp (hUH ) the Doppler shifts
were positive, where fp (hUH) is the plasma frequency at the UH resonance height
hUH of the pump. However, for |fi − fp (hUH )| ≥ 100 kHz the Doppler shifts were
negative and comparable to those for f0 > sfe . Figure 28 shows Doppler phase
t
shifts ϕDi = 0 fDi dt (pump-on is at t = 0) and corresponding SEE spectra for
all three cases. Figures 28(a) and 28(d) show an example of the temporal evolution
of ϕDi for different fi − f0 for f0 = 5.385 MHz > 4fe and the related SEE
spectrum (hr ≈ 225 km). As is typical for f0 slightly above sfe the SEE spectrum
exhibits a DM at downshifted frequencies and a BUM at frequencies upshifted
from f0 . Figures 28(b) and 28(e) display ϕDi and the SEE spectrum for f0 =
5.360 MHz ≈ 4fe (hr ≈ 215 km). The SEE spectrum did not contain the DM
emission and the BUM was weaker compared to the higher f0 in Figure 28(d). The
magnitude of ϕDi was smaller than for f0 further away from 4fe , although ϕDi < 0
for all fi . The AA was also much smaller and the AA evolution time was much
longer compared to for f0 further away from 4fe .
For f0 < sfe and hr < 220 km, ϕDi was similar to the case f0 > sfe displayed
in Figures 28(a) and 28(d). However, for hr > 220 km ϕDi was positive for the
diagnostic wave frequencies −80 kHz < fi − fp (hUH ) < 150 kHz. For fi further
away from fp (zUH ), ϕDi was negative and similar in magnitude to the case f0 >
sfe , as seen in Figure 28(c). The SEE spectrum in Figure 28(f) exhibits again the
DM emission but not the BUM, as is typical for f0 < sfe . The asymmetry in the
sign of ϕDi , with respect to f0 above or below sfe for sufficiently high interaction
altitudes, was not observed to be correlated with any corresponding asymmetry in
max
= max|ϕDi (t = τi )|, τi , and the DM intensity depended
the SEE. However, ϕDi
max
both on hr and f0 − fe . For hr increasing from 205–215 km to 225–235 km, ϕDi
and the DM decreased by approximately a factor of 2 and 4–6 dB, respectively, and
STIMULATED ELECTROMAGNETIC EMISSIONS
279
Figure 28. Phase shifts ϕDi versus time t and fi − f0 (a, b, c) for t = 2 s (dotted lines) and t = 8 s
(solid lines) and related SEE spectra (d, e, f) obtained at the Sura facility. Panels (a) and (d) are for
f0 = 5.385 MHz > 4fe . Panels (b) and (e) are for f0 = 5.360 MHz ≈ 4fe . Panels (c) and (f) are
for f0 = 5.280 MHz < 4fe . The narrow spikes in the SEE spectra are caused by the Doppler radar
pulses. Taken from Grach et al. (1997).
τi increased from 2–3 s to 5–9 s. This altitude dependence was determined from
cases with the same BUM frequency upshift in the SEE spectra (s = 4) implying
the same f0 − sfe . Measurements for the same hr ≈ 223 km but different f0
(s = 4) showed that for the BUM frequency increasing from f+ = 18 kHz to
f+ = 42 kHz, corresponding to an increase of f0 − 4fe of 24 kHz or more, the
max
increased by approximately a factor of 2, and
DM increased by 10–15 dB, ϕDi
τi decreased from 5–10 s to about 2 s, but no systematic changes in the AA was
observed.
5.5. L ANGMUIR TURBULENCE ( PLASMA AND ION LINES )
A commonly used diagnostic tool during ionospheric HF pump experiments is the
ISR. Whereas the SEE may result from a range of wave vectors of electrostatic
fluctuations, an ISR gives measurements of pump excited electrostatic fluctuations
280
T. B. LEYSER
at essentially two given wave vectors, namely parallel and anti-parallel to the radar
line of sight. Scatter from pump-enhanced Langmuir fluctuations is referred to as
HF enhanced plasma lines (HFPL) and scatter from IA fluctuations give the HFIL.
The SEE and ISR techniques are therefore in some sense complementary to one
another.
The first simultaneous measurements of SEE and Langmuir turbulence as studied through HFPL used the EISCAT-UHF radar system in Tromsø and Kiruna
(Nordling et al., 1988). The geometry of the experiment implied that the probed
Langmuir waves had a wave vector directed approximately along the ambient geomagnetic field for the Tromsø UHF station and about 23◦ to the magnetic field for
the Kiruna station. The HFPL and SEE exhibited a similar temporal evolution on a
time scale of several seconds, in that an initial overshoot in the signal strength was
followed by a decay and thereafter an increase back to a high level a few seconds
after pump-on (the temporal resolution in the HFPL measurements was 2 s). The
HFPL persisted throughout entire pump periods of a minute or more, which was
different from previously reported results.
For a low f0 = 3.515 MHz the SEE was well developed while the HFPL were
very weak and only observed during the first few seconds of the 30-s pump period,
indicating that the plasma wave vector probed by the EISCAT-UHF radar did not
contribute significantly to the SEE.
For f0 = 5.423 MHz two excitation regimes were observed in the HFPL and
SEE spectra. In the first case the SEE exhibited a BUM and a DM feature, which
was observed during one hour. During that same hour the HFPL spectra (Tromsø
station) contained narrowly separated cascade lines shifted by about 50–90 kHz
toward the radar frequency in both the up- and downshifted HFPL, in addition to
the usual purely growing line, decay line, and one or two additional cascade lines.
The frequency separation between the discovered narrowly separated cascade lines
was close to the associated IA frequency, i.e., the frequency of IA waves at half the
radar wavelength, whereas the frequency separation of the usual cascade lines was
twice as much. Further observations are necessary before a possible correlation
between the BUM and the narrowly separated cascade lines can be discussed. The
SEE and HFPL spectral features did not overlap in frequency, the BUM being
upshifted from f0 and the narrowly separated cascade lines being downshifted
from f0 . Also, whereas the BUM emission depends on electron gyro harmonic
effects, involving electron dynamics transverse to the geomagnetic field, the HFPL
measurements were made essentially parallel to the magnetic field.
In the second case at f0 = 5.423 MHz, the SEE did not contain a BUM but
only the DM and continuum at downshifted frequencies. In this case the upshifted
HFPL spectrum exhibited broad and widely separated secondary lines up- and
downshifted from the decay line by approximately 50 kHz. The downshifted HFPL
did only exhibit the purely growing line, decay line, and first cascade line. Again,
further observations are necessary before a possible correlation between the SEE
and HFPL can be discussed.
STIMULATED ELECTROMAGNETIC EMISSIONS
281
The different SEE spectra observed for the same f0 = 5.423 MHz was attributed to different ionospheric conditions. It is now understood that the BUM
was excited during conditions when f0 was slightly above 4fe while the spectrum without a BUM was obtained when either f0 was sufficiently above 4fe or
f0 < 4fe .
A detailed comparison has been made of the initial temporal evolution of SEE
and HFPL with the 430-MHz ISR of Arecibo Observatory, for f0 = 3.1745 MHz
and cycling the pump 5 s on/5 s off, which implies a preconditioned ionosphere
(Thidé et al., 1995). The Arecibo ISR typically detects the electrostatic fluctuations
at an angle of approximately 42◦ to the geomagnetic field. At the used low f0 the
SEE was consistently stronger than at higher f0 while the HFPL was weaker than
at higher f0 and disappearing completely after an initial overshoot. Estimations of
fe indicated that f0 ≈ 3fe + 117 kHz was sufficiently far from 3fe so that no
gyro-harmonic effects were expected in the SEE.
Figure 29 shows simultaneously recorded HFPL (left) and SEE spectra (right)
from the first 500 ms after pump-on. The top spectral pair was obtained 50 ms
after the arrival of the pump wave in the ionosphere and the successively lower
spectra were obtained with 150-ms separation. Both the HFPL and SEE had the
maximum intensity within 50 ms of pump-on, after which both the SEE and HFPL
intensities decreased. As seen in Figure 29 the HFPL spectrum was relatively broad
and diffuse, exhibiting a weak purely growing line, decay line, and a few cascade
lines. The SEE spectrum exhibited the FNC extending from f− ≈ 30 kHz to
f+ ≈ 10 kHz. The subsequent HFPL spectra were weaker and 100–250 ms after
pump-on the cascade peaks were generally most pronounced. After 500 ms the
HFPL disappeared completely. The SEE spectra also became more structured after
the initial overshoot. In particular, notches developed in the spectra at about f0 −
3 kHz and f0 − 6 kHz, as indicated by arrows in the bottom spectrum in Figure 29.
The SEE frequency component at f− = 3 kHz coincides with the frequency
shift of the decay line in the HFPL, both of which weaken significantly with time
after the overshoot. It should be recalled here that the HFPL is due to electrostatic oscillations at the one wave vector probed by the radar while the SEE is an
electromagnetic emission which may be excited by a wide range of wave vectors
of the electrostatic oscillations, including that probed by the radar. The notch at
f− = 6 kHz, coinciding with flh in the F region above Arecibo, later developed
into the high frequency flank of the DM.
The reason for the HFPL overshoot within 50 ms remains an open question.
On the basis of the SEE results, it appears that a broad spectrum of plasma wave
vectors are involved (Thidé et al., 1995). Further, it may be noted that the initial
overshoot in the SEE within 50 ms after pump-on occurred on the same time scale
as that observed at the Sura facility for f0 = 5.828 MHz and cycling the pump
10 s on/50 s off (Waldenvik, 1994). The disappearance of the HFPL after several
100 ms was attributed mainly to Landau damping at the low plasma densities in the
interaction region when pumping at f0 = 3.1745 MHz, which was similar to the
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T. B. LEYSER
Figure 29. Simultaneously recorded upshifted HFPL (left) and SEE spectra (right) from the Arecibo
facility. The top spectral pair was obtained 50 ms after the pump wave arrived in the ionosphere and
the successively lower pairs were obtained with 150-ms separation. Taken from Thidé et al. (1995).
STIMULATED ELECTROMAGNETIC EMISSIONS
283
results of the EISCAT-UHF measurements at the low f0 = 3.515 MHz discussed
above (Nordling et al., 1988).
As mentioned in Section 5.3, simultaneous measurements of SEE and the HFIL
with the EISCAT-UHF radar have been performed for f0 near 3fe (Honary et al.,
1999). The pump wave was transmitted from the EISCAT-Heating facility, which
was cycled 2 min on/2 min off. The HFIL persisted with varying intensity throughout the pump period for f0 ≈ 3fe . The HFIL, attributed to nonlinear interactions
between Langmuir and IA oscillations, usually persist only during the first few seconds after pump-on, which has been attributed to increased electron Landau damping of the Langmuir waves as the electron temperature increases during pumping
(Hagfors et al., 1983). The persistence of the HFIL throughout the pump period
for the case f0 ≈ 3fe is consistent with the minimum in pump-induced AA and
electron temperature enhancements for f0 ≈ sfe .
6. Interpretation of SEE Features
In this section theoretical models, numerical computations, and simulation results
which have been propose to interpret SEE are discussed.
6.1. FNC
6.1.1. Low Pump Duty Cycles
A two-scale model for the interaction between the electromagnetic pump wave and
the electrostatic turbulence has been formulated to interpret SEE on time scales
less than a few 100 ms after a cold pump-on (for which thermal nonlinearities
can be neglected), which involves a local small scale level of Langmuir turbulence
described by a one-dimensional electromagnetically driven and damped Zakharov
type model and a large-scale electromagnetic level (Mjølhus et al., 1995). Emphasis was put on predicting signatures in SEE spectra resulting from cascading
and cavitating Langmuir turbulence. For typical experimental parameter values
the numerical solutions show that Langmuir turbulence in the cavitation regime
is possible near the pump reflection height where the difference f0 − fp is small.
For a given f0 and decreasing fp , i.e., lower altitudes in a monotonic ionospheric
plasma profile (see Figure 1), the turbulence changes gradually to the cascading
regime. Further, for increasing pump strength the region with cavitating turbulence
extends downwards towards lower fp . It is interesting to note that even in the
cascading regime the numerical solutions show weak excitations of the anti-Stokes
components upshifted from f0 , in addition to the downshifted cascade products.
A clear distinction was found between the frequency spectra in the two turbulence regimes. The spectra resulting from cavitation are significantly wider than
those from cascading turbulence, both upshifted and downshifted from f0 . If the
altitude range in which cavitation exists extends over many standing pump wave
284
T. B. LEYSER
maxima below the reflection height, the width of the downshifted SEE spectrum
could be typically 200 kHz and the spectrum would be modulated as a result
of interference between the radiation emanating from the successive pump wave
lobes.
Further, it is pointed out that the free mode observed in ISR measurements at
Arecibo drops within a time scale of a few tens of milliseconds (Mjølhus et al.,
1995). In this context it was noted that the most important weakness of the used
local model for the Langmuir turbulence may be that the feedback of the electron
heating and tail formation on the turbulence was not taken into account. A particularly interesting experimental result in this context is the decreasing decay time
τd of the FNC at pump-off with the development of the overshoot, both of which
depend on the pump power and, thus, indicate a dependence on modifications of
the electron velocity distribution during the pumping (Sergeev et al., 1998).
Evidence of strong Langmuir turbulence via SEE have been claimed to be
demonstrated by comparing results from low pump duty cycle experiments at
HIPAS Observatory (f0 = 2.85 MHz and an ERP of 64 MW) with numerical
solution of a one-dimensional driven and damped Zakharov model (Cheung et al.,
1997). The weak type of spectrum shown in Figure 3(a) is consistent with Langmuir turbulence of the coexistence type with cascades and random collapse. The
strong type of spectrum in Figure 3(b) is consistent with Langmuir collapse processes (cavitation regime) in standing pump wave lobes near the pump reflection
height. The modulations of the strong type of spectrum are attributed to constructive and destructive interference between the up-going and down-going electromagnetic waves emitted by the Langmuir turbulence in the dominating first
standing pump wave maximum below the reflection height. However, it may be
noted that whereas the strong type of spectrum exhibited modulations both upshifted and downshifted from f0 , the numerical solutions shows modulations only
at higher frequencies than f0 . The occurrence of weak or strong type of spectrum
was suggested to be determined by variations in the ionospheric absorption of
the pump wave, with a stronger pump strength in the reflection region leading to
cavitating turbulence.
For sufficiently high pump power, the decay time τd as measured in the lower
sideband of the FNC after pump-off is significantly shorter than that expected for
collisional damping. Preliminary estimates indicate that this short τd is due to the
interaction of super-thermal electrons with the plasma waves (Sergeev et al., 1998).
Also, it is interesting to note that strong Langmuir turbulence theory predicts that
the decay time after pump-off of cavitating Langmuir turbulence is 1 ms or less
(DuBois et al., 1990, 1993). In cavitating turbulence the cavitons are nucleated by
direct interaction with the pump. When the pump is switched off the cavitons will
rapidly decay by transit time damping.
The results of the fast decay of the SEE appear consistent (Sergeev et al., 1998)
with those obtained for the broad type of spectrum in HFPL measurements with
the Arecibo ISR (Fejer et al., 1991; Sulzer and Fejer, 1994). When cycling the
STIMULATED ELECTROMAGNETIC EMISSIONS
285
pump 50 ms on/950 ms off at 80-MW ERP, the decay time of the broad spectrum from near the pump reflection height was less than 1 ms, which is shorter
than that expected for collisional damping and was therefore attributed to collisionless damping, presumably involving caviton burnout following collapse in
cavitating Langmuir turbulence. However, simultaneously with excitation of the
broad spectrum near the reflection height, a cascade line spectrum was observed
at a few hundred meters lower altitude, near and above the radar matching height.
The line spectrum exhibited a successively longer decay time after pump-off for
frequency components closer to the radar frequency, which is consistent with cascading Langmuir turbulence. It is interesting to notice that the line spectrum could
be significantly stronger than the broad type, although its growth time was longer
(Fejer et al., 1991). A stronger line spectrum than broad spectrum was also found in
HFPL measurements with the EISCAT-VHF ISR at high latitudes (Rietveld et al.,
2000). It remains an open question why nevertheless the FNC feature appears to be
associated with the broad spectrum and not with the line spectrum, unless the radiation efficiency of the cavitating turbulence giving the broad type is significantly
higher than the cascading turbulence giving the line spectrum.
6.1.2. Higher Pump Duty Cycles
It has been suggested that the experimental data for the FNC (Figure 4) indicate
that the radiation is exited by the parametric decay instability immediately below
the pump reflection height (Frolov et al., 1997d). Particularly, it was reported that
for f0 not near sfe the FNC is excited when the pump power exceeds the threshold
of the parametric decay instability. Further, from studies of the two-scale model by
Mjølhus (1995), briefly described in Section 6.1.1, it was concluded that the short
time scale experimental results presented in Waldenvik (1994) (Figure 4) and Thidé
et al. (1995) (Figure 29) are compatible with cascading Langmuir turbulence in a
broad height range, since the observed frequency upshifted radiation extends only
a few tens of kilohertz above f0 , which is smaller than that expected for cavitating
Langmuir turbulence. However, it remains on open question whether cascading
turbulence is consistent with the experimentally observed decay dynamics of the
FNC at pump-off. When cycling the pump 0.2 s on/9.8 s off it was found that
the decay time is approximately the same for f− = 13 kHz and f− = 20 kHz
(Frolov et al., 1997d). For the parametric decay instability, or cascading turbulence,
the energy from higher frequency components should continue to cascade down
toward lower frequency components after the pump is turned off, so that the decay
time of successively lower frequencies should be successively longer. Increasing
decay times with increasing downshifts from f0 have been observed for pumpexcited Langmuir waves detected with the Arecibo ISR at 430 MHz (Wong et al.,
1983; Fejer et al., 1991).
It is interesting to note that the spectrum of the FNC when cycling the pump 10 s
on/50 s off was relatively narrow and symmetric around f0 during the first 10 ms
after pump-on, which was more pronounced for lower pump powers (Waldenvik,
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T. B. LEYSER
1994). The question arises whether this effect may be related to the double humped
HFPL spectrum observed the very first few milliseconds after pump-on in measurements with the Arecibo ISR (Sulzer and Fejer, 1994). The HFPL observations are
consistent with model predictions (Hagfors and Gieraltowski, 1972; Larsson et al.,
1976) of scattering of the electromagnetic pump by thermal ion acoustic waves
into Langmuir waves, which has been observed also for pump powers well below
instability threshold (Fejer et al., 1991). In order to detect such a mechanism as
SEE the Langmuir waves would have to scatter back into electromagnetic radiation.
6.2. SNC
The SNC is sensitive to the vicinity of f0 to sfe , which suggests that the emission
is related to UH and/or EB turbulence. Since the FNC is attributed to Langmuir
turbulence the FNC should not be sensitive to f0 being near sfe , although such experimental results have not been reported in the literature yet. Further experiments
are thus needed in order to distinguish between effects of Langmuir and UH/EB
turbulence on short time scales after pump-on, both in high and low duty cycle
experiments.
In a discussion of a model for the DP and UP features (see Section 6.3) by
Huang and Kuo (1995) it was suggested that the nonlinear evolution of the thermal
oscillating two-stream instability (OTSI) involving negatively dispersive EB waves
should be instrumental in the excitation of the SNC feature, although an explicit
study of this case was not presented.
6.3. DP, nDP, AND UP
The DP (Figures 6(b) and 6(c)) and UP were initially suggested to be a result of
the parametric decay instability involving Langmuir and IA waves at the first and
strongest standing pump wave maximum below the reflection height (Stubbe et al.,
1984). The model was further developed by including the effect of the magnetic
field on the position of the standing pump wave maxima and by taking into account
the horizontal stratification of the ionospheric plasma density profile as a result of
the ponderomotive force of the standing pump wave (Leyser and Thidé, 1988).
Particularly, it was found that contributions from the excited Langmuir waves in
the created density wells at the successive pump wave maxima could overlap in the
frequency domain. Whereas such overlapping contributions from Langmuir turbulence at the successive pump wave maxima in the horizontally stratified plasma
could occur during ionospheric HF pumping the considered process is not relevant
to the electromagnetic DP nor SNC features, since these emissions were found to
be sensitive to f0 near sfe so that they must be due to electron dynamics perpendicular to the geomagnetic field and not along the magnetic field as for Langmuir
turbulence.
An extensive study of Langmuir turbulence as applied to HF pumping of the
ionospheric plasma has been presented, involving numerical solution of a driven
STIMULATED ELECTROMAGNETIC EMISSIONS
287
and damped Zakharov model with the driving electric field polarized along the
ambient magnetic field (DuBois et al., 1990). Computations of power spectra of
the partial current |nE (ω)2 | ≡ |(nE)k=0,ω |2 , related to the complete SEE source
current, showed a spectral peak downshifted from f0 by approximately 2–5 kHz
and so, it was suggested, might correspond to the DP feature. Further, the partial
current spectra contained a strong peak at f0 with a long tail for downshifted frequencies, but very little spectral energy upshifted from f0 . As mentioned above,
since the DP depends on f0 near sfe , this feature is not likely due to Langmuir
turbulence.
The initial parametric decay of an EB wave into another EB wave and a nearly
perpendicularly propagating IA (or electrostatic ion cyclotron) wave has been theoretically modeled by coupled mode equations in a kinetic approach in the long
wavelength regime, to interpret the DP, nDP, and UP features (Huang and Kuo,
1995). The decaying EB wave has the frequency f0 and is produced through a
thermal OTSI in which the small-scale FAS is the decay mode. The resulting frequency downshifted EB wave scattering off the FAS into electromagnetic radiation
is then suggested to produce the DP. The parametrically excited downshifted EB
wave can itself decay into another EB wave and IA wave. By scattering of the
resulting EB wave off the FAS the 2DP emission can be produced. The initial
instability analysis also indicates that the downshift of the parametrically excited
EB wave from f0 decreases as f0 increases toward 3fe from below, consistent with
the observed dependence of the frequency shift of the DP on 3fe − f0 .
The excitation of the DP for f0 preferably near the gyro harmonic s = 3 and not
at higher s is attributed to the requirement of quasi-neutrality for the existence of
the nearly perpendicular IA waves in the magnetized plasma, which implies that the
electron thermal speed must exceed the parallel phase velocity of the IA wave and
makes it increasingly difficult to excite the IA waves at higher s (Huang and Kuo,
1995). Similarly, if f0 is too far separated from 3fe the condition of quasi-neutrality
is also violated, which is consistent with that the DP is only observed to be excited
in a narrow f0 range near 3fe . It is further noted that the proposed scenario for
the DP excitation is consistent with the experimentally observed AA minimum
for f0 ≈ sfe . AA of electromagnetic waves interacting with FAS is generally
believed to be due to linear conversion through the formation of thermal cavitons, a
process for which the relevant HF electrostatic waves have to have a positive group
dispersion. The thermal OTSI considered to excite FAS involves negative group
dispersive EB waves, which therefore is not associated with significant AA. However, it is not clear that the model is consistent with the experimentally observed
minimum in the intensity of FAS for f0 ≈ sfe (Ponomarenko et al., 1999). It could
be that there is a low residual level of FAS when f0 ≈ sfe , which are due to the
negative group dispersive EB waves as predicted by the model.
The UP has been interpreted within the same scenario as the DP, involving
parametric interaction between EB waves and oblique IA waves (Huang and Kuo,
1995). The DP was interpreted by parametric decay of one of the EB waves in-
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T. B. LEYSER
volved in the thermal OTSI process suggested to excite FAS, into another EB wave
and an IA wave. Scattering of the oppositely propagating EB wave in the thermal
OTSI interaction, off the decay IA wave gives a frequency upshifted EB wave,
which when scattered off FAS produces electromagnetic radiation at the frequency
of the UP.
In order to study experimentally whether the DP family of features is related to
the SNC, the temporal evolution of the DP, 2DP, and UP features following pumpon and -off should be investigated. For example, a question is whether the overshoot
associated with the growth of 10-m FAS is also absent for the DP features, similar
to the case of the SNC.
6.4. BC
The BC (Figure 7) has been interpreted in terms of a model involving, so called,
double transformation, in which UH waves are excited by the pump wave scattering
off a spectrum of FAS, and the broad UH wave spectrum resulting from induced
scattering on thermal ions scatters off FAS back into electromagnetic radiation
which can escape the plasma and be observed as SEE on the ground (Grach, 1985;
Shvarts et al., 1994, 1995; Sergeev et al., 1995; Grach et al., 1998). The simulations
are based on a theoretical model for the BC describing multiple induced scattering
of UH waves in the weak turbulence approximation (Grach, 1985) over a threedimensional dispersion surface. The computations involve a given empirical model
of the FAS spectrum and its temporal evolution after pump-on and -off, which
is based on data from HF and VHF backscatter radars. (Erukhimov et al., 1987;
Frolov et al., 1997c).
The detailed study includes the influence of the FAS intensity, steady state
FAS spectrum, pump wave energy density relative the threshold for the induced
scattering of the plasma waves, scale length of the ambient plasma density profile,
and pump power on the BC spectrum and temporal evolution (Grach et al., 1998).
The simulations show a nonlinear dependence of the BC intensity on the pump
power as well as a complex dependence of the BC spectrum on the FAS spectrum
at different stages of the temporal evolution. The FAS intensity not only determines
the excitation of the plasma waves and the conversion between the electromagnetic
pump and the plasma waves as well as the re-conversion of the plasma waves into
electromagnetic radiation, but also the AA of the pump wave and SEE itself as
it propagates downward away from the excitation region, so that there is a competition between processes that excite SEE and those which absorb SEE. As a
consequence of this significant dependence on the FAS intensity and spectrum,
it is pointed out that the SEE can be used as a diagnostic of the FAS (see also
(Erukhimov et al., 1987)).
The following pump cycle was simulated: CW pumping at an f0 not near sfe
for 60 s during which a steady state in the FAS spectrum is obtained. The decay
stage of the empirically modeled FAS spectrum is then followed for 40 s after
STIMULATED ELECTROMAGNETIC EMISSIONS
289
Figure 30. Comparison of experimental (thick lines) and numerical results (thin lines) for the BC
feature. Temporal evolution at (a) f = 27 kHz, (b) f = 55 kHz, and (c) f = 82 kHz. (d) Steady
state spectrum from experiment (circles) and numerical solution (solid line). For the comparison
between the experimental and numerical results the intensities were set equal at f− = 55 kHz and
t = 60 s. Taken from Shvarts et al. (1995).
pump-off by means of DSEE, i.e., the temporal evolution of the expected SEE
from the plasma waves in the decaying FAS is computed. The experimentally
observed dynamics of the BC during such a pump cycle involve an overshoot in
the radiation intensity during the first few seconds after pump-on after which the
intensity approaches a steady state, as seen in Figure 30 which displays a comparison between experimental and numerical results for the temporal evolution
at f− = 27, 55, and 82 kHz in Figures 30(a–c), respectively, and the steady
state spectrum in Figure 30(d). Following the CW pump-off, the DSEE intensity
increases during several seconds, which is followed by a slow decay during a few
tens of seconds. The simulation results for the BC agree qualitatively with a number
of experimental results (see Figure 30), including the magnitude of the SEE energy
flux on the ground, the steady state spectrum for f− ≈ 15–70 kHz with a slope
of 0.4–0.6 dB kHz−1 for decreasing f− , an overshoot in the radiation intensity
within a few seconds after CW pump-on, and an overshoot in the DSEE intensity
following the transition from CW to pulsed pumping. The overshoot in the DSEE
intensity after pump-off is interpreted by that the AA of the pump and SEE is due to
FAS with smaller l⊥ while the radiation generation is due to somewhat larger scale
FAS, which have a longer decay time than the smaller scale FAS. The overshoot
within 5–10 ms after pump-on during QCW pumping (Sergeev et al., 1995) can,
however, not be explained within the model.
The predicted dynamics after pump-off is approximately consistent with the
measured delay and decay times of the BC (Sergeev et al., 1994, 1995, 1997). The
decay time is consistent with the collisional damping of the plasma waves, although
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T. B. LEYSER
the observed diurnal variations cannot be fully explained. The resulting flattening
of the SEE spectral shape, due to the faster decay of frequency components closer
to f0 , is consistent with the cascade type turbulence and similar to that observed for
Langmuir turbulence with the Arecibo ISR (Wong et al., 1983; Fejer et al., 1991).
The model predicts a triangular shaped spectrum (on a logarithmic power scale)
(Grach et al., 1998), which agrees with the BC at all but the highest intensities. However, whereas the model captures a number of important experimental
results and clearly brings out the complex dependence of the SEE spectrum on
the different l⊥ of the FAS spectrum, there are certain aspects which may need
further development. The model does not appear to account for the plateau of
the BC which occurs at downshifts of a few tens of kilohertz below f0 , and the
associated slightly lower intensity between the DM and f0 compared to that at the
plateau. This could be an indication that the used weak turbulence approximation
for the induced scattering of the plasma waves is not fully applicable to the case
of UH waves driven by a powerful pump wave during long time scales. Also,
the model uses a given empirical expression for the FAS spectrum so that the
resulting plasma wave spectrum interacting with the given FAS spectrum is not
consistent, in the sense that it is the FAS spectrum which should be a response to
the driven plasma turbulence. In addition, the interesting dependence of the BC on
f0 near sfe remains an open question. Some of the gyro harmonic effects of the
BC may probably be explained within the present model by involving the proper
linear dispersion relations for the UH/EB mode near sfe . However, it is not clear to
the author whether the absence of the BC in the relatively wide f0 range between
approximately sfe and (s+0.1)fe could be explained by simply including the linear
dispersion properties of the wave modes near sfe . It may be that other possibly
simultaneously occurring nonlinear processes need to be included together in the
same model and are allowed to interact. For example, the absence of the BC is
correlated with the occurrence of the the BUM and nDM features for f0 slightly
above sfe . Finally, the delay time for the BC to appear following cold pump-on
indicates a highly nonlinear growth process, which is likely to be related to the
formation of FAS, also needs to be studied theoretically.
In a completely different scenario, the BC is attributed to transition radiation
resulting from the interaction of super-thermal electrons with small-scale FAS
(Ermakova and Trakhtengerts, 1995). An effective dielectric permittivity tensor
is derived for the case of a single magnetic field-aligned density inhomogeneity
in a homogeneous external pump electric field, which demonstrates a strong connection between electromagnetic waves and quasi-electrostatic eigen modes in the
inhomogeneous plasma. The intensity of the transition radiation is maximum when
the resonance frequency, near the UH resonance, is near an electron gyro harmonic.
The radiation spectrum is asymmetric around f0 , with upshifted radiation for the
case of density enhancements and downshifted radiation for density depletions.
To properly assess the importance of the transition radiation it is emphasized that
simultaneous measurements of SEE and super-thermal electrons are necessary.
STIMULATED ELECTROMAGNETIC EMISSIONS
291
Another scenario for the BC involves UH turbulence trapped in pre-formed
density depletions associated with the FAS (Mjølhus, 1997, 1998). The used model
is briefly described in Section 6.5.2 in which interpretations for the DM and UM
features are discussed. Strong driving of the trapped oscillation decay (TOD) instability in the cavitation regime of the UH turbulence leads to broad source spectra
which are claimed to be consistent with the experimentally observed BC. The
present author notes that whereas the numerically computed source spectra display
excitation also at frequencies upshifted from f0 , the experimentally observed BC is
not associated with upshifted emissions. However, it may be interesting to consider
the BUS feature in the upper sideband of the pump, which is excited for f0 away
from sfe , as belonging to the BC in the lower sideband, despite their different
growth and decay characteristics following pump-on and -off, respectively. Such
spectra appear to resemble the numerically computed source spectra from trapped
FAS-nucleated cavitating UH turbulence. However, again it is not clear that the
driven UH turbulence can be considered to be decoupled from the FAS during long
time scales at high power pumping, as is the case when considering pre-formed
density depletions, since it is the UH turbulence itself which excites the FAS.
6.5. DM, nDM, AND UM
6.5.1. Wave Interactions
The DM (Figure 8) has been proposed to result from the interaction of UH or EB
and LH oscillations. The frequency downshift of the DM high frequency component, i.e., the minimum f− , has been associated with the LH resonance frequency
flh (Leyser et al., 1989, 1990), where
flh2 =
fpi2 fe2
fp2 + fe2
and fpi is the ion plasma frequency (the ions are predominantly O+ in the bottomside ionospheric F region). For typical bottom-side F-region parameter values,
flh ≈ 6–8 kHz. For a given f0 , the minimum f− of the DM is smaller at Arecibo
than at Sura or Tromsø, consistent with the weaker magnetic field (smaller fe ) at
the lower latitude of Arecibo (Thidé et al., 1989; Leyser et al., 1990).
In early theoretical models initial instability analyses were performed for an
UH wave at the frequency f0 parametrically decaying into an ordinary mode electromagnetic wave and a LH wave (Leyser et al., 1989; Leyser, 1991, 1994). The
model for the DM discussed in Leyser et al. (1989) and Leyser (1991) was taken
from Murtaza and Shukla (1984) in which the theory is developed and applied to
generation of electromagnetic waves in the magnetosphere. In the ionospheric case
the UH wave was considered to result from the pump wave scattering off FAS and
the parametrically excited electromagnetic wave was suggested to give rise to the
DM feature in the SEE spectrum. With the involvement of UH waves in exciting
the DM a strong link was established between the excitation of the DM and the
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T. B. LEYSER
formation of FAS, since the FAS too are thought to be excited through UH waves.
Further, the LH waves could account for the downshift of the HF components of
the DM from f0 and the only weak dependence of this downshift on f0 . Further, the
importance for the DM of electrostatic fluctuations essentially perpendicular to the
geomagnetic field is consistent with the observed lack of correlation between the
SEE and EISCAT-UHF ISR measurements of Langmuir turbulence approximately
parallel to the magnetic field (Nordling et al., 1988). It should be noted that the
high-harmonic ion Bernstein mode discussed in addition to the LH mode in Leyser
(1994) should instead be the thermal dispersion branch of the LH mode, since at
the considered frequency well above the ion gyro frequency the ions are effectively
unmagnetized whereas the electrons are magnetized. This would, however, probably not affect the conclusion of the study, that the spectral range of the excited
electromagnetic waves increases with increasing f0 , which is consistent with the
increasing width of the DM for increasing f0 . At the time of writing, this is the
only attempt in the literature to explain the widening of the DM with increasing
f0 , apart from an early model involving Langmuir–IA wave interaction (Stubbe
et al., 1984), however, which cannot account for the observed electron gyro harmonic effects (Leyser et al., 1989). A crucial test for any potential DM model is to
predict the robust empirical result that, at least for high pump ERP, the DM peak
intensity occurs at f− = fDM ≈ 2 × 10−3 f0 (Stubbe et al., 1984; Leyser et al.,
1994), which is also associated with an increasing spectral width of the DM with
increasing f0 .
However, although UH and LH oscillations are likely to be important ingredients in the DM excitation mechanism, the specifically considered coherent threewave interaction is not a likely candidate for exciting the DM. Because of the fine
wave vector matching condition which is required to couple resonantly the small
scale electrostatic UH and LH waves and large-scale electromagnetic waves, it
appears difficult to maintain phase matching over many electromagnetic wavelengths in the inhomogeneous ionospheric plasma. Thus, it is unlikely that the
parametrically excited electromagnetic wave would be significantly enhanced in
the considered process. Also, the propagation of the electromagnetic wave through
the interaction region occurs in a time scale which is shorter than that of the ion
dynamics and, thus, shorter than the growth time of the instabilities (Mjølhus et al.,
1995).
The first particle-in-cell (PIC) simulations to interpret SEE involved an electromagnetic pump wave propagating into an over-dense, strongly magnetized plasma
with a linear density gradient and used a one-and-a-half dimensional, electromagnetic, open boundary code (Goodman et al., 1994). A localized three-wave interaction was observed, involving the pump decaying into UH and LH waves.
The parametrically excited UH wave converted into electromagnetic radiation on
pump-induced density depletions. Further, when f0 ≈ sfe + flh all excited waves
vanished, consistent with the experimental results for the DM emission in SEE
spectra.
STIMULATED ELECTROMAGNETIC EMISSIONS
293
The linear stage of the parametric decay of an UH/EB wave into an UH/EB
wave and a LH wave has been studied by analyzing the Vlasov equation analytically by a perturbation expansion procedure in the long-wavelength regime
(Zhou et al., 1994). The UH/EB pump wave having the frequency f0 is excited
through a thermal OTSI together with small-scale FAS (Huang and Kuo, 1994a).
The process with the EB pump wave is found to have a lower instability threshold
than that of a similar decay process which instead involves UH waves, when the
instability is excited in a region away from the double resonance layer where the
local fuh = sfe . When invoking scattering of the frequency-downshifted UH/EB
off the FAS, this instability along with its cascading products is proposed to give
electromagnetic emissions which give rise to the DM, 2DM, 3DM, etc. The instability threshold increases as f0 approaches sfe , implying that the 2DM emission
should be quenched before the DM, which is consistent with experimental results.
However, the dependence of the f0 range in which the DM cannot be excited on the
harmonic s was not discussed. The scattering of the oppositely propagating UH/EB
wave excited in the thermal OTSI, off the LH wave produces a frequency-upshifted
wave which can be converted to electromagnetic radiation, and hence the UM, after
scattering off the FAS. With regard to models of the DM and 2DM being excited
by cascading type of processes it is interesting to notice that the 2DM intensity
is not simply a function of the DM intensity, but the cascade sometimes appears
to be truncated. The DM emission may be strong without being accompanied by
a 2DM, which has been observed to occur during conditions of spread F during
which large-scale density irregularities are present in the ionosphere as observed in
ionograms (Leyser, 1989; Leyser et al., 1990). The observation of a single strong
DM at high pump ERP concern f0 = 4.04 MHz, at the Tromsø facility. It may be
argued that the DM was excited for f0 sufficiently near 3fe , such that the 2DM but
not the DM was suppressed. However, in this case the BSS or BUM feature should
have been excited, but this was not the case (Figure 3.28 in Leyser (1989)).
A similar coherent parametric three-wave interaction in which an UH wave
decays into another UH wave and a LH wave in a locally homogeneous plasma has
been studied, emphasizing the importance of oblique UH waves and the electron
and ion Landau damping of the LH waves (Shvarts and Grach, 1997). The presence
of oblique UH waves in the HF pumped turbulence is due to multiple scattering of
the UH waves off FAS. It is proposed that the 2DM and 3DM can be more easily
explained in terms of the considered cascade process. The overshoot in the 3DM
which has been observed for f0 slightly above sfe , where the BC is not excited,
can be interpreted in terms of the double transformation in the same way as the
overshoot of the BC at the same f− for f0 away from sfe .
The DM and 2DM from a pump at f01 = 7.8164 MHz has been observed to be
enhanced by a second pump at f02 = 8.2400 MHz ≈ f01 + 0.4 MHz (Bernhardt
et al., 1994). These results are explained in terms of plasma density irregularities
produced by the pump at f02 which affect the excitation of electromagnetic waves
by the pump at f01 . Either FAS, excited near the UH height of the pump at f02 ,
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T. B. LEYSER
extended down the near vertical geomagnetic field into the interaction region of
the lower pump at f01 , or the standing pump wave of the second pump introduced
horizontal stratification of the plasma density in the pump–plasma interaction region of the first pump, so that the additional irregularities enhanced the conversion
efficiency between electrostatic and electromagnetic oscillations necessary for the
DM and 2DM emissions. Particularly, it is noted that both the DM and 2DM
were enhanced by the same factor (2 dB) by the pump at f02 (Bernhardt et al.,
1994), which is consistent with the model of DM excitation by an UH wave that
parametrically decays into a second UH wave and a LH wave (Shvarts and Grach,
1997). The 2DM could then be excited by the further cascading of the UH wave
into another UH wave and LH wave (Shvarts and Grach, 1997). However, the
experimental results are not consistent with the 2DM being excited by the pump
decaying into an UH and a LH wave, and the subsequent decay of the UH wave
into a LH wave and an electromagnetic (Leyser, 1991), which, thus is downshifted
from the original pump by twice the LH frequency, since that scenario does not
involve FAS and therefore should not be affected by a second pump at f02 .
A coupled set of equations has been derived to model the nonlinear interaction
of a powerful dipole pump electric field with UH and/or EB waves and LH waves
for f0 near sfe in a locally homogeneous plasma (Istomin and Leyser, 1995). The
HF wave equation is of the second order in the time derivative since it contains
both EB and UH waves, which makes it appropriate for studying phenomena near
sfe . The expression for the ponderomotive force of the HF fields includes a new
term which is important for the EB waves near sfe . The model is applied to the
initial parametric decay of the dipole pump field into an EB or UH wave and a LH
wave. The instability threshold is lowest for the decay of the pump into EB and
LH waves for f0 near sfe and s > 3 while it is lowest for decay into UH and LH
waves for s = 3 as well as for f0 sufficiently far from sfe . This is consistent with
theoretical results for the thermal OTSI near 3fe , showing that UH waves are more
easily excited than EB waves (Huang and Kuo, 1994a), as further discussed in Section 6.5.3. Further, for f0 approaching sfe + flh the instability threshold increases
due to the decreasing ponderomotive force resulting from the increasing EB and
UH wavelength component perpendicular to the ambient magnetic field, so that the
instability cannot be excited in a narrow f0 range around sfe + flh . The importance
of EB waves compared to UH waves for exciting the DM in the vicinity of sfe is
manifested by that the narrow f0 range in which parametric decay into EB and LH
waves cannot occur decreases with increasing s in a power law manner, which is
consistent with the experimentally determined f0 range in which the DM feature
cannot be identified (see Figure 12). The theory predicts this forbidden f0 range to
decrease with increasing pump power, which could be tested in experiments. On the
other hand, for the decay into UH and LH waves the f0 range in which the instability cannot be excited does not depend strongly on s, which is not consistent with the
experimental results. In order that the parametrically excited EB or UH waves give
rise to electromagnetic radiation, it is conjectured that the plasma waves scatter off
STIMULATED ELECTROMAGNETIC EMISSIONS
295
weak plasma density irregularities into radiation which can escape the ionosphere
and be detected on the ground. Further, for a given pump power there exists a
maximum frequency mismatch between the parametrically interacting wave modes
at which instability still can occur. This allowed frequency mismatch increases with
increasing pump power. As a result the parametric decay instability is suppressed
at a sufficiently high pump electric field, when the frequency mismatch around
the eigen frequencies of the participating wave modes within which parametric
interaction can occur is of the order of the frequency separation between the EB
and UH mode near sfe .
6.5.2. Trapped Oscillations
The above survey concerns wave–wave interactions in a locally homogeneous
medium. However, recent theoretical and numerical advances stress the importance
of plasma inhomogeneity for the long time scale SEE excitation and suggest the
relevance of UH oscillations localized in the density structures associated with
FAS. The basic mechanisms considered in the different wave–wave interaction
models above may well have their counterparts in localized interactions similar
to the ones discussed in the present section.
The trapping of UH oscillations in a pre-formed density cavity and their parametric decay into UH and LH waves have been suggested to excite the DM, nDM,
and UM features (Gurevich et al., 1997; Mjølhus, 1997, 1998). The UH oscillations are excited by linear mode conversion of the electromagnetic pump wave on
the small scale FAS. The case of UH turbulence trapped in a pre-formed density
depletion corresponds to an experimental situation in which the ionospheric plasma
is preconditioned to contain FAS before a relatively weak pump wave is applied to
excite the SEE. Such experiments have given well-developed and relatively narrow DM features (see Figure 10) which are distinct from the broad DM features
obtained during high power CW pumping (Cheung et al., 1998).
Coupled mode equations for the parametric decay of an UH wave into a LH
wave and a frequency downshifted UH wave inside a pre-formed density depletion
have been derived (Gurevich et al., 1997). Whereas both the pump and decay
UH waves are trapped in the density depletion, the LH waves radiate out from
the cavity. Based on further analysis of previous results for the stationary state of
FAS (Gurevich et al., 1995a, b) it is suggested that the amplitude of the trapping
density depletion remains small while the electron temperature inside the depletion
may be several times higher than the ambient temperature. The density depletion
and electron temperature are assumed to depend both on the coordinate along and
across the ambient magnetic field. By considering the case of weakly nonuniform
plasma, such that solutions to the coupled mode equations in the WKB approximation can be found, the instability threshold for the decay of a trapped UH wave into
another trapped UH wave and LH wave is calculated and the nonlinearly stabilized
amplitude of the decay UH wave is estimated. The instability threshold is found to
depend on the collisional damping of the UH waves and the convection of LH wave
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T. B. LEYSER
energy out from the interaction region. Further, it is discussed how leakage of the
trapped UH oscillations into radiation in the Z mode (Dysthe et al., 1982; Mjølhus,
1983) affects the parametric instability. Particularly, sufficiently many FAS are
required in the pump–plasma interaction region in order that the effective Z mode
leakage be suppressed (Gurevich et al., 1996).
A different model for the parametric interaction of UH oscillations trapped in
pre-formed density depletions based on the general framework for SEE generation
developed in Mjølhus et al. (1995) has been formulated for both one- and twodimensional geometries with a mathematical approach characterized by a Galerkin
method (Mjølhus, 1997) and studied numerically (Mjølhus, 1998). A feature of the
model is the existence of trapped UH oscillation resonances, which allow the existence of a standing, undamped, and undriven UH wave in the density well across
the ambient magnetic field. With nonzero damping the driven UH oscillations at
these resonances will have a large amplitude.
Approximate dispersion relations for the trapped UH oscillations are derived,
which lead to the identification of two types of parametric instabilities. In the TOD
instability, the growing mode has a frequency downshifted from f0 by slightly more
than the LH frequency and is at the frequency of a trapped UH resonance, while the
decaying mode is not necessarily at a resonance. The threshold for this instability
in the pre-existing density well is predicted to be very low. It is further noted that
no wave number matching conditions enter for this trapped instability, which is
in contrast to the case of parametric instabilities in a uniform or slowly varying
medium. The numerical results reproduce source spectra compatible with the DM,
2DM, 3DM, as well as weak UM feature in the SEE spectrum. The second instability, termed the Stokes–anti-Stokes instability, is suggested to be the counterpart
of the OTSI. It occurs when f0 is slightly below the mean value of the frequencies
of two trapped UH resonances, such that the Stokes component of one resonance
will be in resonance with the anti-Stokes component of the other.
Experiments have been performed to explore the excitation of plasma turbulence in pre-formed density irregularities. Following a period of high power pumping, a strong and discrete DM was observed when the pump was switched to low
power as seen in Figure 10 (Cheung et al., 1998). This preconditioning effect is
taken as evidence of the low power narrow DM feature being generated by the
trapped UH decay instability in pre-formed FAS. Also, the very broad spectra
observed during the overshoot of the SEE intensity just after turn-on of the second high-power pulse is consistent with model predictions of cavitation in the
pre-formed FAS.
Further, the decay time τd of the lower sideband SEE may be significantly
shorter than that expected to result from electron–ion collisions. It was estimated
that the pump electric field required to obtain the short non-collisional damping
for the UH turbulence in the UH resonance region is E0 ≈ 0.14 V m−1 , while
to obtain the non-collisional damping for the Langmuir turbulence in the pump
reflection region requires E0 ≈ 0.7–1.8 V m−1 , which is larger by almost an order
STIMULATED ELECTROMAGNETIC EMISSIONS
297
of magnitude (Sergeev et al., 1998). The lower pump electric field to excite the
non-collisional damping for the UH turbulence is consistent with the importance
of resonance type excitation in pre-formed FAS for that type of turbulence.
The fact that both small-scale FAS and the SEE are considered to be excited
by UH waves motivates a unified treatment of the widely separated transport and
ponderomotive time scales, in order to explore the possible complexity of the interacting processes. The first theoretical model of the self-consistent interaction
between the slow plasma transport and rapid ponderomotive processes of UH oscillations driven by a powerful dipole pump electric field has been presented, which is
applicable to case of the DM during high power CW pumping (Istomin and Leyser,
1998, 1999). The slow transport processes lead to a self-localization of UH oscillations in density structures associated with FAS while the ponderomotive processes
concern the parametric interaction of different UH oscillations self-localized in
the same cavity and LH waves which radiate out from the cavity. The theoretical
model explicitly shows that a stationary solution only exists for a self consistent
cavity profile, which is determined essentially by the thermal nonlinearity. This
is different to the finding in Mjølhus (1997) that a stationary solution is found
for sufficiently small pump fields in a pre-formed density depletion. Further, it
is found that the self-consistent and nonlinearly stabilized density cavities have
a quantized amplitude and quantized l⊥ , so that for a given pump electric field
and background plasma, a range of different l⊥ and amplitudes as described by the
quantum number as well as different cavity shapes (e.g., double wells) are possible.
The model is based on a model for the excitation and nonlinear stabilization of a
single small-scale FAS driven by a pump wave (Istomin and Leyser, 1997). Only
the perpendicular dynamics in the transport equations for the magnetized electron
fluid and unmagnetized ion fluid is included. The dominance of the perpendicular
transport over the parallel transport for the FAS formation inside the pump–plasma
interaction region occurs for sufficiently short l⊥ of the FAS and high pump powers.
The single self-localized UH state which is not associated with any other UH oscillations and LH wave, is described by the following equations for the normalized
UH electric field ε1 and normalized electron density perturbation η0 (Equations (9)
and (10) in Istomin and Leyser (1998))
∇⊥ ε1 = η0 (1 + ε1 ) + σ ε1 ,
(4)
ε2 2
∇⊥ η0 = −Q ε1 + 1 ,
2
(5)
2
where σ = /|| = ±1, = 4(fuh − f0 )/fp , Q ∝ ε0 |E0 |2 /(NTe 2 ), and
the collisional damping of the UH oscillations has been neglected. The thermal
parametric instability (soft excitation) which has been considered to initially excite
the FAS (Grach and Trakhtengerts, 1976; Das and Fejer, 1979; Dysthe et al., 1983)
corresponds to the first term in the right-hand side of Equation (5), i.e., the case
ε1 ε12 . The resonance instability (hard excitation) which has been considered
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T. B. LEYSER
Figure 31. Schematic illustration of a double UH state with the associated density cavity. The density
cavity is self-consistently created by the pump electric field and the UH oscillations. The frequency
difference between the primary and secondary trapped UH oscillations differ by approximately the
LH wave frequency. Notice that the density cavity is flatter near the bottom in the presence of the
secondary UH oscillation, which is localized to a smaller region than the primary UH oscillation.
Taken from Istomin and Leyser (1998).
as a nonlinear stage of the evolution of the FAS (Vas’kov and Gurevich, 1976;
Inhester et al., 1981; Dysthe et al., 1982; Gurevich et al., 1995a, b) corresponds
to the second term in the right-hand side of Equation (5), i.e., the case ε12 ε1 .
It may be noted that the high electron temperature enhancements inside the FAS
of several times the ambient temperature that has been found for hard excitation
(Gurevich et al., 1995a, b, 1997) is a consequence of that highly nonlinear solutions
were considered, corresponding to a source in the right-hand side of Equation (5)
for which ε12 ε1 . However, by considering both terms in the right-hand side of
Equation (5) nonlinearly stabilized solutions are found with only moderate electron
temperature perturbations (Istomin and Leyser, 1997, 1998).
For a sufficiently high pump electric field, the single UH state discussed above
is not stable, but instead a double UH state can form, consisting of two selfconsistently localized UH oscillations with a frequency difference of approximately
flh in the same cavity which parametrically interact with LH waves convecting out
from the cavity. Figure 31 illustrates schematically a double UH state, containing
the primary (ε1 ) UH and secondary (ε2 ) UH oscillations. Whereas the single UH
state is characterized by a single quantum number, the double UH state is characterized by two independent integers. It is conjectured that the electrical currents of
the secondary UH oscillations, downshifted from f0 by approximately flh , in the
cavity radiate electromagnetic waves which could escape the ionospheric plasma
and give rise to the DM feature in the SEE spectrum.
STIMULATED ELECTROMAGNETIC EMISSIONS
299
In order to interpret the DM, the model assumes the existence of a distribution
of different cavities with different quantum numbers. Assuming a binomial distribution (which has a power-law tail) of the quantum numbers source spectra are
reproduced which are consistent with the experimentally observed DM (Istomin
and Leyser, 1999). It may further be noted that the nonlinearly stabilized amplitude
of the decay UH oscillation estimated in a pre-formed density depletion (Gurevich
et al., 1997) differs from the calculated amplitude in the self localized double UH
state (Istomin and Leyser, 1998).
The possibility of structure quantization in plasma remains to be experimentally
verified. This could be done by in situ measurements from sounding rockets or
satellites in the ionospheric plasma or possibly in laboratory plasma experiments.
Further, it is tempting to note that the stimulated emission of waves in multiple UH
states appears as a classical and nonlinear analogue of the quantum mechanical
stimulated emission from, e.g., excited atoms, where the role of the electron wave
function instead is played by the nonlinear self-localized UH state.
Finally, both the BC and DM features are attributed to UH turbulence. The DM
has been observed to be the first spectral feature to appear as the ionospheric fcrit
increases toward f0 from below, as is the case during sunrise (Leyser et al., 1990).
This seems inconsistent with models for the DM involving trapped UH oscillations
while models for the BC involve freely propagating UH waves, since to achieve
trapping of waves requires a higher ambient plasma density. Further experimental
investigations with f0 near fcrit as well as of the excitation mechanisms for the BC
and DM are therefore required.
6.5.3. Additional Models for Electron Gyro Harmonic Effects
Several additional models have been suggested to interpret the suppression of the
DM emission for f0 ≈ sfe . Initially, the absence of the DM was attributed to
cyclotron harmonic damping (Leyser et al., 1989, 1990; Leyser, 1991) which,
however, was based on much too high electron temperature. With realistic electron
temperatures cyclotron harmonic damping comes into play only at the higher s,
where the experimentally measured suppression bandwidth is of the order of a
kilohertz or less (Mjølhus, 1993).
By considering the linear dispersion equation for UH waves and noting that
in a plasma with opposite gradients in the ambient plasma density and magnetic
field strength, as in the bottom-side ionospheric F region, it was found that there
exists a height range (i.e., plasma density range) near the double resonance where
the local fuh = sfe , in which UH waves with their frequency equal to the double
resonance frequency cannot exist (Grach et al., 1994; Leyser et al., 1994). Thus, no
UH waves can be excited with the frequency f0 = fuh = sfe , which could explain
the suppression of the DM. Specifically, there exists a phase space in which UH
waves cannot exist near the double resonance, which translates into a forbidden
height range in a monotonic ionospheric plasma density profile. The spatial extent
of the forbidden zone decreases with increasing s, which translates into a forbidden
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T. B. LEYSER
frequency range consistent with the experimental results for the f0 range in which
the DM cannot be detected as displayed in Figure 12. Also, the occurrence of
height variations of the plasma density profile as well as the existence of density
inhomogeneities inside the forbidden zone should decrease the f0 range in which
the DM is suppressed (cf. the spread of the data for a given s in Figure 12).
The excitation of UH and LH waves by a uniform pump electric field has been
studied in one-dimensional PIC simulations (Scales et al., 1997). For a sufficiently
large pump amplitude, sidebands shifted below as well as above f0 at multiples of
the LH frequency were produced, with the lower sideband containing more energy
than the upper sideband. For f0 = fuh = sfe , the excitation of the electrostatic waves was suppressed, which was attributed to the limited frequency range
available for the UH mode near sfe . The present author notes that contrary to the
simulation results, there does not appear a simple relation between the DM and UM
intensities in the experimental results, since a larger DM intensity is not necessarily correlated with a larger UM intensity (Leyser et al., 1994). The question thus
arises whether this could be due to possibly different roles played by the density
irregularities observed in the PIC simulations and the thermal structuring of the
ionospheric plasma into FAS observed in experiments. The excitation of the DM
is closely associated with the thermal self-structuring into FAS, which may lead to
localized states for the UH oscillations and very different conditions for frequency
downshifted and upshifted oscillations. Particularly, frequency downshifted UH
oscillations may be trapped while upshifted UH oscillations may not be trapped
in a given density depletion.
Further, in addition to being important for the DM emission, UH waves are
instrumental in exciting small-scale FAS, as shown in initial instability studies of
thermal parametric instability from infinitesimal density perturbations (soft excitation) in homogeneous (Grach and Trakhtengerts, 1976; Dimant, 1978; Lee and
Kuo, 1983) and inhomogeneous ambient plasma (Grach et al., 1978, 1981; Das and
Fejer, 1979; Dysthe et al., 1983), resonance instability from a finite level of density perturbations (hard excitation) (Vas’kov and Gurevich, 1976, 1984; Inhester
et al., 1981; Dysthe et al., 1982), as well as in studies of the nonlinearly stabilized
state during hard excitation (Gurevich et al., 1995a, b) and in a unified treatment
of nonlinear stabilization of soft and hard excitation (Istomin and Leyser, 1997).
Therefore, by considering the linear dispersion characteristics of UH waves it was
conjectured (Leyser, 1989; Leyser et al., 1990), theoretically predicted (Mjølhus,
1993; Gurevich et al., 1995b), and experimentally verified (Ponomarenko et al.,
1999) that FAS weaken significantly for f0 ≈ sfe . Related effects of the weakening of FAS for f0 ≈ sfe have been observed in AA measurements of low-power
diagnostic HF waves traversing the pump–plasma interaction region (Stocker et al.,
1993; Stubbe et al., 1994), electron temperature enhancements (Honary et al.,
1995; Robinson et al., 1996), and in Doppler sounding of density profile modifications near the UH resonance height (Belyakova et al., 1991; Lobachevsky et al.,
1992; Grach et al., 1997).
STIMULATED ELECTROMAGNETIC EMISSIONS
301
A thermal OTSI has been suggested to explain the excitation of EB/UH waves
by the electromagnetic pump wave (Huang and Kuo, 1994a). The four-wave interaction involves the electromagnetic pump wave decaying into a purely growing
mode together with HF sidebands which can be either EB waves, UH waves, or
EB-UH waves. The model concerns both the excitation of the DM and small-scale
FAS since both depend on UH waves. It was found that the height range in which
the thermal OTSI excites UH waves reduces considerably when f0 approaches 3fe ,
which is consistent with the suppression of the DM as well as the FAS. The suppression of the thermal OTSI is basically due to the linear dispersion characteristics
of the UH waves. The parametric instability having the EB waves as sidebands is,
in general, not as strong as that having the UH waves as sidebands.
FAS have been proposed to be strongly enhanced by the trapping of UH oscillations inside density depletions, where the UH oscillations are generated by linear
conversion of the electromagnetic pump wave on the local density irregularities
(Dysthe et al., 1982; Inhester, 1982; Vas’kov and Gurevich, 1984). The trapping
of the UH oscillations decreases successively as f0 approaches sfe from below
(s > 2), due to the existence of a second cutoff in the UH dispersion relation
where the positive group dispersive UH mode is transformed into the negative
group dispersive EB mode as obtained from a kinetic description of the electrostatic
waves (Mjølhus, 1993). This implies that the enhancement of FAS is suppressed
and thereby also the DM intensity near sfe . However, the model predictions for
the f0 range in which the DM cannot be detected in the SEE spectrum do not
match the experimental results in detail although the model could account for some
weakening of the DM intensity as well as the related weakening of the FAS and
wideband absorption (Leyser et al., 1994).
The suppression of the DM for f0 near sfe has also been attributed to the mode
conversion of propagating UH waves to resonantly excited non-propagating EB
waves, by studying analytically the linear kinetic dispersion equation for the electrostatic electron motion in a homogeneous plasma in the limit of long wavelengths
(Rao ad Kaup, 1990, 1992). By taking the perpendicular and parallel wavelength
components of the UH wave to match the corresponding scale lengths of the FAS,
the model predictions for the f0 range in which the UH wave are mode converted
to EB waves at f0 ≈ sfe were found to be consistent with the experimental results
for the suppression of the DM at s = 3, 4, and 5 (Rao and Kaup, 1992). The
mode conversion process relies on the propagation of the UH waves in the weakly
inhomogeneous geomagnetic field. Although the mode conversion does not explicitly depend on the plasma density gradient, it significantly affects the propagation
of UH waves and should therefore be included when discussing the conversion
efficiency (Leyser et al., 1994). The density gradients are significantly larger than
that of the geomagnetic field (Mjølhus, 1993). Further, it appears unlikely that
pump-excited UH waves can exist in the ionosphere with a parallel wavelength
matching that of the FAS, which could be several tens of kilometers (Hedberg
et al., 1983; Robinson, 1989). It appears more reasonable that the excited UH
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T. B. LEYSER
waves inherit the parallel wavelength of the electromagnetic pump wave, which
is a few tens of meters. In addition, it has been shown that the pump-excited UH
waves rapidly scatter on the FAS such that even shorter parallel wavelengths are
obtained (Grach et al., 1981).
Given that EB and UH waves cannot be excited when their frequency is near
sfe , by one or another mechanism, different models differ in the f0 relative to sfe
at which the DM is predicted to be absent. For the pump wave decaying into EB
or UH waves and LH waves (Goodman et al., 1994; Istomin and Leyser, 1995),
the DM would be suppressed at f0 ≈ sfe + flh . For the case of the pump wave
first scattering off FAS into EB (Zhou et al., 1994) which then have the frequency
f0 , the suppression of the DM occurs for f0 ≈ sfe . Also, if the EB or UH mode
decays into another EB or UH mode and LH waves (Zhou et al., 1994; Istomin
and Leyser, 1998, 1999), the instability would be suppressed at both f0 ≈ sfe
and f0 ≈ sfe + flh . Whether such a double intensity minimum exists for the DM
could be tested in experiments at the higher harmonics s > 5, where the f0 range
in which the DM cannot be identified is smaller than the frequency separation flh
between the two expected minima. The precise relation between f0 and the value
of sfe at which the DM is suppressed can be used to test the DM models and has
implications for the experimental determination of the local fe in the pump–plasma
interaction region.
The picture thus emerges that both EB and UH modes could contribute to the
DM emission, depending on how close f0 is to sfe . For f0 sufficiently near sfe
(s > 3) EB waves contribute to the DM, whereas otherwise the UH mode contributes to the DM. As discussed above, UH oscillations take part in the formation
of FAS as well as can interact nonlinearly with, e.g., LH waves. The self-consistent
interplay of the thermal and ponderomotive nonlinearities can lead to the formation
of complex multiple self-trapped UH states, which are a result of the UH mode being very sensitive to variations in the plasma density. However, the EB mode is less
sensitive to the plasma density. Apart from studies of the thermal OTSI in the initial
instability stage (Huang and Kuo, 1994a), the nonlinear evolution of EB waves
self-consistently with the formation of FAS remains an open research problem.
Particularly, for f0 near sfe where the EB mode is suggested to be more important
than the UH mode in exciting the DM feature, the intensity of small-scale FAS is
low. Also, a possible difference in the efficiency for electromagnetic radiation from
self-consistent EB and UH oscillations in the inhomogeneous plasma remains to be
studied.
6.6. BSS
The dependence of the BSS (Figure 6) on the vicinity of f0 to 3fe suggests that the
excitation mechanism involves EB waves (Stubbe and Kopka, 1990). A parametric
decay instability of a standing EB wave into electromagnetic sidebands and LH
waves has been studied analytically to interpret the BSS (Kuo, 1992). Whereas the
STIMULATED ELECTROMAGNETIC EMISSIONS
303
coherent parametric interaction probably suffers too large convective damping to
be significantly excited with presently available ground-based HF pump transmitters, it is proposed that the electromagnetic radiation is instead generated through
stimulated scattering based on the framework of the parametric coupling process
considered, provided background LH waves of sufficiently large amplitudes exist
in the plasma.
A modulational instability of EB waves, involving the decay of a pump EB wave
into both a Stokes EB sideband and an anti-Stokes EB sideband together with a LH
decay mode, has also been considered to interpret the BSS (Huang et al., 1995).
The EB sidebands are then proposed to scatter off FAS into frequency upshifted
and downshifted electromagnetic radiation which could escape the plasma and
propagate to the ground. The initial instability analysis is based on the Vlasov
equation for the electrostatic waves, which is analyzed by a perturbation expansion
of the electron distribution function. It is found that the instability threshold near
3fe can be exceeded in ionospheric HF pump experiments. Moreover, the threshold
increases with increasing gyro harmonic number s, such that for f0 near 4fe the
threshold is about twice that near 3fe , which could explain why the BSS is only
observed for f0 near 3fe .
Further, using a similar approach the corresponding modulational instability involving UH instead of EB waves was analyzed (Huang et al., 1995). The instability
threshold is significantly higher than that involving EB waves, which is consistent
with that the BSS is only observed near a gyro harmonic. The present author notes
that the BSS has been observed during conditions of more than 10 dB of AA of
a diagnostic radio wave which propagated through the pump–plasma interaction
region (Stubbe et al., 1994). This appears to indicate a high intensity of small scale
FAS. Thus, although the considered modulational instability explains a number of
detailed experimental results, albeit in an initial instability analysis, it may nevertheless be important to study potential mechanisms for the BSS while allowing
instead an inhomogeneous plasma. Further, UH oscillations are significantly more
sensitive to plasma density inhomogeneities than EB waves, which may lead to
entirely new eigenstates for the UH turbulence, such as self-trapped oscillations (Istomin and Leyser, 1998). Also, instability thresholds involving trapped oscillations
can be significantly different from those of associated parametric wave interactions
in a homogeneous plasma (Mjølhus, 1998).
6.7. BUM
The BUM (Figure 11) has been proposed to consist of two components referred to
as the BUM1 and BUM2 , as obtained in experiments for f0 near 4fe (Frolov et al.,
1998). The existing models for the BUM all concern the BUM2 component. An
early theory (Goodman, 1991) for the BUM has been found to be invalid (Stubbe,
1992).
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T. B. LEYSER
A four-wave decay with two pump photons, frequency upshifted UH plasmons,
and downshifted EB plasmons has been studied (Bud’ko and Vas’kov, 1992). This
four-wave process was found to be intensified when a combination occurs with a
three-wave parametric decay involving the pump wave, an UH or EB wave, and a
LH wave. It was assumed that the excited HF electrostatic waves scatter off FAS
into electromagnetic radiation, in order to be observed as SEE.
A two-step process involving a combination of resonantly excited UH waves
and the parametric decay of the pump wave into EB and LH waves has been proposed to excite the BUM (Goodman et al., 1993; Tripathi and Liu, 1993). The UH
waves excited by direct conversion of the dipole pump electric field on FAS and
the parametrically excited LH waves then merge to give electromagnetic radiation,
which is claimed could explain the BUM. The obtained instability threshold for
s = 3–5 can be exceeded in present ionospheric pump facilities. However, for
s = 6, 7 the thresholds may be too high to be exceeded with present facilities,
which is not consistent with experimental results since the BUM is excited also at
the higher s. A result of the involvement of LH waves in the excitation process is
the occurrence of a cutoff frequency upshift from f0 below which the BUM cannot
be excited. This cutoff frequency increases linearly with increasing s in agreement
with experimental results (Goodman et al., 1993).
It has been pointed out that the first-order four-wave process, involving the
decay of the pump into a LH mode, a frequency upshifted UH wave, and a downshifted EB wave (Bud’ko and Vas’kov, 1992), cannot occur resonantly (Huang and
Kuo, 1994b). A new second-order four-wave interaction is therefore suggested,
involving a second harmonic pump oscillation, an UH plasmon, and an EB plasmon to interpret the BUM (Huang and Kuo, 1994b). The second harmonic pump
oscillation is inherent in the nonlinearity of the plasma. When f0 > sfe , frequency
upshifted UH waves and downshifted EB waves can be excited above the UH
resonance layer. Moreover, in the considered interaction process, a low frequency
driven and non-resonant electrostatic oscillation in the frequency range of LH
waves is excited through the nonlinear coupling of the pump and HF electrostatic
waves. The excited upshifted UH wave can then scatter off FAS into ordinary mode
electromagnetic waves with frequencies around 2f0 − sfe , consistent with the frequency of the BUM as given by Equation (2). The UH waves could also scatter off
the driven low frequency oscillations into electromagnetic waves with frequencies
around fBUM ± flh , which is suggested to be the generation mechanism for the
2BUM feature. The excited downshifted EB wave in the four-wave interaction has
a much smaller amplitude than the UH wave and, thus, the scattering products
are also much weaker, so that the upshifted emissions dominate in the spectrum.
The present author notes that since the model suggests that the excitation of the
BUM depends on the presence of FAS while the 2BUM does not depend on FAS,
but instead on the driven low frequency oscillation, the 2BUM could have a much
shorter growth time after pump-on than the BUM, which could be experimentally
investigated.
STIMULATED ELECTROMAGNETIC EMISSIONS
305
It is noted (Huang and Kuo, 1994b) that the two-step process considered in
Goodman et al. (1993) and Tripathi and Liu (1993) is difficult to use because
of the assumption that the frequency downshifted electromagnetic wave must be
strongly cyclotron damped in order to explain the asymmetric BUM features. It is
argued that the proposed mechanism is more promising than the two-step process,
since the asymmetry between the upshifted and downshifted emissions is a direct
result of the theory and not a result of the assumption mentioned above. Also, the
two-step process cannot give rise to the 2BUM. However, by using the criterion
suggested in Goodman et al. (1993) the BUM cutoff can be explained (Huang and
Kuo, 1994b).
The theoretical initial instability analysis in Huang and Kuo (1994b) has been
supported by one-space dimension electrostatic PIC simulations (Hussein and
Scales, 1997; Hussein et al., 1998). The simulations show that for the used parameter values, the initial evolution is well described by the theoretical model. Further, it
is found that the full nonlinear evolution of the four-wave parametric decay interaction is important in interpreting the asymmetrical SEE spectrum. Specifically,
the amplitude of the downshifted EB and upshifted UH wave grow roughly at
the same rate, whereas the asymmetric spectrum results first after saturation since
the downshifted EB wave is more damped than the UH wave due to increasing
cyclotron damping resulting from wave–particle heating. The temporal evolution
in the PIC simulation should be compared with experimental measurements of
the BUM evolution. It may be noted that the one-dimensional model is restricted
to consideration of only one altitude and one propagation angle to the ambient
magnetic field.
The above mentioned models concern three- or four-wave parametric interactions. A completely different scenario for the BUM excitation has been suggested,
involving pump-accelerated super-thermal electrons. Grach (1999) proposed that a
model for solar radio emission (Zheleznyakov and Zlotnik, 1975) could be used for
the BUM excitation in ionospheric plasma experiments. The model determines the
steady state density of the accelerated electrons taking into account both turbulent
trapping in the acceleration region due to scattering of the electrons on plasma
waves as well as return of escaped electrons to the acceleration region due to
collisions. For f0 close to sfe a significant anisotropy in the perpendicular electron
distribution is formed, which relaxes into a bump-on-tail distribution perpendicular
to the ambient magnetic field due to Coulomb collisions with electrons outside
the acceleration region. The resulting cyclotron instability excites plasma waves
with frequencies above f0 which scatter off FAS into electromagnetic radiation
that could result in the BUM. The involvement of super-thermal electrons in interpreting the BUM emission is consistent with the experimentally observed decay
time τed ≈ 60–300 ms of the BUM enhancement as additional pumping is turned
off, with the observed time delay between variations in the additional pumping and
correlated changes in the BUM, and with the increase in the BUM decay time as
the diagnostic pump is turned off during additional pumping (Frolov et al., 1997b).
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T. B. LEYSER
Further, for super-thermal electron energies of 50–70 eV the model prediction is
consistent with the linear dependence of the BUM cutoff frequency on the gyroharmonic s. It remains to be found out whether this interesting model could account
for all the differences between the BUM and DM concerning the dependence on
FAS, preconditioning, and additional pumping. For example, when stepping the
pump power during CW transmission, the BUM responds abruptly to the power
steps while the DM responds slowly, similar to the response expected for smallscale FAS, which implies that the temporal evolution of the BUM is unrelated to
that of the FAS during high duty cycle pumping (Wagner et al., 1999).
6.8. LPC
The LPC feature (Figures 22(b) and 22(c)) is excited for a low ERP of less than
100 kW following a period of CW pumping at significantly higher ERP. It was
proposed that the evolution of the LPC during the 30-s low ERP period is associated
with the decay of small-scale FAS after the pump is turned from high to low ERP.
Since the smallest FAS have the shortest decay time (Hedberg et al., 1983; Frolov
et al., 1997c), when the smallest FAS have decayed the pump excited plasma
waves suffer less convective damping, which enables further cascading toward
lower frequencies and, thereby, a wider LPC. This interpretation implies that the
cascading turbulence which is exciting the LPC requires a lower pump ERP than
the formation of the smallest FAS, when such FAS already exist. In a homogeneous
plasma the threshold for exciting a thermal parametric instability, which could give
rise to small-scale FAS, is lower than that of a ponderomotive cascading of plasma
waves. However, in an inhomogeneous plasma with pre-existing small-scale FAS
the threshold for ponderomotive cascading may be significantly lowered, as has
been found for the TOD instability threshold of trapped UH oscillation (Mjølhus,
1998), and lower than the pump power required to sustain the FAS.
Probably the theoretical (Grach, 1985) and numerical model (Shvarts et al.,
1994, 1995; Grach et al., 1998) discussed in Section 6.4 for the BC should be
applicable to the LPC. First, the pump–plasma interaction concerns the excitation
of HF plasma turbulence in pre-existing FAS. Second, the pump wave has a low
power, such that the FAS may not be significantly affected by the pumping. Third,
the used weak turbulence approximation of the induced scattering of the plasma
waves off thermal ions may be more suitable for the case of a low power pump.
The triangular shaped LPC spectrum agrees with that predicted by the model. The
BC at its highest intensities does typically not exhibit the triangular shape as the
model predictions and the LPC.
6.9. S ECOND HARMONIC EMISSIONS
To interpret SEE near 2f0 the merging of plasma waves has been studied theoretically (Karashtin et al., 1986; Erukhimov et al., 1987). From calculations of
the expected emission intensity near 2f0 it was concluded that significantly higher
STIMULATED ELECTROMAGNETIC EMISSIONS
307
intensity would result from plasma waves merging in the UH resonance region
than in the plasma resonance region, for the magnetic field angle relevant to the
experiments at the Zimenki facility.
The second harmonic emissions in Figure 23, with the SEE symmetrically
around 2f0 , have been attributed to a parametric decay instability combined with
a Raman up-conversion occurring near the pump reflection height (Derblom et al.,
1989). The decay instability involves an ion cyclotron wave as the low frequency
ion mode (the ion gyro frequency of the O+ ions is 46 Hz for fe = 1.35 MHz).
6.10. AMP
No theoretical models directly related to the electromagnetic AMP feature have appeared in the literature. However, from linear theory it was predicted that two electromagnetic pump waves could parametrically excite a standing Langmuir wave
at the arithmetic mean of the two pump frequencies together with a standing low
frequency wave at half the difference between the pump frequencies (Fejer et al.,
1978a; Cragin et al., 1978). Measurements with the Arecibo ISR have demonstrated both the standing HF Langmuir wave (Showen et al., 1978) and the standing low frequency ion acoustic wave (Sulzer et al., 1984). However, the slow
growth of the electromagnetic AMP indicates a dependence on the formation of
FAS, i.e., excitation of thermal processes, and therefore UH waves, in addition to
ponderomotive interaction between high and low frequency waves. It remains an
open question whether the considered electrostatic AMP, excited by the interaction
of Langmuir and ion acoustic waves, has a counterpart involving UH instead of
Langmuir waves.
6.11. A DDITIONAL THEORETICAL AND NUMERICAL MODELS
Theoretical models have been proposed which may be useful to interpret SEE in
general, not focusing on particular spectral features. An early work on incoherent
scattering from semi-infinite unmagnetized plasma assuming a pump frequency
exceeding the electron plasma frequency indicates that density discontinuities may
be important for the backscattering of a powerful electromagnetic wave in the
ionosphere (Stenflo and Yakimenko, 1978). It is stressed that the considered problem of sharp density discontinuities, including both incident, reflected, refracted,
and incoherently scattered waves, is very different from the theory of slightly
non-uniform plasmas.
It has been stated (Stenflo et al., 1990a, b) that the SEE was theoretically predicted (Stenflo, 1979) before the experimental discovery. Stenflo (1979) concerns
stimulated scattering of large amplitude waves in magnetized plasma in which
the electromagnetic pump wave interacts coherently with low-frequency electrostatic waves to excite a scattered electromagnetic wave, such as, e.g., in stimulated Brillouin scattering, stimulated Raman scattering, and stimulated resonance
line scattering. The coupling coefficients describing the nonlinear interaction of
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T. B. LEYSER
three arbitrary waves in magnetized plasma were discussed. It has also been noted
(Goodman et al., 1993) that the described scattering processes (Stenflo, 1990a)
would yield frequency shifts of the scattered electromagnetic waves which are either orders of magnitude too small or too large compared to those usually observed
in SEE measurements. Extensive experimental searches have shown that the theoretically predicted stimulated Brillouin and Raman scattering by an HF pump wave
do not contribute significantly to the SEE. However, possibly stimulated Brillouin
scattering has been detected as faint spectral components downshifted only a few
tens of hertz from f0 (Thidé et al., 1983). Also, one uncertain observation of what
may be stimulated Raman scattering has been reported (Derblom et al., 1989).
Further, stimulated Brillouin scattering was theoretically predicted to be observed
by the EISCAT UHF and VHF incoherent scatter radars as well as the Jicamarca
incoherent scatter radar (Drake et al., 1974; Dysthe et al., 1977; Fejer, 1977), which
operate at much higher frequencies than an HF pump wave. Unambigous experimental verification of the theoretical predictions were obtained with the 50-MHz
Jicamarca radar (Fejer et al., 1978b). Thus, observations of stimulated scattering
by an HF pump wave in the ionosphere appear to remain as interesting problems
for continued research. It is well known that stimulated scattering processes are
important in laser plasma experiments in the laboratory and are associated with a
number of complex nonlinear effects.
In the early literature experimental results were presented which showed a pronounced overshoot of approximately 200 ms duration following pump-on and pumpoff (Thidé et al., 1983; Stubbe and Kopka, 1983). The anomalously long persistence of the SEE after pump-off has been interpreted in terms of turbulent expansion of a cloud of super-thermal electrons (Trakhtengerts, 1983). However, the
long life time of the SEE has not been further discussed in more recent experimental investigations of the temporal evolution (Sergeev et al., 1997). The early
experimental results may thus either require special ionospheric conditions such as
interaction with a free energy source or, possibly, were an instrumental effect.
Nonlinear dispersion relations have been obtained for the coherent three- and
four-wave parametric coupling involving an electromagnetic pump wave and, e.g.,
low-frequency electrostatic collisional modes in magnetized plasma to excite electromagnetic waves (Stenflo, 1990a). A system of equations related to the conventional Zakharov equations have been derived, including a magnetized electron fluid
and temperature fluctuations, thereby extending the validity of the conventional
isothermal Zakharov equations into thermal time scales of pumping and providing a possibility to study the interaction of the electromagnetic pump wave with
low-frequency collisional modes such as gradient drift modes (Stenflo, 1990b).
A nonlinear dispersion relation for the parametric decay of an UH wave into a
left-hand circularly polarized electromagnetic wave propagating along the ambient
magnetic field and a LH convection mode in an inhomogeneous magnetized plasma
has been derived (Stenflo and Shukla, 1992). For a sufficiently strong plasma inho-
STIMULATED ELECTROMAGNETIC EMISSIONS
309
mogeneity a new electron convection mode is introduced which may participate in
the parametric interaction.
The trapping of plasma waves inside small-scale FAS has been suggested to
significantly increase the dissipation of pump energy inside the FAS and thereby
amplifying the resonant instability exciting the FAS as well as giving rise to electromagnetic radiation (Vas’kov and Gurevich, 1984). It is stressed that the effect of
the wave trapping is important to cylindrical density irregularities but not to planar
irregularities. During the explosive stage of the resonant instability resulting in the
FAS formation, the frequency of a trapped mode may adiabatically decrease. These
frequency downshifted plasma waves are suggested to be one of the sources for the
observed wideband SEE. The present author notes that such a mechanism would
be most efficient in an initial non stationary stage of the pump–plasma interaction. Since the adiabatically downshifted plasma waves loose their coupling to the
pump wave, these waves would rapidly decrease in intensity. However, the initial
dynamic nonlinear evolution of the plasma response to the HF pumping is poorly
understood, at least for UH turbulence for which both thermal and ponderomotive
nonlinearities are important. In the stationary nonlinearly stabilized state of the
FAS evolution it has been found that the actual density irregularity is strongly
coupled to the trapped UH oscillations, such that the irregularity and trapped field
appears as a unit by itself (Istomin and Leyser, 1997; Istomin and Leyser, 1998).
The dynamics of the trapped modes in the initial FAS evolution requires further
investigation.
Resonant emission of electromagnetic waves by UH solitons has been suggested
to be an additional channel for energy dissipation in strong plasma turbulence
in magnetoactive plasmas, which may be more efficient than collisional damping
(Mironov et al., 1988). However, the properties of UH turbulence in general, and
the role of solitons in particular, remain open questions, partly because of the
difficulty of making direct measurements of electrostatic fluctuations across the
geomagnetic field in the ionospheric experiments.
A sixth-order dispersion relation for linearly unstable electromagnetic waves
from a pair of counter-propagating Langmuir waves in a driven plasma has been
derived (Glanz et al., 1993). The model includes two independent density gratings,
which can be resonant ion-acoustic waves and which are produced by the beating of
the HF waves. The model predicts both downshifted (Stokes) and upshifted (antiStokes) electromagnetic radiation at different frequencies.
A number of SEE features are proposed to depend on the excitation of UH
waves. In this context, the direct conversion of the ordinary mode electromagnetic
pump wave into UH waves on the density gradients of FAS has been studied theoretically (Antani et al., 1991, 1996) and in PIC simulations (Ueda et al., 1998).
The electric field and the average power absorbed by the excited UH waves was
calculated for a coherent distribution of FAS as well as an incoherent broadband
FAS spectrum (Antani et al., 1991). The geometry considered in the theoretical
investigation concerns a given linear background plasma density profile perpendic-
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T. B. LEYSER
ular to the ambient magnetic field (Antani et al., 1996). In the linear regime of the
conversion process, i.e., without considering nonlinear effects which could saturate
the UH wave growth, it was found that the UH wave excitation is stronger for the
case of large-scale density gradients than for small-scale irregularities.
Two-dimensional electromagnetic PIC simulation were performed with a prearranged density striation along the ambient magnetic field (Ueda et al., 1998).
This situation thus corresponds to experiments in which the ionospheric plasma
has been preconditioned to contain FAS and the pump wave has a relatively low
power so as to not affect the FAS intensity. The direct conversion of electromagnetic to electrostatic waves was found to be significantly less efficient in the PIC
simulations than theoretically predicted (Antani et al., 1991). The simulations gave
an UH electric field amplitude of a few 10% of the pump electric field.
7. Discussion
7.1. G ENERAL COMMENTS
SEE is a complex nonlinear response to electromagnetic HF pumping of the ionospheric plasma, as manifested in the wide range of time scales for different temporal features, the detailed spectral structure, and the dependence of the SEE on
parameters of the pumping such as frequency, power, duty cycle, and pumping by
additional waves. As for the temporal evolution, the time scales range over four
orders of magnitude, from less than a millisecond for the decay time after pumpoff of certain features to tens of seconds for growth times. The delay time of the
appearance of long time scale spectral features (SNC, BC, DM, UM, and AMP)
indicates a highly nonlinear growth dynamics. It is interesting that the rich SEE
spectral structure occurs only on the long thermal time scales and is associated
with the formation of small-scale FAS, i.e., dissipative structures, due to thermal
nonlinearities in the plasma. The importance of the FAS for the long time SEE is
also clearly seen in that the SEE intensity downshifted from f0 exhibits a minimum
when f0 ≈ sfe .
As for the spectral structure, the stationary SEE depends strongly on the ratio
f0 /sfe , which is schematically depicted in Figure 32. With stationary is meant
the state several seconds to a few tens of seconds after pump-on, depending on the
pump power, after which the spectral structure does not change significantly during
CW pumping. It is important to recall that the spectral structure within a time scale
of a few tens of milliseconds may be totally different from the long time SEE,
particularly after a cold pump-on, which indicates significantly different responses
of the plasma to the pumping. The stationary SEE spectrum is asymmetric both
with respect to f0 and with respect to f0 relative to sfe (nDM, BC, BDM, and
BUM). For f0 within a few tens of kilohertz above sfe (s > 3) the SEE spectrum
typically consists of a SNC, a weak DM, and a BUM feature with a pronounced
STIMULATED ELECTROMAGNETIC EMISSIONS
311
Figure 32. Schematic diagram of SEE spectra for different f0 relative to sfe (s > 3). Notice the
asymmetry of the SEE in the individual spectra as well as for f0 above and below sfe .
cutoff at its low frequency side. From experiments with f0 near 4fe it has been
suggested that the BUM with a cutoff should be referred to as a BUM1 (Frolov
et al., 1998). For f0 several tens of kilohertz above sfe the DM has reached its
maximum intensity and 2DM and 3DM features are commonly excited. In the
upper sideband a UM is present and the BUM has moved further above f0 . This
BUM feature without a cutoff at its low frequency flank should be referred to as a
BUM2 , which is distinct from the BUM1 (Frolov et al., 1998). For still higher f0 ,
of the order of 100 kHz (or 0.1fe ) above sfe , a BC feature develops in the lower
sideband, the BUM is not present anymore, and the BUS feature has appeared in the
upper sideband. On the other hand, a few tens of kilohertz below sfe both the DM
and BC are well developed. Approximately 0.1fe below sfe both the BC and DM
weaken significantly while for f0 lower than 0.1fe –0.2fe below sfe the BC and
DM are again stronger. The schematic diagram in Figure 32 is an improvement
of the diagram in Leyser et al. (1993). No results of systematic experiments for
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T. B. LEYSER
different f0 near fe nor 2fe have been reported. This is mainly due to limitations
in the available f0 at most of the existing HF facilities. Also, D region absorption
is higher for the lower f0 .
The association of the excitation of the long time scale SEE with the formation
of small-scale FAS indicates the importance of UH turbulence in constituting the
plasma response to the electromagnetic driving. It may be noted that SEE excited
by the Tromsø facility but recorded at Kiruna in Sweden already in 1981, led to
the suggestion that the SEE must emanate from a few kilometers below the pump
reflection height in order to reach the Kiruna receiving site approximately 200 km
from Tromsø (Thidé et al., 1983), which is consistent with the present paradigm of
the importance of UH phenomena since the UH resonance region is typically a few
kilometers below the pump reflection height (Figure 1). The first more direct experimental evidence of SEE being correlated with perturbations in the UH resonance
region appears to have been found in MDR measurements (Lobachevsky et al.,
1992). Further, FAS have been proposed to play a triple role for the long time scale
SEE (Frolov et al., 1994). In the first stage of the HF pump–plasma interaction the
FAS participate in the transformation to plasma waves and the diffusion of these
waves over the wave vector spectrum. In the second stage the FAS participate in
the reverse transformation of the plasma waves into SEE. In the third stage the FAS
determine the AA of the SEE as it propagates through the pump–plasma interaction
region.
UH turbulence is interesting for the study of complex nonlinear processes at the
moderate driving strengths used in ionospheric plasma experiments, since the turbulence involves the interaction of ponderomotive and thermal nonlinearities. For
magnetized electron dynamics, i.e., electron motion across the ambient magnetic
field, instability involving thermal nonlinearities which couple the HF electron
motion to the slow ion motion has a significantly lower threshold than instability involving ponderomotive nonlinearities to couple the fast electron motion to
the slow ion dynamics (Grach and Trakhtengerts, 1976). On the other hand, for
unmagnetized electron dynamics, i.e., electron motion along the magnetic field in
a magnetized plasma, the case is reversed in that the instability threshold involving
thermal nonlinearities is significantly higher than for ponderomotive nonlinearities.
Therefore, for a pump electric field which typically slightly exceeds the threshold
for ponderomotive instabilities, UH turbulence appears as a more complex plasma
response than Langmuir turbulence. As is typical for processes involving interacting mechanisms in a hierarchy of time scales the slower processes slave the
evolution of the faster processes. With other words, processes at widely different
time scales are tightly coupled in that the slow thermal processes significantly
influence the evolution of the fast ponderomotive interactions. An example of such
an interaction is considered in the model of self-localized multiple UH states (Istomin and Leyser, 1998, 1999), which suggests that the self-structuring into FAS
is associated with trapping of the UH turbulence in density cavities. The result is
a strongly inhomogeneous distribution of both the plasma itself and the UH turbu-
STIMULATED ELECTROMAGNETIC EMISSIONS
313
lence, which is associated with absolute instability in the inhomogeneous plasma.
Further, the multiple time scale interactions give new forms of turbulence in that
the occurrence of plasma inhomogeneity leads to multiple eigen states of plasma
oscillations. The model suggests that the self structuring of the plasma into FAS is
an intrinsic ingredient in the actual electromagnetic radiation mechanism.
Several theoretical models of electrostatic wave–wave interactions in a locally
homogeneous plasma have been considered as an intermediate stage in the excitation of SEE. The electrostatic waves are then thought to interact with the thermally
striated weakly inhomogeneous plasma to give rise to the electromagnetic radiation (e.g., Grach, 1985; Shvarts et al., 1994, 1995; Zhou et al., 1994; Huang and
Kuo, 1995; Istomin and Leyser, 1995). This separation of the short time scale
ponderomotive and long time scale thermal processes may be artificial, at least
for sufficiently high pump powers. It is well known that a single wave mode in
a homogeneous plasma can be transformed to a multiple mode in a sufficiently
inhomogeneous plasma.
Further, a number of parametric wave–wave interaction models which require
wave vector matching conditions to couple resonantly the small scale electrostatic
oscillations to the large-scale electromagnetic mode have been suggested to interpret SEE (e.g., Sstenflo, 1990a; Stenflo and Shukla, 1992; Leyser, 1991, 1994;
Glanz et al., 1993). Such required spatial coherency between the electrostatic and
electromagnetic waves is probably not efficient, since the plasma turbulence must
be correlated over many electromagnetic wavelengths for significant radiation to
occur, although counter parts of the considered electrostatic modes may be relevant
for the SEE in other types of nonlinear processes. Also, an electromagnetic wave
propagates through the interaction region in a time scale which is shorter than that
of the ion dynamics and, thus, shorter than the growth time of the instabilities
(Mjølhus et al., 1995).
SEE during extraordinary mode pumping (Thidé et al., 1983; Leyser, 1989)
is a largely unexplored research area. Results of extraordinary mode pumping are
difficult to interpret unless f0 > fcrit . For fcrit < f0 < fcrit +fe /2, the extraordinary
mode pump wave would reflect from the ionosphere whereas the possible ordinary
mode leakage during the extraordinary mode transmission would pass through the
ionospheric plasma profile with minimum interaction. Early experimental results
at f0 = 2.759 MHz in extraordinary mode indicate SEE spectral peaks shifted
from f0 by multiples of 45–50 Hz (Thidé et al., 1983). These results have been
interpreted in terms of parametric decay into UH and ion Bernstein waves (Sharma
et al., 1993). However, in the model the extraordinary mode wave propagates
perpendicular to the ambient magnetic field, which does not appear applicable to
high-latitude experiments. When the extraordinary mode wave propagates nearly
parallel to the magnetic field it does not reach the UH resonance region where UH
waves can be strongly excited, but reflects at lower plasma frequencies.
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T. B. LEYSER
7.2. S UGGESTIONS FOR FUTURE WORK
The most important experiments to increase our understanding of the plasma response to electromagnetic pumping are to perform in situ measurements of the
driven Langmuir and UH turbulence by sounding rockets and satellites, similar to
the mid-latitude Arecibo sounding rocket experiments (Kelley et al., 1995) which
concerned the nighttime ionosphere but unfortunately not directly the pump–plasma
interaction region. The remote techniques of SEE and radar scatter measurements
all only give an integrated picture of the turbulence. Further, it would be most useful
to perform VHF and UHF radar studies of the UH and EB turbulence spectrum
similar to what has been done for Langmuir turbulence at smaller angles to the geomagnetic field. This requires mid- or low-latitude HF pump and radar facilities if
ground-based radars are to be used. One consequence of the trapped UH oscillation
picture in which the UH turbulence and density structures are strongly localized in
space, is that the relative strength of the plasma line and the centerline backscatter
should change in favor of the plasma line with increasing radar frequencies, i.e.,
decreasing length scales (Mjølhus, 1993). Such a trend has been experimentally
observed (Minkoff et al., 1974), but more detailed investigations of the UH plasma
line spectrum remains an important task, e.g., to look for evidence of LH oscillations in the frequency spectrum. As for the investigation of experimental results,
high resolution SEE measurements should be analyzed with higher order statistical
tools to study, e.g., the coherency of the different emissions and correlations between different parts of the frequency spectrum. Below follow some more detailed
suggestions for future work.
The details of the temporal evolution of the SEE are not well understood and
require further experimental, theoretical, and numerical investigations. This concerns, e.g., the overshoot in the SEE intensity within a few tens of milliseconds
and the undershoot in the BUM intensity during pump power stepping, an anticorrelation of the SEE in the lower and upper sideband, respectively, at f− =
5 kHz and f+ = 5 kHz for f0 = 4.04 MHz on a time scale of several seconds
(Lobachevsky et al., 1992), and at times an anti-correlation while at other times
a correlation between the DM intensity and SEE at 2f0 . The FNC exhibits an
intensity overshoot within a few tens of milliseconds, which for low pump duty
cycles depends on the pump power (Frolov et al., 1997d; Sergeev et al., 1998)
but for higher duty cycles is independent of the pump power (Waldenvik, 1994).
These overshoots have not been addressed in the literature when interpreting the
SEE. The time scale of the SEE overshoot is similar to that of the mini overshoot
of the HFPL observed in 430-MHz radar measurements at Arecibo (Showen and
Kim, 1978; Djuth et al., 1986). The mini overshoot has been interpreted in terms
of Langmuir turbulence and local plasma density modifications due the standing
pump wave maxima just below the reflection height on time scales of approximately 100 ms (DuBois et al., 1993). As the density depletions at the standing
wave maxima form the density in the surrounding plasma grows. This transient
STIMULATED ELECTROMAGNETIC EMISSIONS
315
density profile is most effective at trapping and intensifying the local pump electric
field, so that the strongest local turbulence is excited. On longer time scales the
transient density accumulations may relax so that the pump field becomes less
localized and weaker, which implies that the Langmuir turbulence too is weaker
corresponding to the decay of the overshoot. The mini overshoot has also been
interpreted in terms of the growth and saturation of Langmuir waves propagating in
pre-existing plasma density ducts (Muldrew, 1988). However, it is unlikely that the
SEE overshoot would depend on the presence of pre-formed density ducts, since
the overshoot is seen at the lowest pump duty cycles at which preconditioning is
minimum. The development of the SEE overshoot is correlated with a decrease in
the non-collisional decay time of the SEE at pump-off, which suggests that both
effects are related to modifications of the electron velocity distribution. Also, it
may be interesting to note that the SEE overshoot occurs on a similar time scale
as the disappearance of the free mode observed with the Arecibo 430-MHz radar
(Fejer et al., 1991; Sulzer and Fejer, 1994).
There are a number of other SEE spectral features that also exhibit decay times
after pump-off that are shorter than that expected for collisional damping of electrostatic and electromagnetic waves, as, e.g., the DM, BUM, and BUS features.
Again, these short decay times are open problems to theoretical interpretation and
suggest that non-Maxwellian electron velocity distributions develop in the plasma
turbulence. Also, it would be interesting to further study the dependence of the
decay time on the pump duration and power. Decay times shorter than the electron
collision time have also been measured for Langmuir oscillations with the Arecibo
ISR (Wong and Taylor, 1971; Fejer et al., 1991; Sulzer and Fejer, 1994).
For f0 near 3fe either the symmetric BSS feature or the asymmetric BUM
and BDM features have been observed. Presumably it is the conditions in the
ionospheric plasma or possibly the interaction height which determines which
feature is excited. It is clearly interesting to investigate whether these spectral
features are closely related or not. Whereas the temporal evolution of the BUM
has been studied in some detail, such investigations have not been reported for the
BSS. Therefore, measuring the dynamic properties of the BSS should shed light on
whether the BSS may be related to the BUM and BDM.
It is proposed to perform experiments with f0 near fcrit . Measurements at Tromsø
during sunrise have shown that the first SEE feature to appear when fcrit approaches
f0 from below is the DM (Leyser et al., 1990). The DM even appeared as fcrit was
a few 100 kHz below f0 . However, it may be noted that the models that have
been suggested for the BC involve freely propagating UH waves while those for
the DM involve UH oscillations trapped in density depletions, which implies that
the BC should be excited at lower plasma densities than the DM and, therefore,
should develop at lower fcrit than the DM. The experiments at Tromsø mentioned
above were performed at f0 = 4.04 MHz at which the BC has not been observed
to be developed even for f0 < fcrit . Thus, it is suggested to repeat these type of
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T. B. LEYSER
experiments at higher f0 not near sfe in order to study, e.g., conditions for the BC
and DM development.
An experimentally and theoretically rather unexplored area is that of multifrequency pumping, which constitutes an entirely different driving regime than
the usual single-frequency pumping so that it may excite different nonlinear responses of the plasma. Examples from two-frequency pumping are the AMP and
the quenching of spectral features which are excited by a single pump wave (Leyser,
1989). Another example is the nonlinear electromagnetic sideband generation by
two pump waves with closely spaced frequencies (Ganguly and Gordon, 1986;
Huang and Fejer, 1987). By transmitting two HF pump waves with a frequency separation from 0.5 Hz to more than 100 Hz, electromagnetic sidebands shifted from
the pump frequencies by the frequency difference of pump waves can be generated. The experimental results have been attributed to the dependence of the phase
delay of the reflected pump wave on the amplitude of the incident waves (Huang
and Fejer, 1987). Such nonlinear sideband generation does not appear to involve
significant effects of plasma turbulence. Further, excitation by varying degree of
non-monochromatic pump waves is another most interesting area which could give
information on, e.g., correlation times in the excited nonlinear processes, as has
been used in VLF wave stimulation experiments in the magnetosphere (Helliwell,
1988).
The role of FAS is different for the BUM and the DM, e.g., as observed in the
hysteresis effects for the DM during pump power stepping but the absence of such
effects for the BUM, in the transients in the BUM emission at the power steps,
and in the different dependence on preconditioning for the BUM and DM. During
high duty cycle pumping the dynamics of the BUM is not related to the intensity
of small scale FAS, which is contrary to the DM intensity. As a result the DM
intensity can be used to monitor the FAS intensity (Frolov et al., 1997c; Wagner
et al., 1999), whereas this is not the case for the BUM. Further, during additional
pumping the behavior of the BUM and DM is also different, as discussed above. All
these experimental results must be reflected in the models for the BUM and DM.
In the parametric wave interaction models for the BUM that have been suggested
(Bud’ko and Vas’kov, 1992; Goodman et al., 1993; Tripathi and Liu, 1993; Huang
and Kuo, 1994b; Hussein and Scales, 1997; Hussein et al., 1998) the role of FAS
is to either convert the electromagnetic pump wave into electrostatic waves or to
convert electrostatic waves back into electromagnetic waves which can be observed
as SEE. However, if the role of the FAS is only to convert between electrostatic and
electromagnetic waves the emission intensity should depend on the FAS intensity
in a straight forward manner and similar to the case for the DM, which is not
consistent with the experimental results.
Several problems in the interpretation of the experimental results obtained from
ionospheric HF pumping appear to be related to our lack of understanding of nonlocal and global plasma response. To model the SEE radiation intensity on thermal
time scales a non-local approach appears necessary, to incorporate the multiple role
STIMULATED ELECTROMAGNETIC EMISSIONS
317
of FAS in AA of the pump wave, excitation of the SEE by small scale electrostatic
turbulence, as well as the AA of the SEE as it propagates out from the excitation
region. Further, the dispersion branch associated with the UH mode is not purely
electrostatic, but involves a transition from the UH to the electromagnetic Z mode
with decreasing wave number component perpendicular to the ambient magnetic
field. This may lead to a leakage of energy in trapped UH oscillations out from the
trapping density depletion (Dysthe et al., 1982; Mjølhus, 1983, 1990). To consider
the effects of this Z mode leakage for UH oscillations trapped in FAS is an important extension of the work on the localized turbulence (Mjølhus, 1997, 1998),
particularly since the Z mode leakage out from a single isolated density depletion
is significantly higher than the collisional damping of the UH oscillations which
decreases the amplitude of the stationary of FAS and increases the threshold for
parametric decay of the trapped UH oscillations (Gurevich et al., 1996, 1997).
The actual self-consistent radiation mechanism from the electrostatic turbulence
has not been studied in detail. For example, the role of Langmuir turbulence for
the SEE appears poorly understood. Whereas the electromagnetic radiation from
Langmuir turbulence is strong within a time scale of the order of 100 ms or less
after pump-on for low pump duty cycles, the radiation at longer time scales appears significantly weaker. When f0 ≈ sfe the FAS intensity and the associated
AA of the pump wave is minimum, implying that the Langmuir turbulence in the
pump reflection region should be driven also on the long time scales, however,
apparently without resulting in significant SEE during steady state. It is therefore
suggested to study the actual radiation mechanism from plasma turbulence, e.g.,
with particle velocity distributions which are self consistent with the turbulence.
Further, in many of the models for the SEE the focus is on the interaction between
electrostatic oscillations and it is only assumed that the HF electrostatic mode converts into electromagnetic radiation in order to give rise to SEE. It is essential that
such conversion between electrostatic and electromagnetic radiation is studied in
some detail for realistic HF pumped ionospheric plasma. For example, electrostatic
oscillations strictly perpendicular to the ambient magnetic field cannot produce
electromagnetic radiation which can escape from the plasma along the magnetic
field (Tidman, 1960), but a parallel wave number component must be introduced in
the models. Such a component could be inherited from the electromagnetic pump
wave itself.
Finally, the bulk HF pump experiments have been performed during geomagnetically quiet conditions which have given evidence of a wide range of plasma
physical phenomena. However, a most interesting and unexplored research field is
the interaction of localized HF pump-induced plasma perturbations with free energy sources in the ionospheric plasma and naturally driven geophysical processes
(Belenov et al., 1997; Yeoman et al., 1997; Blagoveshchenskaya et al., 1998a, b).
Although the energy involved in the naturally driven phenomena, whether it involves the dynamics of the neutral atmosphere or the magnetosphere, is orders of
magnitudes larger than that which can be supplied by an electromagnetic wave
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T. B. LEYSER
transmitted from an HF pump facility, the pump-induced perturbation could affect
the larger scale geophysical phenomena during certain critical conditions. Evidence
of interaction between HF pumped SEE and naturally driven ionospheric perturbations has probably been seen in the occurrence of the BDE emission briefly described above. In general, such stimulus–response type of experiments concerning
geophysical processes in the near-earth space plasma could help in understanding
some of the complex and dynamic interaction between different plasma in the
ionosphere and magnetosphere and, possibly, the role of the ionosphere for the
large-scale processes associated with the magnetosphere–solar wind interaction.
8. Summary
The study of SEE has revealed an enormous richness of plasma processes that are
excited by transmitting an HF electromagnetic pump wave into the ionospheric
plasma. The complexity of the driven plasma turbulence is exhibited in the wide
range of time scales of the radiation evolution and the development of numerous
SEE spectral components. The type of plasma processes excited, although far from
fully understood, depend on experimental parameters such as the pump power,
frequency, duration of pumping, pump duty cycle, and usage of additional pump
waves at different frequencies in various modes. The SEE is particularly strong
within on the order of ten milliseconds after a cold pump-on. These emissions emanate from a non-stationary plasma state, in the sense that the SEE intensity decays
to a lower level if the pumping continues beyond the overshoot time. For continued
pumping, the SEE intensity is again strong on long thermal time scales (on the
order of seconds), although intensity overshoots also occur on these time scales.
The dependence of the SEE on these long time scales on electron gyro harmonic
effects suggests that an ambient magnetic field in the plasma is important for the
plasma to dissipate into electromagnetic radiation during long time quasi-stationary
turbulence conditions. Further, the ambient magnetic field facilitates the slow symmetry breaking structuring of the plasma into field-aligned striations, which is
strongly associated with the excitation of the rich long time SEE spectrum. In view
of the increased complexity of the SEE on the thermal time scales, when the slow
thermal type processes slave the evolution of the fast ponderomotive interactions,
SEE appears as a self-organization of the plasma in response to the electromagnetic
HF pumping, which depends intrinsically on the interaction of processes at widely
different time scales to determine the nonlinear plasma states. It is reasonable to
expect that the knowledge gained from studying SEE in the ionosphere should be
relevant for the understanding of non-thermal electromagnetic radiation from other
plasmas in the laboratory and space.
STIMULATED ELECTROMAGNETIC EMISSIONS
319
Acknowledgements
The author gratefully acknowledges collaboration and discussions over many years
with Bo Thidé and the present and past members of the wave group in Uppsala,
including Tobia Carozzi, Harald Derblom, Simon Goodman, Bengt Lundborg, Åke
Hedberg, Johan Nordling, Mattias Waldenvik, and others, as well as with Lev
Erukhimov, Vladimir Frolov, Savely Grach, Yakov Istomin, Georgy Komrakov,
Helmuth Kopka, Michael Rietveld, Evgeny Sergeev, and Peter Stubbe. The author
also thanks Jan Bergman, Tobia Carozzi, Roger Karlsson, Bo Thidé, and Christer
Wahlberg for their valuable comments on the manuscript. This work was supported by the Swedish Natural Science Research Council. Partial support during
the final preparations of the manuscript was also obtained from the Semiconductor
Manufacturing Alliance for Research and Training Program of the University of
California.
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