1
Investigations of Remote Plasma Irregularities by Radio Sounding:
Applications of the Radio Plasma Imager on IMAGE
Shing F. Fung1, Robert F. Benson1, Donald L. Carpenter2, Bodo W. Reinisch3, Dennis L.
Gallagher4…
1
NASA Goddard Space Flight Center, Greenbelt, MD
Stanford University, Palo Alto, CA
3
University of Massachusetts, Lowell, MA
4NASA Marshall Space Flight Center, Huntsville, AL
2
Abstract. The Radio Plasma Imager (RPI) on the IMAGE mission operates like a radar. It is
usually assumed that echoes of electromagnetic sounder waves are reflected at remote plasma
cutoffs. Thus analyses of RPI observations will yield the plasma parameters and distances to
the remote reflection points. This assumption is valid only if the reflecting plasma surfaces
are sufficiently smooth such that they behave effectively as plane mirrors to the incoming
sounder waves. This implies that geometric optics or the WKB approximation must hold for
the propagation of the sounder waves. In addition, in order to overcome potentially low signal
strengths in long-range magnetospheric sounding, pulse compression and coherent integration
techniques perfected in ionospheric digital sounders will be employed to enhance signal-tonoise ratio. Therefore, the ability of the RPI to transmit and receive coherent pulses is
essential to the applicability of these techniques. When there exist plasma irregularities,
however, either in the medium in which the sounder waves propagate or in the remote
magnetospheric plasmas that are being probed by the sounder waves, echo signatures will
become complicated. In this paper, we investigate the conditions for which coherent signals
can be observed by the RPI and the effects of multi-scale irregularities, such as scattering and
plasma resonances, that will affect the RPI signals. Sounding of irregular plasma structures in
the plasmapause and plasmasphere is also discussed.
2
1.0 Introduction
The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite, scheduled to
be launched on February 15, 2000, will carry six remote sensing instruments to study the
global response of the Earth's magnetosphere to changes in the solar wind. One of the
scientific instruments to be flown is the Radio Plasma Imager (RPI) which is an advanced
digital radio sounder that operates like a radar. The design of the RPI is based on that of the
Digisonde Portable Sounder (DPS) developed at the University of Massachusetts Lowell, for
ground-based observations of the ionosphere. By transmitting coded electromagnetic pulses
and receiving the echoes reflected from remote magnetospheric plasmas, measurements by
the RPI can be used to determined the global-scale electron density structure and dynamics of
the magnetosphere.
1.1
RPI Measurement Capabilities
The RPI operates like a radar and assumes that echoes of sounder waves are reflected at
remote plasma cutoffs. This assumption is valid only if the reflecting plasma surfaces are
sufficiently smooth such that they appear effectively as plane surfaces to the incoming
sounder waves. This simply means that geometric optics or the WKB approximation must
hold for the propagation of the sounder waves. A detailed description of the RPI is given in
Reinisch et al [1999a]. We summarize briefly the RPI measurement capabilities here in order
to discuss the analysis of certain RPI observations to ascertain information on plasma
irregularities present in the remote magnetospheric plasmas.
The RPI transmits coded electromagnetic pulses with a minimal pulse length of 3.2 ms. Thus
the corresponding range resolution is 480 km. Pulse transmission is accomplished by using
two 500-m crossed-dipole wire antennas in the satellite spin plane. These dipoles and a 20-m
dipole antenna in the satellite spin (z) axis are then used for echo reception. Three 300Hzbandwidth receivers are used for echo signal reception. The three-axis dipole antenna
configuration allows the determination of phase and polarization, and thus the angle of
arrival, of the incoming signals [Reinisch et al, 1999a,b]. The angular resolution of directionfinding is about 1º, limited primarily by the noise level of the short (z) antenna [Calvert et al.,
1995; 1997].
For each sounding measurement at frequency f, the RPI will measure the echo signal
amplitude, phase and Doppler shift. In this paper, we will discuss how magnetospheric
plasma waves and irregularities can affect these RPI echoes
.
1.2
Radio Sounding in Cold Plasma
Discussions of long-range sounding of magnetospheric plasmas have usually been focused on
the use of the so-called “free-space”, high-frequency electromagnetic waves, the ordinary (LO) and extraordinary (R-X) modes [Stix, 1992]. This type of sounding is based on the coldplasma approximation which is valid when the phase speed of the sounder wave far exceeds
the electron thermal speed of the plasma medium. Transmitted pulses in the L-O and R-X
modes are reflected when they encounter their respective plasma cutoffs (k = 0), fL-O and fR-X,
with
3
f L O  f pe
(1)
and
f R X 
fc e 1
 4 f pe 2  f c e2
2 2


1/ 2
(2)
where fpe and fce are the electron plasma and gyro frequency, respectively. By displaying the
apparent ranges of the resulting echoes as a function of the sounding frequency f, a
plasmagram is produced. The plasmagram is the magnetospheric analog of the ionogram
display produced by ionospheric sounders [Benson et al., 1998]. Using inversion techniques,
the plasmagram echo traces will be converted to electron density profiles from the sounder to
remote plasma regions as is done in the case of ionospheric sounding [Jackson, 1969; Huang
and Reinisch, 1982]. Using this sounding technique, it is feasible to obtain global-scale
density structures of the magnetosphere on time scales shorter than the plasma dynamical
time scales [Calvert et al., 1995, 1997; Reiff et al., 1996; Fung and Green, 1996; Green et al.,
1999].
1.3
Thermal Plasma Effects
The cold plasma approximation is by and large valid for describing the propagation of
sounder waves in most ionospheric and magnetospheric plasmas. This in fact has allowed the
radio sounding technique to be applied successfully to observe the structure and dynamics of
the ionosphere [Hunsucker, 1992; Jackson et al., 1980]. It is important to note, however, that
both ionospheric and magnetospheric plasmas have finite temperature, varying from 0.1 eV
in the ionosphere to ~1 keV in the outer magnetosphere. Thermal effects and plasma
instabilities can lead to the generation and propagation of plasma waves and turbulence in
ionsopheric and magnetospheric plasmas [Akhiezer et al., 1975; Keskinen and Ossakow,
1983].
Although the natural ionosphere has mostly been treated as a cold plasma, its finite
temperature supports the propagation of many plasma waves, such as Langmuir and ion
acoustic waves [Kelly, 1989; Hargreaves, 1992]. As discussed in the next section, these
eigenmodes and other larger-scale plasma irregularities can interact with high-frequency
sounder waves to produce observable signatures in radio sounding observations. We will
illustrate in section 3 some examples of radio sounding observations resulting from
ionospheric plasma waves and irregularities.
2.0 Effects of Multi-Scale Irregularities on Sounder Measurements
It is generally assumed that long-range propagation of a sounder wave is supported by the
background plasma such that the validity of the WKB approximation is maintained [Budden,
1985; Stix, 1992]. That means that any inhomogeneity scale length Li of the background
plasma must be large compared to the wavelength of the sounder wave, i.e.,
  Li  n0 n0 
1
(3)
4
with n0 being the background electron density. In a magnetized plasma, such as the
ionosphere and magnetosphere, a similar condition on the variability of the background
magnetic field B0 is also required.
Due to the occurrence of plasma instabilities and dynamical effects, plasma waves and
irregularities are often present in ionospheric and magnetospheric plasmas. These plasma
fluctuations can affect the propagation of a sounder wave, particularly at magnetospheric
boundaries such as the plasmapause and magnetospause. Effects of plasma irregularities on
radio sounder signals will depend on the specific interactions between the sounder waves and
the irregularities.
In the linear-wave approximation, perturbations (e.g., n, B, etc.) associated with different
plasma wave modes in a plasma are too small to result in significant changes in the
background density n0 or magnetic field B0. This approximation is considered to be well
satisfied at the distant echo regions where the amplitudes of the sounder waves are small.
Thus, the distant sounder waves and the magnetospheric plasma wave modes can be treated
as independent of one another, except when they satisfy the conditions for resonant
interactions. The nature of the interaction between the sounder wave and a given plasma
wave mode depends on the frequency fi and wavelength i of the plasma wave. When Li >
> i >> De (Debye length), the plasma wave perturbations can coherently scatter the
sounder wave.
Indeed, a lot of ground-based radar observations of ionospheric irregularities are results of
coherent scattering [see Kelley, 1989]. These observations have also been called “incoherent”
because they were obtained by incoherent scatter radar (ISR) facilities. Although these ISR
have been designed (originally) to observe incoherent scattering, such as Thomson scattering
by individual electrons, they do detect signals returned by anomalous reflection. In this paper,
we will adhere to the physical definition of incoherent scattering as scattering by randomly
distributed particles and that of coherent scattering as scattering by collective plasma
oscillations.
The weak turbulence approximation is not likely to be satisfied in the vicinity (~ 1 km) of the
spacecraft [Pulinets and Selegey, 1986]. In the nonlinear or strong turbulence regime, density
fluctuations associated with Langmuir or ion acoustic waves can induce significant density
perturbations in the background plasma. When Li (and i), geometric optics (the WKB
approximation) fails and physical optics must be used to describe wave propagation. In
analyzing sounder measurements, such as those obtained by the RPI on IMAGE, one may
have to consider the effects of multi-scale irregularities on the sounder signals.
2.1
Signal Coherence and Spatial Resolution
Before considering further the effects of multi-scale plasma fluctuations on a sounder wave,
we need to review the conditions under which coherent signals can be received by the RPI.
One of the techniques used by the RPI to perform magnetospheric sounding is coherent pulse
compression [Calvert et al., 1995, 1997; Reinisch et al., 1999a]. This technique requires that
the echo received by the RPI receivers is coherent such that it retains the phase code of the
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transmitted pulse. The coherent signal requirement dictates that all the signals of a given echo
received by the three orthogonal RPI antennas must be returned from within a Fresnel zone
2RF at the target plasma, as illustrated by the solid-line portion in Figure 1.
i
R eflecting
RF
r + /4
surface
RF
r
r + /4
IM A G E /R P I
Figure 1.
When probing a remote plasma region at a range r with a sounder wave of wavelength , all
the signals returning from within an area of radius RF at the target will be coherent if
2
r  RF   r  4 
2
2
(4)
or for r >> , we have
RF  r / 2
(5)
where RF is usually referred to as the Fresnel radius in optics.
As discussed in Reinisch et al [1999a,b], analysis of echo signals relies on the capability of
the RPI to distinguish each echo observed. This is accomplished by differentiating each echo
by its frequency, range, polarization, arrival direction and Doppler shift. Signals reflected by
two neighboring target plasma regions that are located at the same range r may have similar
wave properties and Doppler shifts. Echo differentiation then depends on the angular
resolution of the instrument. Because of finite angular resolution  the RPI can
resolve two closely located target regions only if they are separated by a distance greater than
6
R = r. Thus, R effectively represents a linear measure of the minimum target area
discernable by the RPI. In order for an echo from a given target to be coherent, the minimum
target size must be smaller than or equal to the diameter of a Fresnel zone (2RF). Thus, by
equating R to the Fresnel zone we can determine the maximum range rc at which a coherent
echo can be received by the RPI. We then have
rc 
2
2
(6)
The corresponding maximum Fresnel radius that can be resolved by the sounder observation
is simply given by RFc = / such that we can obtain the following relationship:
2
rc 2RFc 2RF 2 
R 


  Fc 
r
R  R   RF 
(7)
There have been questions concerning the feasibility of detecting coherent echoes from the
magnetosphere [Greenwald, 1997]. Although these questions have been addressed by Calvert
et al [1995; 1997], it is instructive to consider the condition for echo coherence. When r > rc,
we have ∆R > 2RF > 2RFc such that the signal received within an angle  would be
incoherent see Figure 1). Table 1 lists the values of rc and RFc for various sounder wave
frequencies and  appropriate for both the RPI and ground-based sounders [Reinisch
et al., 1995; 1999b]. For example, bottom-side sounder pulses at 3MHz have coherent range
rc = 656 km, which is larger than typical ranges r encountered in bottom-side sounding
observations.
30 kHz
300 kHz
1 MHz
3MHz

10 km
1 km
0.3 km
0.1km
rc
10.3 RE
573 km
1 RE
57.3 km
0.3 RE
17.2 km
0.1RE
5.7km
RFc
Table 1.
It is important to note from Table 1 that it is as feasible to detect coherent echoes near 30 kHz
from the magnetopause and near 300 kHz from the plasmasphere as it is from the ionsophere
near 3MHz. In all these cases the ratio RFC/ = 57.3, which is only a function of the
instrument angular resolution. Since the RPI and modern ground-based digital sounders have
similar angular resolutions, ≈ 1º, sounding of the magnetopause and plasmasphere from
within the magnetospheric cavity should be analogous to bottom-side ionospheric sounding.
It is important to note that Equation (6) assumes a plane reflecting surface. In the presence of
plasma irregularities or nonlinear plasma waves, large density gradients and large-amplitude
fluctuations normal to the average surface of reflection at r can introduce additional phase
path lengths, causing phase mixing and scattering of sounder waves (dotted-line portion in
7
Figure 1). Therefore, for discussion purposes we assume that the irregular density surfaces
being probed by the RPI signals satisfy the planar condition:
1
1 n 
i  
n r 

(8)
in which n = n0 + n is the total electron plasma density and n/r is the electron density
gradient along the normal to the reflecting surface (see Figure 1).
2.2
Ordering of Scale Sizes
It is apparent that the echo characteristics to be received by the RPI are determined by the
various characteristic scale sizes mentioned. Within the limits of Equations (3) and (8), we
need to consider the ordering of  i, De, and RFc for a given sounding observation from a
range r and sounder frequency setting. The RPI operates in the frequency range 3 kHz –3
MHz, thus the corresponding wavelength range is 0.1 <  < 100 km (see Table 1 above).
Since the electron density and temperature encountered by an RPI transmitted wave range
between 0.1–105 cm-3 and 10-3–1 keV, respectively, the typical Debye lengths are given
below:
Region
Nominal no, T
De
Magnetospheric lobe or
cavity
Magnetopause boundary
n0 = 0.1 cm-3, T < 1 keV
< 743 m
n0 = 10 cm-3, T = 1 keV
~ 74.3 m
Within the plasmapause
n0 > 103 cm-3, T < 1 eV
< 0.25 m
Table 2.
For linearly propagating sounder waves that satisfy Equations (3) and (8), the parameters
provided in Tables 1 and 2 indicate that the condition Li > RFc > ( i) >> De is satisfied in
most magnetospheric plasma environment to be observed by the RPI. When  i, the
sounder wave will simply behave as if it is traversing a medium with large-scale irregularity
under the WKB approximation. On the other hand, when  i, the sounder wave can be
coherently scattered by the plasma wave.
2.3
Coherent Scattering of Sounder Wave
Scattering refers to the interaction of a sounder transmitted wave with a scattering center that
is small compared to the wavelength (i.e., i <<  Li), such that after the interaction the
scattered wave propagates into a direction different from that of the transmitted wave.
Incoherent scattering occurs when  De such that individual charged particles act as the
scattering center. Due to the low transmitter power of the RPI and low magnetospheric
densities, incoherent scattering is not a significant effect to be considered. Coherent
8
scattering occurs when  i >> De such that the scattering centers are provided by the
collective action of the perturbations associated with the plasma waves.
2.3.1 Raman and Brillouin scattering
The most common processes by which high-frequency electromagnetic waves are scattered
by a plasma are the stimulated Raman and Brillouin scattering [Kruer, 1988; Swanson, 1989].
These processes are quite similar in that they are both three-wave processes involving an
incident electromagnetic wave (0, k0), a scattered electromagnetic wave (s, ks), and a
plasma wave (i, ki). In Raman scattering, the plasma wave is a Langmuir wave; whereas in
Brillouin scattering, the corresponding plasma wave is an ion acoustic wave.
The scattering processes can be generally described as a three-wave decay process in which
the following wave resonance conditions are satisfied (conventionally expressed in terms of
the wave angular frequencies  and wave vectors k):
k0  ks + ki
(9)
0   s  i
(10)
and
As the sounder wave encounters a plasma medium in which plasma waves are present, it can
be scattered by the plasma wave when (9) and (10) are locally satisfied.
The physics of the scattering process is fairly straightforward. When the high-frequency wave
propagates through a plasma medium, its oscillating electric field with amplitude E0 may
encounter rippling density perturbations associated with a plasma wave along its propagation
path. In responding to the incoming wave field, the plasma electrons will oscillate with a
velocity v = eE0/mw0, which in turn generates a transverse current J = -evn. When the
phase-matching conditions (9) and (10) are satisfied, the transverse current can then generate
the scattered wave field. It can be shown that the scattering cross section maximizes for the
case of backscattering of unpolarized wave [Akhiezer et al., 1975].
For backscattering, we have ks = –k0 such that (9) implies that ki = 2ko. This is a general
result pertaining to backscattering from irregularities or plasma waves. In the case of Raman
scattering, since i of a Langmuir wave is near the local plasma frequency pe, and the
scattered wave frequency is s ≥ pe, therefore we must have o ≥ 2pe. This implies that
Raman backscattering would occur at a density near ncr/4, where ncr is the critical electron
density defined by the L-O mode cutoff (Equation 1) of the transmitted wave (0, k0). Within
the limit (kiDe)2 << 1, Brillouin (and Raman) backscattering causes the returned signals to be
shifted in frequency (s = o – i). Since the ion-acoustic wave frequency is given by i =
ki√(Te/Mi) ≈ 6.15104√(Tei) rad-s-1, where Te is the electron temperature in electron-volts
and iis the wavelength in meters, we can estimate the frequency shift as f = i/2 Using
the backscattering condition i≈ /2, we thus have f/√(Te) = 2, 20 and 65 Hz eV-1/2 for the
9
sounder wave frequency of 30 kHz, 300 kHz, and 1 MHz, respectively. The actual frequency
shifts, dependent on the electron plasma temperature, can be measured by the Doppler shift
measurements [Reinisch et al., 1999].
2.3.2 Aspect-sensitive scattering
As discussed above, the ionospheric spread-F phenomenon is the result of scattering of radio
sounder signals by ionospheric field-aligned irregularities (FAI). High-frequency
electromagnetic waves propagating nearly transversely with respect to the background
magnetic field can be scattered by the FAI. A coherent condition selects scattering from
irregularities whose spacing equals half the sounder wavelength [Hanuise, 1983; James,
1989]. This condition is simply the backscattering condition (ki = 2ko) discussed in the last
sub-section.
2.4
Nonlinear Processes
We have so far treated all wave modes and the plasma medium to be well described by the
linear or weak turbulence theory. In fact, when the wave energy (W) becomes comparable to
the plasma thermal energy (Wth = nTe), the plasma medium in which the waves propagate can
become a nonlinear medium, such that the dielectric tensor becomes a function of the wave
amplitudes. In this case, perturbation techniques no longer provide adequate descriptions of
the wave-plasma interactions [see e.g., Treumann and Baumjohann, 1997]. When W/Wth > 1,
the plasma medium is significantly modified by the presence of the sounder wave [Pulinets
and Selegey, 1989]. Strong plasma turbulence effects become important when 1 > W/Wth >
(kiDe)2. Density variations, such as the generation of cavitons, associated with strong ionacoustic and Langmuir turbulence may be involved in the formation of density striations in
the auroral and equatorial upper ionospheres [Treumann and Baumjohann, 1997]. Detailed
treament of strong plasma turbulence is beyond the scope of this paper. Suffice it to note,
however, that effects of plasma nonlinearities may be manifested in the RPI signals,
particularly in the ionosphere and plasmasphere.
3.0
Detection of Ionospheric Irregularities by Space-Borne Radio Sounders
In the terrestrial ionosphere and magnetosphere, irregularities in electron density sometimes
form in the direction perpendicular to the ambient magnetic field B. These density
irregularities are referred to as field-aligned irregularities (FAI) because they are usually
maintained for long distances along B. FAI are easily detected by observing the radio waves
that are either scattered by them or are guided (ducted) along them. The most common
ionospheric signature of FAI is in the form of spread F, i.e., the spreading in both range and
frequency of the F region reflection traces on ionograms from both ground-based and
satellite-based ionosondes. Topside ionograms from the latter also display spectacular
signatures of ducted echoes resulting from radio waves guided along FAI. We review in this
section some signatures of ionospheric plasma irregularities observed in topside ionograms,
which can serve as a guide to the analogous magnetospheric observations by the RPI.
3.1
Spread F
10
Ionogram spread-F signatures are attributed to aspect-sensitive scattering (see subsection
2.3.2) from FAI with dimensions transverse to B small relative to the wavelength of the
probing sounder wave. The scattered signal often has an intensity equal to or greater than the
signal from the totally reflected wave as illustrated in Figure 2, which also illustrates that the
spread-F can extend over a great altitude range. In this example, the spread-F signature
extends from the altitude of the electron density F-peak to the altitude of the spacecraft. The
spread F provides valuable information concerning the presence and nature of FAI, but can
greatly complicate the inversion of ISIS 2 ionograms to produce electron density profiles
[Hagg et al., 1969]. This ionogram also displays whistler-mode noise during the fixedfrequency portion of sounding operations (in this case at 0.12 MHz for about the first 3.3 s of
the ionogram) and continuing into the swept-frequency operation until the local plasma
frequency is encountered near 0.37 MHz. In addition, noise signals from the ionospheric
breakthrough of ground-based radio transmitters is observed at frequencies above about 3.0
MHz.
Figure 2. Digital ISIS 2 high-latitude ionogram
3.2
Ducted Echoes
When a topside sounder is immersed in a FAI that extends from one hemisphere to the other,
long-range non-vertical echoes [Muldrew, 1969; Calvert, 1995] can be received as
schematically illustrated in the top half of Figure 3. In this schematic, a satellite-borne
sounder immersed in a FAI associated with an equatorial plasma bubble is illustrated with ray
paths of expected ionospheric echoes. The expected ionogram traces produced by these
echoes are illustrated in the schematic ionogram inserts adjacent to each reflection point. The
ionogram insert for trace #1 corresponds to the radio waves that travel the shortest distance to
their total reflection directly below the satellite, i.e., to vertical propagation in a horizontally
stratified ionosphere. The ionogram insert for trace #2 corresponds to propagation along the
FAI in the local hemisphere. This trace is of longer duration than trace #1 because the waves
along the non-vertical path have a longer distance to travel to encounter the same n0 value
compared to the vertically-propagating waves in a horizontally stratified ionosphere. Note
that the FAI depicted in the figure only needs to correspond to a n/n0 of a few percent to
give rise to ducted echoes [Dyson and Benson, 1978].
11
The ionogram insert for trace #3 is of much greater duration and has a different shape. It
corresponds to reflections of the ducted waves in the conjugate hemisphere. In this case, the
first signal return (corresponding to the wave cutoff frequency) occurs at a finite delay time
because no echo is returned from the regions of lower n0 above the satellite along this ducted
propagation path into the conjugate hemisphere. This first signal return corresponds to the
location in the conjugate hemisphere where n0 is the same as the value corresponding to the
wave cutoff conditions (Equation 1 or 2) at the satellite location. The delay (for trace 3) then
decreases with increasing frequency above wave cutoff due to decreasing wave retardation
with increasing frequency. The ionogram insert for combinations of these ducted echoes is
shown at the very top of Figure 3. Combining these multiple echoes with trace #3 (right
schematic insert) yields an epsilon signature exactly as observed in the low-latitude, lowaltitude ISIS 1 ionogram reproduced in the lower portion of Figure 3.
Figure 3. Schematic and ISIS 1 observations of equatorial-bubble duct [adapted from Dyson
and Benson, 1978].
12
3.3
Sounder-Generated Plasma Resonances
The relatively modest gradients in n0 and B near a space-borne sounder give rise to shortrange echoes of sounder-generated electrostatic waves at characteristic frequencies of the
ambient medium. These echoes, which have been given the name plasma resonances [Benson
and Osherovich, 1992], can be used for in situ diagnostics of plasma parameters.
Sounder-generated plasma resonances are sometimes the dominant signal returns on
ionospheric topside ionograms as illustrated in Figure 4. Most of the prominent resonances
are due to echoes of sounder-generated electrostatic waves that are returned due to the
extreme sensitivity of the plasma-wave dispersion curves to small gradients in n0 and B near
resonant frequencies (kic/i >>1) [Benson, 1977]. Those designated with subscripts Qn are
attributed to a matching of a component of the wave group velocity to the satellite velocity.
Those designated with subscripts Dn (only the one with n = 1 appears in this figure) are
attributed to sounder-stimulated plasma emissions. All of these sounder-generated resonances
will play an important role in the analysis of RPI plasmagrams obtained from the IMAGE
satellite because they provide information on the ambient values of n0 and the magnitude of B
near the spacecraft. In addition, there is evidence that the Qn and Dn resonances are
stimulated by natural processes in the magnetosphere and that the plasma-to-gyrofrequency
ratio in the magnetosphere is often similar to the value corresponding to the ionospheric
conditions represented by the ratio of about 2.5 shown in Figure 4 [Christiansen et al., 1978;
Benson et al., 1999].
Figure 4. Alouette-2 ionogram illustrating plasma resonances [adapted from Benson, 1977]
3.4
Effects of Plasma Irregularities on Signal Strengths of Long-Range Echoes
There is some concern with regard to the effect of plasma irregularities in the reflecting
medium on the ability to receive the long-range echoes that are anticipated in magnetospheric
sounding. There have been investigations of this question in case of ionospheric soundings in
the presence of spread F [Franklin and Maclean, 1969; Jelly and Petrie, 1969]. These studies
13
indicated that there was an increase of up to 10 dB in the signal level, as compares to that
expected assuming a mirror reflection, under such conditions. One question that we plan to
address in this paper is how such plasma irregularities may effect the signal processing
techniques that are available to RPI on IMAGE that were not available in the case of the
topside-sounders on the Alouette/ISIS satellites.
Bob will expand on this section.
e.g. this can be a good start of the URSI presentation and a follow-up paper by Benson
et al [1999] ???.
4.0 Observing Magnetospheric Plasma Irregularities
In situ observations by ISEE 1 and more recently CRRES satellites during passages through
the equatorial plasmasphere have revealed remarkable variability and plasma irregularities
near the plasmapause and outlying regions. The irregularities at or near the plasmasphere
surface may appear at essentially any local time. Although the conditions under which
irregular density structure develop in the plasmapause and in the outer plasmasphere are still
unknown [Lemaire and Gringauz, 1998], their interpretation and that of many phenomena
studied remotely will be enhanced by the ability of IMAGE to make local measurements
along its orbit and as it penetrates or closely approaches the plasmasphere boundary
[Reinisch et al., 1999; Green et al., 1999; Carpenter, ???? (Don, what references should I
use here?)].
Figure 5 shows a near-equatorial density profile along an ISEE 1 orbit in the afternoon sector.
The position of the satellite in geocentric distance as a function of MLT is shown in the inset.
The profile shows irregular structure at the plasmapause (L < 4), an only slightly structured
outlier near L = 6, and a patchy, irregular region of dense plasma beyond L=7. The plasma
frequencies corresponding to the density scale on the left are shown at the right. This is the
likely density setting in which the RPI operates in the early phase of IMAGE. We should also
note that the corresponding densities involved in off-equatorial probing by RPI may be
somewhat larger, but probably not much more than a factor of ~2 above those indicated.
14
30 OCT 83
ISEE 1
0709-1016 UT
12
8
1000
500
12RE
E
4
18
06
100
100
00
50
10
1
10
PLASMA FREQUENCY (kHz)
ELECTRON DENSITY (el/cc)
10000
5
0.1
2
4
6
L
8
10
Figure 5. A near-equatorial density profile along an ISEE 1 orbit in the afternoon sector
[adapted from Carpenter et al., 1993].
As an example of the rapid density change in a given local time sector, Figure 6 shows two
plots of electron density along two consecutive CRRES orbits near the equator in the postmidnight/morning sector, showing how much the plasmasphere structure can change within
10 hours during a calm period. Orbits 145 (panel a) and 146 (panel b) occurred in succession
during a period of quieting following an earlier episode of plasmasphere erosion. The two
curves mark the quiet plasmasphere and the nighttime plasmatrough region according to an
empirical model [Don, can you please provide the correct model reference here?].
Logarithmic n0 is plotted vertically in coordinates of satellite L value as a function of MLT.
During orbit 145 the form of the density distribution was very simple, with a well-defined
plasmapause and well-behaved profiles within the plasmasphere and in the trough region.
However, during orbit 146, dense plasma extended farther in L value and in general the
profiles were highly structured. Local density troughs appeared both inbound and outbound in
the L = 3–4 range. Much of what was seen on orbit 146 may be attributed to density structure
imposed in an earlier MLT sector and transported into the MLT sector of the CRRES orbit by
the effects of the Earth’s co-rotation electric field. Recall that CRRES was in a lowinclination orbit, and moved quasi-synchronously with the Earth, while IMAGE, in a high
inclination orbit, might be expected to see how a change such as that from 145 to 146 occurs
at a given meridian as the “changes” rotate into “view.” (From EUV near apogee, a global
perspective on the changes should be obtained.)
15
Figure 6. Successive CRRES orbits (145 and 146) showing drastically different plasmapause
extents and plasmaspheric density structures [adapted from reference???].
The frequency ranges required for RPI sounding of the plasmasphere are commensurate with
the expected plasma frequencies (see Figure 5) and the R-X cutoff (Equation 2). For density
in the region of plasmapause gradients and beyond the frequency range is ~10–200 kHz.
Fixed or selected frequencies in the range ~80–100 kHz may be useful for efficient probing
of a steep plasmapause on some occasions. For probing the profile within the plasmasphere
from the plasmatrough region, frequencies in the range ~100–500 kHz will be needed. For
sounding from relatively low altitude within the plasmasphere, the range 100 kHz–3 MHz
will be needed, this being a form of ‘topside sounding.’
In order to capture the dynamical variations, one would like to detect changes in plasmapause
equatorial radius at rates ranging from 0 to 200 km per minute. While these are small
velocities compared to magnetopause motions, they need to be considered by accounting for
the spacecraft velocity at various points along the IMAGE orbit. Range resolution is another
important limiting factor in study of the plasmasphere shape and dynamics. Ways to
maximize it as a function of distance from the plasmasphere should be investigated before
IMAGE operations begin. Note that some of the best opportunities for studying the
plasmapause profile and its dynamics will come when IMAGE is relatively close to the
plasmapause, but that the shortest possible pulse and dead time may be needed under those
circumstances.
Small-scale density fluctuations have been observed by the CRRES satellite near the
plasmapause [LeDocq et al, 1994]. On many occasions when the CRRES apogee is near the
plasmapause and the spacecraft is moving eastward more or less at constant geocentric
distance, there appear many individual irregularities, each surrounded on the record by
plasmatrough level densities. An example can be seen in the data for Figure 6b [Carpenter
reference??]. Some of these irregularities may be in width of order of a few hundred km, and
may have finer structure.
There will be a great variety of irregular structures formed at and near the plasmapause as
illustrated in Figures 5 and 6. At times the plasmapause scale width may be less than 50 km
near the equator, and less than 5 km at ionospheric heights. For these cases, it may be fruitful
to combine relaxation sounding observations of irregularities and the plasmapause with local
measurements of density and possibly electron temperature by quasi-thermal noise
16
observations [Reinisch et al., 1999; Green et al., 1999, Meyer-Vernet, 1998]. Measurements
of rapidly changing quantities may best be made by some combination of passive recording,
such as relaxation sounding and antenna impedance measurements in the whistler-mode
frequency range. In the cases of passive recording and relaxation sounding, changes in
plasma parameters with time would be deduced from corresponding changes in resonance
frequencies in the plasma (see section 3.3), such as at the plasma or upper hybrid frequencies.
It is of interest to consider the information expected to be obtained by the RPI along an
IMAGE orbit. Figure 7 shows the IMAGE orbital configuration during the early phase of the
mission. On a given upleg (C, D), RPI should obtain some information on the global average
size of the plasmasphere. In addition, differences in range of echoes arriving from various
bearings may provide information on the large variations in plasmapause L value with
longitude, thus revealing any asymmetry imposed upon the plasmasphere by spatially and
temporally irregular processes that act upon it. On the downleg (E,F,G,H), RPI should be able
to obtain detailed information on the L value and variation in L value with local time
(compared to the upleg measurements) of the plasmapause over a several hour period. It
should be able to measure the density profile between IMAGE and points well within the
plasmasphere, and detect important changes in that profile with time, such as the appearance
of irregularities, refilling of depleted regions, loss of plasma from particular regions, and
changes in density gradient. With apogee in the afternoon sector, the early phase of IMAGE
will offer the best chance to study outlying dense regions, as indicated in Figures 5 and 6 and
discussed in Green et al. [1999].
Figure 7. Schematic of IMAGE orbital configuration during the early phase of the mission,
illustrating favorable positions for RPI observations of the plasmapause and plasmasphere.
The outliers shown in Figure 5 will not necessarily lie in the plane of the IMAGE orbit. Some
of these outlying plasma structures may be tail-like and connected to the main plasmasphere,
while others may be more or less detached from that body. RPI should have a unique
opportunity to investigate these poorly known forms. Discussion of the inner trough effect
indicated in Figure 6 is also given in Green et al. [1999].
17
When IMAGE is at relatively low altitude within the plasmasphere and in the southern polar
region (I,A,B), it should be able to sound the topside ionosphere and thus determine the lowaltitude conditions associated with the observations made at higher altitude on the opposite
side of the Earth.
5.0 Discussion and Conclusions
The varying positions of IMAGE along its orbit affects primarily the ranges and viewing
geometry toward various target plasmas, although spacecraft motion along its orbit also
contributes to Doppler shifts [Reinisch et al., 1999b]. Range variations can change the
effective Fresnel radii RF (Equation 5) for echoes arriving from different directions.
Therefore, we expect the coherence or degree of polarization of an echo at a fixed frequency
to vary with IMAGE positions as r changes from r < rc to r > rc. This effect is expected to be
observed in both magnetopause (boundary layer) and plasmapause (plasmasphere) echoes at
different frequencies, particularly during periods of increased noise level. Unlike in situ
measurements of magnetospheric plasmas, which cannot always distinguish between spatial
and temporal variations, high-time resolution observations by the RPI will yield spatial
structure of magnetospheric plasmas.
Like the ionosphere, plasma irregularities with different scale sizes are expected to be present
in magnetospheric plasmas. Irregularity scale sizes (i) can vary from much larger to much
smaller than the wavelengths () of the sounder waves. When the intervening plasma
irregularities or turbulence has De << i << , the sounder waves can be coherently scattered
to produced spread echoes in both frequency and range, analogous to the spread F signature
in the ionospheric case (see subsection 3.1). Such scattering can effectively reduce the power
of a primary echo and increase the chances of secondary echoes. In order for the RPI to
receive a strong primary echo, it is necessary that only very weak turbulence is present at i
<<  in the plasma medium between the RPI (IMAGE) and the target plasma. Since the
effects of incoherent scattering, which occurs when De >> , are likely to be negligible in
the RPI measurements, we have neglected in this discussion the echo power losses due to this
process.
Longer wavelength irregularities (at i > ) will also scatter the sounder waves. Such
scattering, however, will not likely alter significantly the propagation direction of the sounder
wave (see Equation 3). Large-scale (i >> ), low-frequency plasma irregularities in the
target plasma can also affect the coherence of the sounder echoes. Reflected signals from a
given target location are coherent when i (or Li) of the reflecting area is large compared to
the Fresnel radius (Equation 5). When i < RF and r > rc, echoes from neighboring target
regions can superimpose to produce a strong, but randomly phased, signal at the RPI receiver.
From these considerations, it is expected then that the echo characteristics measured by the
RPI can be quite complex, depending on the nature (scale sizes and amplitudes) of the
irregularities present in the intervening and target plasmas. Coherent integration technique is
applicable only when the target range r is less than rc (Equation 6). These effects need to be
accounted for when analyzing RPI data.
18
RPI has the ability to make repetitive measurements of magnetopause and plasmasphere
echoes from a remote location. Origin of the echoes can be determined by angle-of-arrival
measurements. Plasma density measurement will be derived from the frequency of each echo.
In addition, RPI has the ability to make Doppler and spectral measurements, and thus the
echo characteristics for each Doppler/spectral component can be analyzed separately.
Consistent and repetitive measurements of the magnetopause/boundary layer echoes at
different RPI positions along the IMAGE orbit (with varying RF) can yield information on the
characteristic scale sizes of the irregularities present in this region. Determination of
characteristic irregularity length scales can lead to the identification of their generation
mechanisms. Leading theories include: shear driven MHD instabilities, drift-wave
instabilities, cross-field streaming instabilities, and reconnection.
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