Science Case for a Laser Experiment to study
Stimulated Electromagnetic Emission.
The phenomenon of Stimulated Electromagnetic Emission (SEE) is well-known in the
solar-terrestrial physics community, particularly among researchers working in the
field of ionospheric modification, in which high-powered HF transmitters are used to
modify the naturally occurring ionospheric plasma via wave-wave and wave-particle
interactions [Leyser, 2001]. Such experiments are routinely performed at the EISCAT
facility in Norway, in which the UK is a founding member. These experiments are
designed to understand the fundamental wave-plasma interactions, which commonly
occur in astrophysical plasmas.
Ionospheric modification experiments exploit the fact that, at altitudes where the
natural plasma frequency of the ionosphere matches that of the powerful HF pump
wave, a “forced oscillator” effect occurs, where a resonant acceleration of charge is
induced in the ionospheric plasma by the field of the RF wave. In order to conserve
neutrality, electric fields are induced to counter the effects of charge separation, and
these disturbances can propagate in space as induced electrostatic plasma waves. A
variety of electromagnetic to electrostatic wave conversion mechanisms have been
identified, for example: (1) the parametric decay instability and (2) the oscillating two
stream instability, which both result in Langmuir and ion-acoustic waves propagating
near parallel to the magnetic field direction [Fejer, 1979]; (3) the parametric decay
instability of upper-hybrid and (4) electron Bernstein waves propagating near
perpendicular to the magnetic field direction, which both result in lower-hybrid (ion)
waves [Istomin and Leyser, 1995]; as well as (4) the thermal parametric instability of
upper-hybrid waves [Dysthe et al., 1982].
The thermal parametric instability heats the plasma, which amplifies existing density
irregularities by plasma expulsion. This results in magnetic field-aligned meter-scale
density irregularities (called striations), which are routinely observed over EISCAT
by the UK’s CUTLASS SuperDARN radar [e.g. Honary et al., 1999]. The
electromagnetic pump wave then scatters from the striations to form electrostatic
plasma waves, of which the ion-acoustic waves are observable by the EISCAT radars
[e.g. Stubbe et al., 1992]. The striations serve to “trap” the upper-hybrid plasma
waves, which further heats the plasma, reinforcing the plasma density depletion in a
positive feedback process which saturates in a few seconds. The electrostatic waves
thus generated cascade to higher and lower frequencies via multiple and complex
mechanisms, some of which are not well understood [e.g. Carozzi et al., 2002]. The
modified spectrum of plasma waves are then re-converted on the plasma irregularities
into electromagnetic waves, which may be observed on the ground as sideband
signals around the pump frequency. These richly structured spectra are called
Stimulated Electromagnetic Emissions (SEE) [Leyser, 2001].
This picture of “stimulated backscatter” was proposed several decades ago [e.g. Fejer,
1979], and there were theoretical predictions of stimulated backscatter effects from
HF heating long before the first experiments were carried out. Amongst the first HF
heating experiments to search for stimulated backscatter effects were those performed
using the powerful HF facility at EISCAT in Tromso, Norway, in the early 1980s
[Thide et al., 1982]. These experiments illustrated a wealth of wave-wave and waveparticle coupling effects. An unexpected result was that the SEE spectrum was found
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to be dependent on the pump wave frequency relative to harmonics of the electron
gyro-frequency, which varies with the magnetic field strength in altitude. In
particular, it is hard to explain the frequency up-shifted features within the SEE
spectrum [Stubbe et al., 1994]. In addition, recent work suggests that the orientation
of the pump wave with respect to the magnetic field is also important [Isham et al.,
2005].
Typically, the SEE occurs at frequencies separated by tens to hundreds of kilohertz
from the pump wave frequency, and the spectrum of stimulated modes is very
complex. This includes repeatable up-shifted as well as down-shifted spectral
components, with many distinct features, for example, the Down-Shifted Peak (DP),
Down-shifted Maximum (DM), Broad Continuum (BC), Fast Narrow Continuum
(FNC), Up-Shifted Peak (UP), and the Broad Up-shifted Maximum (BUM), among
others [Leyser, 2001]. Another notable feature is that the full range of SEE
phenomena is never observed simultaneously. In particular, when the pump wave
frequency is close to a harmonic of the electron gyro-frequency, prominent
components of the SEE spectrum (e.g. the DM) are much reduced or totally absent.
Other features appear only when pumping near a gyro-harmonic (e.g. the BUM), such
that the spectral content of the SEE undergoes major changes when the frequency of
the pump wave is changed in small steps around the gyro-harmonics [Frolov et al.,
2001]. In addition, pumping just above or below an electron gyro-harmonic does not
produce the same SEE spectrum, e.g. the BUM feature only occurs for pump
frequencies just above an electron gyro-harmonic.
Producing the SEE is critically dependent on the polarisation of the sounding wave,
occurring for O-mode polarisation but not for X-mode. This underlines the
importance of the magnetic field permeating the plasma, which makes the ionosphere
birefringent resulting in the X-mode reflection altitude being below that for O-mode
[Fejer, 1979]. Only the O-mode polarisation reaches the upper-hybrid resonance
altitude in the ionosphere, where the thermal parametric instability occurs, which is
the primary mechanism for creating the plasma density irregularities central to the
generation of SEE. Furthermore, plasma turbulence is only observed above a certain
power threshold, strongly suggesting the importance of non-linear wave coupling in
the production of SEE effects [Leyser et al., 1990]. The time scales required to excite
the SEE are different for various features. Some develop rapidly within milliseconds
(e.g. FNC), whereas others take up to seconds (e.g. DM). The implication is that SEE
effects are primarily generated in two interaction regions, the plasma resonance region
at the pump wave reflection height, and the upper hybrid resonance region, typically a
few kilometres below the HF reflection height. Rapidly developing SEE features are
associated with the pump wave ponderomotive force near the HF reflection altitude
[Boiko et al., 1985]. Slowly developing spectra are associated with field-aligned
density irregularities (striations), which grow as a result of upper hybrid resonance,
i.e. the slowly growing parametric thermal instability [Derblom et al., 1990]. Most of
the SEE spectral features are downshifted in frequency relative to the pump wave.
Despite many years of observations, a detailed theory to explain all the observed
aspects of the SEE has proved elusive, especially for the up-shifted spectral
components. In particular, the behaviour of the BUM remains to be explained. This
feature displays a variable separation from the pump frequency (15-150 kHz),
depending on the pump frequency relative to the electron gyro-harmonic frequency.
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When the pump frequency is within ~15 kHz of a gyro-harmonic, the BUM
disappears altogether. Attempts have been made to localise the source of the BUM
using interferometry, suggesting that it comes from a direction close to the magnetic
field [Isham et al., 2005]. Similar behaviour is also displayed by the first, second and
third orders of the DM, which also disappear for gyro-pumping. The DM occurs ~10
kHz below the pump frequency. One suggestion is that the production of the BUM
and DM might correspond to some kind of four-wave process taking place in density
cavities, in which cavity modes scatter the incoming pump wave [Leyser, 2001]. The
practical difficulties of making in-situ measurements in the F-layer ionosphere (200300 km) and the height ambiguities associated with remote passive sensing have
meant that the mechanisms for several SEE spectral features remain open to
speculation. In particular, only a few of the electrostatic plasma wave modes can be
observed indirectly by radars for set geometries (e.g. upper-hybrid waves nearperpendicular to the magnetic field with SuperDARN, and ion-acoustic waves nearparallel to the magnetic field with EISCAT). Often the measurement geometry
excludes the desired measurements.
In principle, the various theories for the generation mechanism of the SEE, in
particular the BUM feature, can be tested by a laboratory plasma experiment, since
the physical mechanisms are inherently scalable from the ionosphere/HF regime to
the regime of laboratory plasmas pumped by lasers. This means that laser pumping of
a (magnetised) laboratory plasma should produce optical SEE effects with similar
spectral characteristics, relative to the pump wave, as those observed during HF
pumping of the ionosphere. Such an experiment would also allow direct probing of
the plasma waves within the pumped volume, something which is extremely difficult
to do in the ionosphere. This is because the typical ionospheric SEE altitude range
(200-300 km) is above balloon and aircraft altitudes, while satellites and rockets only
give a brief and rare measurement opportunity. To our best knowledge, no such SEE
experiments have ever been performed.
Our proposal is to use the Vulcan laser, in conjunction with an overdense oxygen
plasma (since oxygen is also the dominant species at the ionospheric F-region heights
where SEE effects are produced). We propose to use laser ablation to create the
plasma, which would then be pumped to create the SEE effects. [We need to state
how a (un)magnetised plasma will be generated to demonstrate technical feasibility.]
The primary aims of this proposal are three-fold. One, to allow direct probing of the
pumped volume in the laboratory, i.e. to measure the electrostatic waves that do not
propagate to the ground from the ionosphere, which is essential to understanding the
wave-particle mechanisms that produce the SEE. Two, to test the influence of the
magnetic field by performing laboratory experiments with and without a magnetic
field present, which is impossible in the ionosphere. No magnetic field implies that
the electron gyro-harmonic SEE spectral features will no longer exist, but this remains
unproven. Three, to demonstrate the universality of SEE phenomena in wave-particle
interactions within plasmas. This would serve to relate the ionospheric and laboratory
observations to other environments, e.g. particle accelerators, fusion reactors, and the
solar corona, so that the efficiency of plasma heating and particle acceleration
mechanisms can be quantified in these environments.
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[Rephrase below, include reference, looks too much like a personal note]
The results of the experiment will be modelled using a simulation code created by
Raoul Trines who, together with Bob Bingham, has published a paper on “Photon
Landau damping”, a process which predicts backscatter effects at upshifted
frequencies arising from a turbulent wave field, which seems to hold considerable
promise for predicting the origin and behaviour of the BUM.
{Needs more on Raoul’s code}
{Needs more here on the actual laser experiment design}
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References
Boiko et al., Radiophysics and Quantum Electronics, 28, 259–268, 1985.
Derblom et al., J. Geophysical Research, 94, 10111–10120, 1989.
Carozzi et al., J. Geophysical Research, 107, doi: 10.1029/2001JA005082, 2002.
Dysthe, Mjølhus, Pecséli and Rypdal, Physica Scripta, T2/2, 548-559, 1982.
Fejer, J., Reviews of Geophysics, 17, 135-153, 1979.
Frolov, Sergeev, Ermakova and Komrakov, Geophysical Research Letters, 28, 31033106, 2001.
Honary, Robinson, Wright, Stocker, Rietveld and McCrea, Ann. Geophys., 17, 12351238, 1999.
Isham et al., Annales Geophysicae, 23, 55-74, 2005.
Istomin and Leyser, Physics of Plasmas, 2, 2084-2097, 1995.
Leyser et al., J. Geophysical Research, 95, 17233-17244, 1990.
Leyser, Space Science Reviews, 98, 223-328, 2001.
Stubbe, Kohl, Rietveld, J. Geophys. Res., 97, 6285-6297, 1992.
Stubbe, Stocker, Honary, Robinson and Jones, J. Geophysical Research, 99, 62336246, 1994.
Thide, Kopka and Stubbe, Physical Review Letters, 49, 1561-1564, 1982.
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