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.
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 uniform 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 ion-acoustic and Langmuir
waves. This classical picture of “stimulated backscatter” was proposed several
decades ago, 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 heater in Tromso, Norway, in the early
1980s. These experiments illustrated a wealth of wave-wave and wave-particle
coupling effects, some of which showed the expected classical signatures of
stimulated backscatter. An unexpected result, however, was that the range of
stimulated emission phenomena was found to be dependent on the pump wave
frequency. Recent work suggests that the orientation of the pump wave with respect to
the magnetic field is also important.
The spectral content of the observed stimulated emissions was very different than that
predicted theoretically. Typically, the stimulated emissions occurred at frequencies
separated by tens to hundreds of kilohertz from the pump wave frequency (rather than
the predicted tens of Hertz), and the spectrum of stimulated modes was very complex,
including repeatable up-shifted as well as down-shifted components, with many
distinct features including the Down-Shifted Peak (DP), Down-shifted Maximum
(DM), Up-Shifted Peak (UP), Broad Up-shifted Maximum (BUM) and the Broad
Continuum (BC), among others. Another notable feature was that the full range of
SEE phenomena was rarely observed simultaneously. In particular, when the pump
wave frequency was close to a harmonic of the electron gyro-frequency, important
components of the SEE spectrum, like the DM, were much reduced or totally absent.
Other features, appeared only when near a gyroharmonic such that the spectral
content of SEE underwent major changes when the frequency of the pump wave was
changed in small steps around the gyro-harmonics. 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.
Investigations of SEE soon established that the effect was critically dependent on the
polarisation of the sounding wave. SEE effects were consistently observed when
pumping with O-mode polarisation, but were apparently not produced by X-mode.
This underlined the importance of the magnetic field, and suggested that the effect
arose from an altitude which was never reached by X-mode polarisation, due to the
birefringent nature of the ionospheric plasma. Further observations showed that
turbulent effects were only observed above a certain power threshold, strongly
suggesting the importance of non-linear wave coupling in the production of SEE
effects. The time scales required to excite the SEE features were found to be different
for various features. Some were found to develop rapidly, within milliseconds,
whereas others took hundreds of milliseconds. The implication was 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 resonance height. Rapidly developing SEE spectra are
associated with ponderomotive phenomena near the HF reflection altitude. Slowly
developing are associated with field-aligned density irregularities (striations), which
grow as a result of upper hybrid resonance, i.e. the conversion of the electromagnetic
pump wave into stationary electrostatic waves perpendicular to the magnetic field
line.
The downshifted components of the SEE spectrum were typically more energetic than
the up-shifted components, suggesting that the secondary radiation is caused by mode
conversion of Langmuir waves produced by parametric decay of the pump wave.
Many of the observations are clearly attributable to non-linear wave interactions with
electrostatic wave modes perpendicular to the magnetic field, and the conversion of
pump wave energy into upper hybrid modes appears to be facilitated by the
production of field-aligned density striations, which are well known to be produced
during ionophric pumping, and whose production has been studied by a number of
radar systems at HF and VHF frequencies. These striations serve to “trap” the upper
hybrid modes, making them most susceptible to enhancement by energy conversion of
the pump wave in a positive feedback process which saturates in a few seconds.
However, despite many years of observations, a detailed theory to explain all the
observed aspects of SEE has proved elusive, especially for the up-shifted spectral
components. In particular, the behaviour of the strong feature known as the Broad
Upshifted Maximum (BUM) remains to be explained. This feature displays a larger
separation from the pump mode frequency, the further the pump mode is from an
electron gyroharmonic. When the pump mode frequency is within ~15 kHz of a gyroharmonic, 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 magnetic field-aligned. Similar behaviour is also displayed by the first,
second and third orders of the down-shifted maximum (DM, 2DM and 3DM) which
also disappear during gyro-pumping, though the separation frequencies in this case
are much lower (~10 kHz). One suggestion is that the production of the BUM and
DM might correspond to some kind of four-wave process taking place in density
cavitations, in which cavity modes scatter the incoming pump wave. The practical
difficulties of making in-situ measurements in the F-layer ionosphere (200-300 km)
and the ambiguities associated with remote sensing have meant that the mechanisms
for several SEE spectral features remain open to speculation.
In principle, this explanation of the origin of the BUM is susceptible to testing by a
laboratory plasma experiment, since the physical mechanisms are inherently scalable
from the ionosphere/HF regime to the regime of lab plasmas pumped by lasers. This
means that laser pumping of a magnetised lab plasma should produce optical SEE
effects with similar spectral characteristics, relative to the pump wave, as those
observed during HF pumping of the ionosphere.
Observation of the optical emissions produced by the laser pumping might also shed
light on the phenomenon of “auroral rings”, characteristic annular structures of
artificial optical emissions which are sometimes observed during HF pumping of the
ionosphere. This phenomenon, which again seems to be heavily dependent on the
orientation of the pump wave with respect to the magnetic field, is also thought to be
due to the presence of cavity modes, and possibly arises due to the presence of strong
pump field gradients at the edges of large-scale density structures. It may also,
additionally, be related to the proximity of the pump to a gyroharmonic. Another
interesting aspect for study in a laboratory plasma is the phenomenon of “preconditioning”. When heating a previously unperturbed ionosphere, relatively large
powers can be required to generate the plasma turbulence. However, once the
turbulence is generated, non-linear effects can be created with relatively low pump
wave powers. It would be interesting to test whether the same is true in the laser
plasma regime.
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). The idea would be to use laser ablation to create
the plasma, which would then be pumped to create the SEE effects. [MJK: Do we
need to state how a magnetised plasma will be generated?]
The results of the experiment would also 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}