PARAMETRIC INSTABILITIES IN THE IONOSPHERE

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PARAMETRIC INSTABILITIES IN THE

IONOSPHERE

J. Fejer

To cite this version:

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PARAMETRIC INSTABILITIES IN THE IONOSPHERE

J. A. FEJER

Max-Planck-Institut fiir Aeronomie, D-3411 Katlenburg-Lindau 3, Germany

Rhumb.

-

On prksente un examen des instabilites parametriques excitQs dans l'ionosphbre par transmission depuis le sol, d'onde radio de puissance. L'instabilite parametrique de dkged- rescence est fortement excitee par dc telles transmissions en dessous dc la frequence plasma maximum de I'ionosphGre. Trois sous-produits de cette instabilite sont des lueurs artificielles, des striations de densite alignks le long du champ, et de forte absorption anormale des ondes RF de diagnostic. Des theories tendant a expliquer ces phknomknes sont presentks dans leurs grandes ligncs.

Deux autres instabilites paramktriques, a savoir les diffusions Brillouin et Raman, sont faiblement excitkes par certains radars puissants de retrodiffusion incoherente. On decrit des tentatives pour detecter les effets de ces instabiiites sur les spectres de retrodiffusion incoherente.

Abstract.

-

Parametric instabilities cxcited in the ionosphere by powerful radio wave transmissions from the ground are reviewed. The parametric decay instability is strongly excited by such transmissions below the maximum plasma frequency of the ionosphere. Three byproducts of this instability are artificial airglow, field-aligned density striations and strong anomalous R F absorption of diagnostic waves. Theories attempting to explain these phenomena are outlined.

Two other parametric instabilities, namely stimulated Brillouin and Ranlan scattering, are weakly excited by certain powerful incoherent backscatter radars. Attempts to detect the effects of such instabilities on incoherent backscatter spectra are described.

1. Introduction. - A sufficiently strong electromagnetic wave in a plasma can excite parametric instabilities in which two (or more) new waves grow from the thermal (equilibrium) level to large amplitudes, taking their energy from that of the original wave ; at least one of the new waves is electrostatic in nature.

Interest in parametric instabilities has increased considerably in recent years as a result of their pos- sible role in laserjksion.

In the ionosphere just as in laser fusion, waves produced outside the plasma penetrate into the plasma. Particularly powerful

H F

transmissions have recently been used [ I ] at Platteville near Boulder, Colorado somewhat below the penetration frequency of the ionosphere with incident power dznsities of about 50 pW/m2. It has been pointed out [2] that such transmissions in the ordinary magnetoionic mode must excite the parametric decay instability in the ionosphere near the reflection point of the H F wave where its electric field is greatly enhanced. There is a great deal of indirect evidence that this instability, in which the incident electromagnetic wave decays into a Langmuir wave and an ion acoustic wave, has indeed been strongly excited by the transmissions from Platteville and by similar transmissions [3] with solnewhat lower incidcnt power densities of 20 pW/m2 from Arecibo, Puerto Rico. Most of this

paper will be concerned with this indirect evidence coming from the so-called

H F

heating experiments at Platteville and at Arebico as well as its theoretical interpretation. Observations and theoretical work carried out since an earlier review [4] will be stressed. More recently it has been pointed out [5, 61 that

existing

V H F

and UHF incoherent backscatter radars can weakly excite parametric instabilities of a different kind in which the incident wave decays into an electromagnetic wave and into an electrostatic wave. The latter is either an ion acoustic wave (stimulated Brillouin scattering) or a Langmuir wave (stimulated Raman scattering). There is preliminary but rather direct evidence for the weak excitation of stimulated Brillouin scattering by the VHF incoherent backscatter radar at Jicamarca near Lima, Peru. In the last part of this paper these recent theoretical predictions, the observations that are being carried out to check the theory, and some preliminary observational results are discussed briefly.

2. The HF-enhanced plasma line. - There have been no diagnostic facilities for the direct detection of parametrically excited Langmuir waves by radar backscatter at Platteville where the first HF heating experiments have been carried out.

The prediction [2] of such parametrically excited Langmuir waves prompted the installation of an HF

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C6-56 J. A. FEJER

transmitter at Arecibo with a suitable H F feed in the focal region of the 300 m (spherical) dish [3], adjacent to the 430 MHz feed.

The predictions appeared to have been quickly verified [ 3 ] . Figure 1 shows the type of backscatter spectrum received by the 430 MHz radar (from the Ph. D. thesis of R. L. Showen, Rice University, December 1975) during H F heating using ordinary polarization.

I I I I I I A v t I I l k - r l - - T

-

GENERAL FEATURES OF HF- INDUCED ENHANCEMENT

U)

-

.- C DOWNSHIFTED x [DUE TO UP-GOING PLASMA WAVES) e -

-

.- -F

-

> - t cn Z W - t- z n W - E A 4

i

-

0 Z I I 1430 MHz)

FIG. 1.

-

Backscattcr spectrum observed during HF heating. The words growing mode and decay modr refer to the purely growing (or oscillating two-stream) instability and the parame- tric dccay instability with which the spectral features are believed to be associated. The enhancement in the ion line is due

to ionacoustic waves associated with the parametric decay instability. Courtesy of R. L. Showen (Ph. D. Thesis, Rice

University).

The double-humped ion line of incoherent backscat- ter due to scattering by thermal ion acoustic waves is only slightly enhanced in figure 1. Enhancements by several orders of magnitude above the thermal scattering level take place, however, a t frequencies differing from that of the radar transmitter by about the H F frequency or slightly less, indicating the presence of strong Langmuir waves at and below the H F frequency. It was natural to associate Langmuir waves at the H F frequency with the purely growing or oscillating two-stream instability [ 7 ] (in which the ion wave has zero frequency). The Langmuir waves whose frequency was less than the H F frequency by the ion acoustic frequency were similary taken to be associated with the parametric decay instability [8]. No similar effects are observcd when the H F wave has extraordinary polarization. This iscxpected theore- tically because the extraordinary wave is reflected below the height where Langmuir waves can propagate. The linear threshold of the parametric decay instability is exceedcd by a considerable margin over a height range extending down to about 10 km below the reflection level of an ordinary H F wave. In order to compare experimental results with theory it is therefore necessary to consider the mechanism of

saturation of the instability. Theoretical work 19, 10, 11, 12, 131 shows that the saturation spectrum at Arecibo is the result of a cascading process. The H F heating wave (the pump) has an electric field that is nearly parallel to the magnetic field of the earth for an ordinary wave near the reflection point. Initially this field excites parametrically a Langmuir wave of slightly lower frequency and an ion acoustic wave. When the Langmuir wave (whose electric field is also nearly parallel to the magnetic field of the earth) rea- ches large enough amplitudes it too suffers a decay, thereby exciting a Langmuir wave going in the opposite direction and another ion acoustic wave. The process continues until the last member of the cascade is of insufficient amplitude to decay again.

The actual saturation spectrum obtained by computer calculations [ l l , 121 consists of Langmuir waves that make angles below about 200 with the geomagnetic field as shown by figure 2 [Ill. This calculation is for a uniform medium ; estimates show that in a smoothly varying medium propagation effects do not greatly modify the spectrum of figure 2.

FIG. 2.

-

Distribution of wave energy density over wave number space. A finite number of waves with discreet values of wave number k, angle 8 between k and the pump ficld and azymuth angle have been assumed in the calculation. The energy densities for the different azymuth angles for the same

k and 8 have been summed ; the ratios I,' of thcsc partially summed cncrgy densities to the threshold energy density are shown 1; is the ratio of the pump wavc energy density to thc

threshold energy density. After Fejer and Kuo [ I I].

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figure 2. Figure 3 shows the results of computer calculations [13] of the wave spectrum for an angle of 450, for various ratios of the incident power density to the threshold density. It should be noted that the spectrum is well above the thermal level even below threshold (a ratio of 0.5) and that it then resembles the double-humped spectrum of incoherent backscatter as it should because its Langmuir waves are the result of scattering (of the pump) by thermal ionacoustic waves.

NORMALIZED WAVE NUMBER

FIG. 3.

-

Normalized spectral energy density per unit volume of wave vector space a s a function of normalized wave number, with the ratio of pump power to threshold power a s the para- meter labelling the curves. The wave number A is that of a plasma wave a t the pump frequency ; the wave number B is the fre-

quency-matched wave number. After Fejer and K u o [13].

The spectrum of figure 3 is weaker by 3-4 orders of magnitude than the spectrum of figure 2 ; the observed spectrum shown by figure 1 agrees in intensity and shape reasonably well with the theoretical spectrum shown by figure 3 ; the latter does show something like the decaj) line and the broad bump (the numerical work ignored the purely growing instability).

Clearly the most urgent need is for observations confirming the spectrum of figure 2 ; plans to do this will be discussed later. It would also be desirable to confirm the double-humped curve of figure 3 below threshold ; this probably could be done at Arecibo, using sufficiently low R F power and long integration times.

Recent more detailed observations of the H F enhanced plasma line [14] as well as the observations described in Showen's previously mentioned Ph. D. thesis, give us a more complicated picture than the one just described : its discussion will be left to a later part of this paper.

3. Artificially produced spread-F.

-

The first striking result of the Platteville observations was

the appearance of spread-F (echoes with several time delays rather than a single echo) on the ionogram tens of seconds after switching on the H F heating transmitter using either ordinary or extraordinary polarization [15]. Spread-F irregularities have also been studied using satellite transmissions. In this manner the field-aligned nature of the irregularities was demonstrated, the scale size across the magnetic field being in the range of about 100-500 m [16]. Two seemingly different mechanisms have been suggested to explain spread-F irregularities. In one approach [17, 181 the mechanism is a purely growing parametric instability in which the non-linear force is a partial pressure force caused by collisional dissipation and the excited high frequency wave is an electromagnetic rather than an electrostatic wave. The net result is a modification or modulation of the incident electromagnetic field. The mechanism works equally well for ordinary or extraordinary polarization. Field-aligned irregularities with wave lengths of a few hundred meters or greater in a direction perpendicular to the magnetic field can be explained in this manner.

In another approach [20] the mechanism is a focusing instability also based on collisional heating. Although the two mechanisms appear different on the surface, the mathematical developments are quite similar and the two treatments certainly describe the same phenomenon.

The excitation of the parametric decay instability has been ignored in the above treatments although it must in reality influence the self-focusing process. The opposite must also be true and the parametric decay instability must be more strongly excited in regions of high H F heating wave field strength.

4. Airglow modification.

-

Most observations were carried out on the 6 300

A

atomic oxygen line which is emitted naturally during the F-region recombination process. Some of the reaction rates involved in the recombination process are tempe- rature dependent [21]. Therefore the predicted effect of simple collisional heating is a slight temporary reduction of the normal airglow for a few minutes following the start of H F heating transmissions ;

a similar temporary increase follows switching off as shown by figure 4.

In the Platteville experiment extraordinary polarization of the heating transmitter produced the effects just described. The effects of ordinary polarization

were quite different as shown by figure 5. Enhance- ments of the airglow by 20-30 Rayleigh are observed during the H F transmissions [22].

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C6-58 J. A. FEJER

25-26 Mov 1971

1

X -Mode ~ r o < o g a t i o n

A6300 A Change Transmitter

FIG. 4. .- Changes in the 6 300 h airglow produced by heating transmissions in the extraordinary mode. Courtesy of Haslett

and Megill [34].

Airglow enhancements of the type shown by figure 5 are believed to be caused by collisional excitation of the 0 (ID) state by electrons having energies greater than 2 eV. They are believed to be produced by acceleration from the Maxwellian tail by parametrically excited Langmuir waves [24].

Time (M.D.T.) 2030 2M5 2100 2115 2130 2145 120, I I I I I PDwa = 1.6 MY Excitation hmumcy = 5.3 Mht 01 I , , , , I , . , , I , . , . I , , , , I , . , , I , , . . I . , , . , . . . , , , , , , I , , . . I 1 0 1 2m MOO 4000 5000 Time (seconds)

FIG. 5. - Changes in the 6 300 A airglow produced by heating transmissions in the ordinary mode. Courtcsy of Sipler and

Biondi [21].

Such accelerations of charged particles by random wave fields have been studied numerically [25]. The nature of the acceleration mechanism depends strongly on the ratio rAc/zTR of the correlation time to the trapping time. For zAc/zT, 3 1 the acceleration by velocity diffusion can be calculated using quasilinear theory. For zAC/zT, 9 1 a charged particle tends to be accelerated by a single step process rather than by velocity diffusion as shown by figure 6. With the aid of the simplest form of the saturation spectrum [9] and neglecting magnetic field effects one obtains rAC/zTR P - I k3 h3(mi/m,) where P is the ratio of the power density of the pump wave to threshold power density, k is the wave number of the plasma

FIG. 6. - One-dimensional motion (in velocity space) of a n

electron, for S>\C/TTR = 6.5 in a cornputcr generated spectrum of random electrostatic waves with the indicated spread of phase speeds. The waves were assumed to differ only in wave number, not in frequency. Timc is normalized to reciprocal of the angular wave frequency. Note the sudden accelerations at times 6 500 and 19 500. The particle is trapped in the wave between the times 10 500 and 17 500, for example. In principle the motion must be symmetrical with respect to the non-dimensional specd of unity (particle speed = mean phased speed of

wavc spectrum). After Graham and Fejer [32].

waves and h is the Debye length. Taking kh

-

0.15, P

--

5 results in zAC/rTR

-

20. This means that acceleration to energies of 2-3 eV occurs by the single step process rather than by velocity diffusion in the regions of interest, about 8-1 1 km below the reflection level of the ordinary wave ; crude calculations [24] show that this mechanism can explain the observed airglow enhancements at Platteville.

The fact that at Arecibo the collisional excitation of the 6 300

A

line is relatively seldom observed, does not necessarily mean that the parametric decay instability is not excited. The calculations [24] using the simple single step process indeed show that a reduction of the pump power density by a factor of 4 leads to a reduction of the airglow intensity by a factor of lo3. It is therefore not surprising that enven a reduction of the power density by a factor of 2 at Arecibo in comparison with Platteville leads to a very large reduction in intensity of the collisionally excited airglow which on most occasions is masked by the temporary modification of the natural airglow by heating.

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- 8 0 -

-

TIME AND DATE ( G M T ) 3 . 1 0

- - -

- 9 0

-

SJE

-100- D 0

z

-110-

-

(AMRAD MINIMUM DETECTABLE SIGNAL)

-

_"

- 1 2 0 3 - 0

9

4 Q - 1 3 0

7

0 -140

-

1 I , , I I I I , I I t 1 10 ' O O 3 . J

t+s

= r f 1000

+,

=7

FIG. 7. - Thc observed scattering coefficients b (f) where f is the radar frequency in MHz.

Courtesy of Minkoff [30] where the definition of b can be found.

157.5 MHz backscatter shows a decay time constant of 8 s. during an off period of 28 seconds but a growth time constant of less than a second after switching on. The band width [29] of the scattering is typically less than 10 Hz for 157.5 MHz backscatter and much smaller for lower radar frequencies [26] (1-2 Hz for 40 MHz).

Figure 7 shows the backscatter coefficient as a function of the radar frequency, derived [30] from all the radar measurements. The r.m.s. percentage density derivation derived from these data is about 1

%.

The height range of scatter is about 15 km near the reflection height of the H F wave.

Although the mechanism responsible for these field-aligned short scale striations is not fully unders- tood, it is highly significant that they, like the collisional excitation of the airglow, are only produced when the heating transmissions have ordinary polarization and therefore are almost certainly related to the parametrically generated Langmuir waves. One suggested mechanism [31] is a parametric instability in which the waves of the previously discussed saturation spectrum somehow act as a pump. Another mechanism has been studied in the Ph. D. thesis of Min-Chang Lee at the University of California, San Diego. Calculations show that the collisional dissipation of randomly phased Langmuir waves of the previously discussed saturation spectrum necessarily produces field-aligned striations of about the observed intensity. The medium simply selects those Fourier components of the heating that are nearly field-aligned. The calculations ignore the possible reaction of the striations on the saturation spectrum itself. We shall see later that there is indirect experimental evidence for such a reaction.

6. Wide band absorption. - Ionograms taken during the Platteville heating experiments [IS] show that the ordinary echo trace disappears within 5-10 seconds after the heating transmitter is switched on with ordinary polarization.

This phenomenon of wide band absorption is almost certainly due to scattering of the incident electromagnetic wave by short scale field-aligned irregularities into Langmuir waves [32]. The average observed absorption of about 10 db is well explained on the basis of radar scattering data illustrated in figure 7.

Naturally the heating wave itself must similarly be scattered into Langmuir waves with wave vectors perpendicular to the geomagnetic field. Such Langmuir waves have indeed been detected by the VHF radars from New Mexico [27, 291. This detection involves a double scattering porcess. First the striations scatter the H F heating electromagnetic wave into Langmuir waves, and then the Langmuir waves scatter the UHF radars wave into electromagnetic waves, having about the same bandwidth as the radar signal scattered by the striations [29].

7. Plasma line overshoot.

-

The previous discussion of the HF-enhanced plasma line ignored certain interesting phenomena which the first observations did not reveal. One of these phenomena is the so-called overshoot. If the heating transmitter has been switched off for some time then, after switching on, the H F enhanced plasma line (decay mode) rises at first well above its steady state level within milliseconds and then takes 5-10 seconds to drop to its steady state value. Figure 8, taken from the Ph. D. thesis of R. L. Showen at Rice University, illustrates this behaviour.

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C6-60 J. A. FEJER

7 APRIL 1975

fHF= 5.425 MHz

foF2 = 9 0 M H z

t- DOWNSHIFTED

FIG. 8.

-

Temporal variations of the HF enhanced plasma line (decay mode). After the off period the intensity rises more than an order of magnitude above the previous steady state value although the HF transmitter is operating at half the power. The inset shows the ionacoustic frequency as a function of the time during the overshoot. Courtesy of R. L. Showen (Ph. D.

Thesis, Rice University).

8. Height of the plasma line.

-

Recent investi- gations [14] of the height from which the H F enhanced plasma line comes, resulted in some unexpected and important results. The observations were made in daytime when Langmuir waves are also excited by photoelectrons [33]. Thus the height of the photoelectron-enhanced plasma line (which is present even when the H F transmitter is off) could be compared with the height of the H F enhanced plasma line. Figure 9 [14] illustrates both the overshoot phenomenon and the difference in height of the photoelectron-enhanced and the HF enhanced plasma lines. In these cases the H F enhanced plasma line is only seen following an off-period for a few seconds (in other observations the H F enhanced plasma line was seen during the whole on-period but with a larger initial amplitude) at a height greater by about

5 km than the photoelectron-enhanced plasma line. The natural expectation would be that an H F enhanced plasma line should come from exactly the same height as the photoelectron-enhanced plasma line because at that height and at the frequency set by the filter used in the experiments (and turned to the expected frequency of the decay line) the dispersion relation of Langmuir waves is satisfied for the wave length determined by the U H F radar frequency and the Bragg condition.

190

200 2io 2X) 230 240

HEIGHT (km)

FIG. 9.

-

Received signal strength givcn by Q = 20 loglo[(S

+

N)/F]

, for a series of time intervals and height gates of the Barker code. The * >> represents times when the H F transmitter was off. The value of Q is rounded off to the nearest integer and

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The only seemingly possible explanation for the absence of an H F enhanced plasma line at the height of the photoelectron-enhanced plasma line, is strong Landau damping caused by the photoelectrons. This Landau damping not only causes the threshold to exceed the power density of the incident H F wave but reduces the H F enhanced plasma line that accord- ing to figure 3 should be present even below threshold, to a level below that of the photoelectron-enhanced plasma line.

The presence of the H F enhanced plasma line at a height where the dispersion relation is not satisfied still has to be explained. Muldrew (private communi- cation) suggests that there must be field-aligned small scale ducts of reduczd density in which the dispersion relation can be satisfied at greater heights where the field strength of the H F wave is much stronger (i. e. determined by the first peak in the absolute value of the Airy function which determines the amplitude of the H F wave as a function of height). Presumably these ducts havc relatively little influence on the electroniagnetic wave whose propagation is determined by the large scale structure of the medium. It would be interesting to carry out similar observations at night when there are no photoelectrons. The H F enhanced plasma line should then come from a larger range of heights.

9. Outstanding problems in ionospheric heating theory. - The discussions of the previous section made it clear that the presence of striations should somehow be taken into account in the saturation theory of the parametric decay instability. It should be possible to calculate a saturation spectrum for an individual duct and then consider the scattering properties of the different ducted modes when illu- minated by the Arecibo radar.

Another interesting question is the part played by ducted modes in the development of striations. A further question of interest is the effect of large scale field-aligned irregularities in the excitation of the so-called Z mode waves at the reflection level of the ordinary wave, i. e. the question of linear

conversion. So-called Z mode traces have often been

seen on ionograms during H F heating [IS].

Another interesting experimental fact not mentioned so far is the strong enhancement both in striations [26] and in airglow [34] when the HF heating transmitter is operated at about twice the cyclotron frequency of electrons. In an intersting theoretical paper [35] it is pointed out that previous work [24, 361 has neglected the effect of the goemagnetic field on particle acceleration by random Langmuir waves ;

this effect could be particularly important near the second harmonic of the cyclotron frequency and diffusion in perpendicular rather than parallel velocity space could be important. Further work examining this problem would be very desirable. One of the problems with the above explanation is that it would

only explain the enhancement of the airglow but not of the striations.

10. Stimulated Brillouin scattering.

-

Existing incoherent backscatter radars can transmit high power pulses of relatively long duration (milliseconds). The question arises whether they can excite parametric instabilities in the ionosphere. Only those parametric instabilities in which one of the excited waves is an electromagnetic wave need to be considered because the radar frequency is much higher than the frequency of any electrostatic waves in the plasma. It has been suggested [5] that such parametric instabilities would probably be excited by the EISCAT radar now being built in T r o m s ~ . More detailed considerations [6] show that the threshold for a uniform medium is exceeded by several orders of magnitude by incoherent backscatter radars for stimulated Brillouin scatter, i. e. the decay of the radar wave into an upgoing ionacoustic wave and a backscattered electromagnetic wave.

The radar wave suffers negligible attenuation in

the ionosphere and would travel in a uniform medium having the properties of the F2 peak, distances very much greater than the actual thickness of the ionospheric F2 layer. The ionosphere is therefore certainly not a uniform medium for the purpose of stimulated Brillouin scatter and the relatively low threshold of stimulated Brillouin scatter for a uniform medium is of no practical interest. The effects of the instability are determined by its spatial growth rate [6, 371. The main effect of the instability is a spatial modification of incoherently backscattered waves, on their way down from the altitude of scattering, by parametric interaction with the upgoing radar wave. The waves representing the lower frequency hump of the double-humped incoherent backscatter

-

lhnear theory -- non-linear theory ' \

\

FIG. 10. - The calculated normal incoherent backscatter spectrum for Jicamarca is shown by the solid line. The inter- rupted line shows the spectrum modified by stimulated Brillouin scattering. A peak radiated power of 2 MW is assumed. Thc assumed pulse length is 3 ms and thecalculated received spectrum

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C6-62 J. A. FEJER

spectrum are amplified, the wavcs representing the upper frequency hump are attenuated by the non- linear interaction. The result is an expected asymmetry in the incoherent backscatter spectrum which for the Jicamarca radar (50 MH7) in daytime at full power could amount to 25 "/, as shown by figure 10. Preliminary examination of the data from a controlled experiment carried out in Jicamarca by K. Rinnert, R. F. Woodman and the author, using 3 ms pulses transmitted at full and at quarter power alternately show asymmetries of about the expected magnitude. A more detailed analysis of the data and a comparison with the theory still remains to be carried out. Perhaps it should be remarked that there is a reason to expect good agreement between theory and experiment because the instability is weakly excited and the theory is in a sense linear. This of course is not the case with the H F enhanced plasma line where the (quasilinear) theory of weak plasma turbulence has been invoked (and the turbulence may, in fact, not be weak [38]).

1 1. Stimulated Raman scatter. - In this instability the incidcnt radar wave decays into an upgoing Langmuir wave and a backscattered electromagnetic wave. In principle it, too, can be weakly excited by incoherent backscatter radars. In contrast to stimulated Brillouin scatter, however, where the pondero- motive force (arising from the nonlinear interaction of the incident radar wave and the backscattered electromagnetic wave) satisfies the dispersion relation of ionacoustic waves over a large range of heights, the dispersion relation of the similarly arising ponde- romotive force in the case of Raman scattering only satisfies the dispersion relation of Langmuir waves over a very small range of heights. This range is only about 0.5 km near the flat peak of the F2 region electron density profile and otherwise much smaller. In the case of the Arecibo radar this is somewhat compensated by the higher spatial growth rate. An attempt will therefore be made to use the noise-like signal from a strong extraterrestrial radio source which interacts on its way down with the upgoing radar wave. The interaction should result in an enhancement of the spectrum received from the source in a narrow frequency band near the radar frequency minus the Langmuir wave frequency at the F2 peak, and in a depression of the spectrum near the radar frequency plus the Langmuir wave frequency.

The spectral modulation is expected to be only about 1

o/d

at Arecibo both for Brillouin and Raman scattering and their detection will need far more integration time than at Jicamarca.

12. Conclusions. - High power VHF and U H F incoherent scatter radars are expected to excite detectable but weak stimulated Brillouin and Raman scattering. Preliminary data from Jicamarca indicate an experimental confirmation of the former.

High power H F transmitters excite a much greater variety of parametric instabilities and the excitation is strong. In addition to thc experiments at Platteville and at Arecibo there has been considerable experimental and theoretical work in the USSR [39, 401. The present brief review does not even claim to give a complete account of the U. S. work in this field and no attempt has been made to describe the very important work proceeding partly along parallel lines in the USSR ; a more detailed review covering all the work on ionospheric heating is definitely needed.

The only parametric instabilities excited by heating waves of both magnetoionic modes are focusing instabilities which can also be regarded as collisionally coupled stimulated Brillouin scattering. Large scale (hundreds of meters) fieldaligned irregularities result from this instability. They cause scintillation of satellite signals and spread-F.

H F transmissions with ordinary polarization addi- tionally strongly excite the parametric decay instability and this in turn leads to the formation of short (meters) scale fieldaligned striations and to an acceleration of electrons to energies of several electron volts and thus to the excitation of 6 300

A

airglow. The striations scatter U H F and V H F electromagnetic probing waves (into elcctrornagnetic waves) and make voice transmissions between the California and Texas coasts via the HF-generated striations over Platteville possible. The striations also scatter H F electromagnetic waves into Langmuir waves and thus cause the phenomenon of wide band absorption of H F waves. There seems to be experimental evidence of ducted propagation of parametrically excited Langmuir waves. This should really have been predicted theo- retically because small scale striations were known to exist and are capable of ducting Langmuir waves. The existence of ducting in turn requires a modification of existing saturation theories of the parametric decay instability.

The purpose of this review was to demonstrate that many experiments in non-linear plasma physics, involving parametric instabilities, are possible in the ionosphere. The heating experiment planned by the Max-Planck-Institut fiir Aeronomic in Tromsar [41]

will be capable of producing more than twice as high an ionospheric power density as the Platteville heating installation. In addition to the EISCAT radar many other cooperative observations are planned including probably rocket flights through the heated region. This new installation will be able to make direct measurements of the saturation spectrum of Langmuir waves and will thus be the first real test of the saturation theories. It is hoped that the installation will further contribute to many aspects of non-linear plasma physics.

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using VLF waves. The review was also restricted to electrojet current by modulating the heating transmitter transmissions from the ground and thus ignored now being constructed at Tromsar thus modulating very important experiments that are possible from a the conductivity of the ionospheric D region. This

space craft. modulated current will then act as a VLF transmitter.

The effects of ionospheric heating are not restricted In spite of these omissions it is hoped that the to parametric instabilities. A very interesting planned great interest of ionospheric heating experiments to

application [42] is the modulation of the auroral plasmaphysics has beendemonstrated.

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[4] FEJER, J. A., Phil. Trons. R. Soc. Lond. A. 280 (1975) 151.

[5] DYSTHF, K. B., LEER, E., TRULSEN, J. and STENFLO, L.,

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[6] FEJEK, J. A., Geophys. Res. Lett. accepted for publication. [7] KAW, P. K. and DAWSON, J. M., Phys. Flrtids 12 (1969)

2586.

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