Self-action effects associated with the generation of plasma irregularities during ionospheric modification experiments

Self-action effects associated with the generation of plasma irregularities during ionospheric modification experiments T. R. ROBINSOS Physics Department, University of Leicester. Leicester LEl 7RH, U.K.

Abstract-Five

papers presented by various authors at the URSI Symposium on Actirje (Florence, August 1984) arc previewed. New results from the heating facilities at both Arecibo and Ramfjordmoen are reported. The observations include: (a) heater generated intermediate scale plasma irregularities diagnosed by CHF radio star scintillations, UHF radar incoherent scatter and HF signal fading techniques; (b) UHF plasma line enhancements; (c) anomalous absorption of both heater and HF diagnostic waves. These experimental results are discussed in terms of various self-action processes, such as self-focussing and pump depletion. The results of some recent theoretical investigations into anomalous absorption and reflection are also presented. Experiments

poster

in Space

Plasmas

1. PAPERS PREVIEWED 1. Simultaneous measurements of radio-star scintillations and plasma line intensity fluctuations during ionospheric modification at Arecibo.

terms of the various non-linear feedback processes, both positive and negative, which occur during ionospheric heating. Positive feedback mechanisms drive parametric instabilities of various types (FEIER,

SANT~MAY BASU, SUNANDU BASU, M. P. SULZER

1979). Negative

and H. C. CARLSON.

power thresholds

2. Radio wave self-action effects in the ionosphere. L. M. DUNCAN, J. P. SHEER~Nand W. E. GORDON. 3. HF produced electron density irregularities in the polar ionosphere; diagnosed by UHF radio star scintillations. A. FREY, P. STUBBE and H. KOPKA. 4. An experimental investigation of ionospheric irregularities and pump wave self-action during ionospheric heating. T. B. JONES, T. ROBINSON. A. WILKINSON, P. STUBBE and H. KOPKA. 5. Anomalous absorption and reflection in ionospheric radio modification experiments. EINAR MJOLHLX

2. INTRODLCI.ION

The modification of the ionosphere by means of large amplitude radio waves (heating) is essentially a non-linear phenomenon. This non-linearity takes several forms: (a) different plasma wave modes become coupled; (b) the medium through which large amplitude waves propagate is strongly modified; (c) the heater wave field (pump) which initiates these non-linear effects is itself modified as a consequence of the excitation of new modes and the modification of the medium of propagation. Effect (c) is termed self-action. Pump self-action can be understood in

feedback

mechanisms

lead to pump

in the initial stages of an instability and also bring about its final stabilisation. An important feature of the pump self-action processes which occur during ionospheric heating is the generation of field-aligned plasma irregularities or filaments on a wide range of spatial scales (UTLAUT and VIOLETTE, 1974; MINKOFF et al., 1974). Irregularity

scale sizes are determined

wavelength

of the plasma waves involved

largely by the

in the nonlinear interaction process. The generation of small scale irregularities, for example, with scale lengths of a few metres across the geomagnetic field occurs whenever heater wave energy is able to scatter into electrostatic waves. Intermediate to large scale irregularities, on the other hand. with scale lengths var!~ng between a few hundred metres to several kilometres across the geomagnetic field occur where the heater wave scatters into other electromagnetic waves. During overdense heating (pump frequency > F-region critical frequency) both scattering processes occur. However, during underdense heating (pump frequency < F-region critical frequency) neither electrostatic waves nor small scale irregularities are produced. During the past decade a number of different theories involving parametric instabilities have been proposed to account for the waves and irregularities

1246

T R. R~BINWN

sample of 430 MH7 signals from the radio source 3C166 recorded on 31 January 1984 during a period of continuous overdense heating. when a mean heater power of 30 PW m-’ at 250 km was employed. Two effects contribute to the observed temporal variation in the UHF signal intensity: (i) the scanning speed (east to west) of the UHF radar beam through the irregular plasma density within the heated volume; (ii) the motion of the irregularities as they drift with the background ionosphere. The S, scintillation index computed from these data is 0.03. The average fading period is about 19.5 s. The dominant fading period is identified with the Fresnel scale, 1, (SINGLETON, 1970). BASU et al. estimate that A,=590 m for the irregularities which cause the scintillations in Fig. ?. BASU et al. also obtained evidence of the inhomogeneous structure of the HF pump wave field by observing plasma line echoes and scintillations simultaneously. Figure 3 illustrates the temporai variation of the enhanced plasma line Intensity in the same time interval as in Fig. 2. The plasma line corresponds to the decay mode at a frequency shifted from 430 MHz by & -f,j, where fH is the heater frequency and f, is the frequency of ion acoustic waves which take part in the parametric decay instability. The temporal variations in intensity in Fig. 3 are again due to the radar scanning through drifting spatial inhomogeneities. In this case, however, the intensity modulations are interpreted as the inhomogeneity of pump wave field due to focussing and defocussing in the presence of plasma filaments. Where the plasma line is intense the pump electric field strength exceeds the threshold of the parametric decay instability. Elsewhere the pump field is too weak to strongly excite the instability. The average plasma line fading period in Fig. 3 is 42 s. which 3. EXPERIMENTAL RESULTS corresponds to a spatial scale of 1.5 km. It will be noted that intense fluctuations are only observed between 41 s and 327 s. BASC’ 6’1 al. conclude that the apparent differences in the spatial structure of the scintillation and plasma line effects are due to a Four of the five papers provide independent number of causes. These m&de: (1) the localisation experimental evidence for the occurrence of interin space of the pump field due to focussing; (ii) the mediate scale plasma filamentation in the F-region tendency for the small scale scatterers which produce due to the self-focussing of powerful HF: radio waves. the plasma line to decay much more rapidly than the BASI: CI al. (Arecibo) and FREY rr ol. (Ramfjordmoen) which cause radio star employ radio star scintiliation techniques. DC:~‘CAN large scale irregularities scintillations as they are convected out of the source PI al. (Arecibo) employ incoherent scatter techniques region; (iii) Fresnel filtering effects which involve and Jests (lt ~11.(Ramfjordmoen) observe the fading only the scintillation measurements. This latter of an HF diagnostic signal. Figure 1 illustrates the possibility is evidenced by the fact that low ampligeometry employed by BASU et al. for the 430 MHz tude structure correspoadmg to 1.5 Am scales was radar observations at Arecibo. Figure 2 shows one

produced during ionospheric heating experiments (for reviews see GUREVICH, 1978; FUER, 1979; STUBRE and KOPKA, 1980). Mechanisms which involve only electromagnetic modes are commonly termed self-focussing or filamentation instabilities (LITVAK. 1968; PERKINS and VALEO, 1974; Kuo and SCHMIDT, 1983). A variety of instability mechanisms involving electrostatic modes have been suggested. These include the resonance instability (VASKOV and GLJREVICH, 1975) and the oscillating two stream instabilities (DAS and FWER, 1979; INHESTER er al., 1981; DYSTHE et al., 1983). The parametric decay instability (SILIN. 1965; PERKINS and FLICK, 1971) is also commonly invoked to explain the enhancement of electrostatic wave amplitudes which are observed as enhanced plasma lines in radar backscatter spectra (MINKOFF er al., 1974). It is generally accepted, however, that plasma filamentation of all scales arise, because of thermal conduction along the geomagnetic field (STUBBEand KOPKA, 1980). It is the purpose of the summary paper to highlight some recent observations and theoretical work concerning the self-action effects of powerful radio waves which are associated with the excitation of ionospheric plasma irregularities. The material described in the following sections forms part of a series of poster presentations by a number of different authors at the URSI Symposium of Actit)e kperimews in Spucr Plasmas in Florence during August 1984. A number of different experiments involving the HF heating facilities at both Arecibo, Puerto Rico. and at Ramfjordmoen, Norway, are included. It is to be hoped that the juxtaposition of data and ideas from diverse sources can shed new light on a number of related phenomena which occur when a powerful radio beam acts on the ionospheric plasma.

Self-action

effects associated

18 9

with the generation

!

250 40

of plasma

km Xb~msphsric km CEW) x 80

km

irregularities

1247

path iNSI

188

167. _enO_fZELenrS.-----

4

J 671

670

669

668

667

66.6

664

66 3

662

f lW 1

tcqitods IiHF heater AI0 - Aroc~bo

665

locatim

rawspheric obsnvotwy

Enhanced

eZm-

-

430

0

-

Absense

~losma

observed

the

MHZ scrotlilatlonr of ~Iosma

observed tine OT scintiilotrww

Fig. I. Subionospheric tracks of radio sources through the heated volume on 26 and ii January 1984 above Arecibo. The cross-hatched areas indicate regions of HF enhanced plasma line echoes, whereas the solid blocks indicate regions of 430 hlHz scintillations. The elhprxal curve represents the heater 6 dB contottr. (From BASC er tri I XM6 43ouh

:OS

31 Jan 84

AlBCibO

22:o

22:01:09

AST

Fig. 2. A sample of GO MHz radio star scintillations

during

o\,crderlse

heating

at 4reclho.

(HASI

cq Q/.)

I248

T. R. RO~IINSOS

i

&ST Frp. 3. The UF enhanced

plasma

line

echo

corresponding

found in the scintillation spectra. The scintillations spectral slopes were found to be steep, in agreement with previous measurements reported by BASU et af. (1982) who also discuss the implications of such steep spectral roil-offs. BASU er ui. also report a number of new observatrons of underdense heating. On these occasions the dominant period of the scintillations observed was somewhat longer than that during overdense heating (see Fig. 41 Characteristic scale sizes of 3 km are reported during underdense conditions. Significantly. the enhanced plasma line entirely disappeared during underdense heatmg. This is to be expected if the parametric decay instability is responsible for the plasma line enhancement. The larger scale filamentatron associated with underdense heating is probably due to a rise in the threshold power required to produce the smaller scale irregularities under these conditions (for further details, see BASU et ul., 1964). FREY and DLWCAN (1984) have recently reported observations similar to those of BASU er al. DWCAN ev ul have also obtained convincing evidence that self-focussing occurs during heating at Arecibo. These atithors utilize both plasma line and ion line incoherent scatter measurements to demonstrate the occurrence of inhomogeneities in the plasma density. plasma temperature and pump wave field. The experimental geometry employed by DUNCAN el cd. 13 similar to that of BAstJ P’I(11.(Fig 1j.

to tire scmtlliatconc

in Fiy.

3.

(BMI (V‘ii )

However, unlike RAS~I et (II.. DCINC’AS et ul. do not employ beam scanning, Consequently. temporal variations in the data produced by DUNCAN YI al. art’ entirely due to natural ionospheric drifting. Frgurs 5a ilustrates variations in enhanced piasma hne intensities obtained by D\:NC.AN CJ((11.Thcsc data at-r‘ consistent with pump wave field inhomogenerties with horizontal scales of about a kilometre and arc similar to the observations of BASIL P; rrl. (Fig. 3). Utilizing the high range resolution (- IO0 m) obtsinable with the Arecibo radar, IXrNcAh~ cr (11.additmnalfy obtained measurements of the altitude variations of the enhanced plasma line (Fig. 5b). These data arc consistent with the existence of piasma density inhomo~ene~ti~s which cause the height of frequency ma:ching between the pump and the plasma waves to vary. DUNCAN et ul. point otit that although the data in Fig. 5a, b were obtained simultaneously, contrary to expectations they do not exhibit entirely the same structure. During a separate experiment al Arecibo, ih=~i(.~~ e: nl. obtained important evidence of heater induced filamentation in both plasma density and temperature structure. Figure 63 illustrates a series of incoherent scatter radar po\\er profiles taken at S s intervals in the heaght range 150-220 km during u period when the heater was operating. Inhomogeneities in the power profile due to plasma density filamentation are clearI> visible. Figure hb depicts

Self-action

effects associated

I

23

with the generation

I

of plasma

i

I

I

irregutaritres

I

2O:f9:23

2o:lt:23

20:15:23

1249

AST Fig. 4. As Fig. 2 but during

undcrdense

heating.

3.0

(b) 25-

Fig. 5. (al Variations in the HF enhanced plasma line echo obtained b! D~IXC-AS PI cd. (h) Variations in attitude of the HF enhancsd plasma line (arbitrary zero) corresponding to Fig. 5,. (DWCAN et rrl.)

(B~sti (‘r ai.)

’

I

I

I T./T,

I

I

FOR W-WI PROFM

202540 - 202-a

1250

T. R.

the corresponding (temporal) variations in the ratio of the electron to the ion temperature, T,/T,, derived from the incoherent scatter data. The ion temperature is unlikely to be affected significantly by the heater. Consequently. these data indicate inhomogeneitles in 7, consistent with self-focusslng. It must be acknowledged, however, that the evidence presented by DWCAN et al.for similar irregularity structure in borh .V, and r,. is not entirely conclusive, since only the vertical structure in N, and the horizontal (drift) structure in T, have been determined. However. the 45” dip angle at Arecibo does allow the vertical and horizontal structures to be compared in a reasonably unambiguous manner. DUNCAN ef al. have estimated that the spatial scales of the inhomogenelties in r,. al-e approximately 3 km. The Arecibo observations indicate clearly that selffocussing effects occur during heating at low geomagnetic latitudes. The first evidence of selffocussing at a high latitude site has been obtained only recently by FREV et d. using the high power facility at Ramfjordmoen. Norway. The experimental geometry employed by FRE\I et al. for radio star scintillation observations at 933 MHz is illustrated m Fig. 7. It was possible to tilt the heater beam to within a few degrees of the local magnetic field direction at Ramfjordmoen (dip=77.5 ). The line of sight to the radio source (Cass-A in this case) was also always withln a few degrees of the geomagnetic field direction. This is to be contrasted with the much more oblique angle (-45’) between the geomagnetic field and both the heater and radar beams in the

ROMINSON observations of BAX or al. and D~UCAU ef ul. at Arecibo. Figure 8 illustrates radio star scintillation observations from IO May 1983 during periods of overdense heating when the high pohcr transmitter was operated in the following 40 minute cycle: 4 min at 30 y/, fuullpower; 6 min off; 4 min at 60 Y,,:6 mln off; 4 min at 100%; 6 min off; 4 mln at 60:,; 6 min off. ‘Full power‘ in this case represents a power density of 320 VW mm2 at 250 km, which is considerably higher than that available at Arecibo. The actual powers during heater on periods indicated in Fig. 8 have been corrected for spatial RF power variations within the heater beam pattern. The S, scintillation index calculated during each heater on period is also indicated. It is apparent that from 6.10 to 6.20 UT an HF power level of 40 /IW m * is sufficient to produce noticeable Intensity scintillations at 933 MHz. FREY rf al. also report a second experiment performed on 9 September 1983 employing a different radio star (3C295) which produced similar results. During the experiment on 9 September an HF power density threshold of 22 LLWmm2 for the excitation of scintillations was observed. From considerations of Fresnel filtering effects FRLS CI al. estimate the upper limit of the scale size of the irregularities causing the 933 MHz scintillations to be 700 m. These authors also make two important observations concerning the power dependence of their scintillation results. Firstly, that there is a general increase in the S, scintillation index with heater power. Secondly. that there is a tendency for intensity fluctuation to be faster at lower heater powers. FREY rt (I/. conclude that shorter scale irregularities are produced at lower HF power densities and are not the product of cascading from larger scale irregularities. The HF power density thresholds observed for these shorter scale irregularities arc at least two order5 of magnitude too low to be consistent with Carl) theories. such as that of C‘RAGINc't al.(1977). However, the observations are consistent with the filamentat~on instability of Kuo and Ltt; (19X3) (see also Lrto and SCI-~MIDT.19X3) who predict HF power density thresholds of less than 1 /tW m ~’ for scale sizes larger than 150 m. FREY TV ui. also report that gro\vth and decay times of plasma irregularities are highly variable with average growth times of IO 40 s and deca) times of I ~3 min (for further details of this work, see FRLY (‘I [I/.. 10x4). The observations of Jo\r:s 01 trl. also provide evidence for plasma filamentation during high latetude heating experiments. However, these authors employ HF diagnostic techniques which cannot be

Seff-action elfects associated with the generanon of plasma irregularities TAOMSO

10

MAY 1903

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6:30

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interpreted in such a straightforward manner as the IJHF measurements, because of refraction effects. It is appropriate therfore to deal with these and other HF results in the following section. Tromso

o :49

The experimental geometry employed in the HF measurements performed by JCI~ES VI a/. at Ramfjordmoen during September 3983 is illustrated in Fig. 9. A 30 W diagnostic transmitter is located some 50 km north of the high power facility and the diagnostic signals were received at a site approAimately 40 km south of the heater. This separation 01 transmitter and receiver ensures that the diagnostic

// 1

MHz1

/

0

IO

20

Sauyh stte

C 5 2 MHz-Rx

I

T. R. ROBIXSON

1252

heating cycles are displayed in Figs IO and 11, respectively, The time resolution of the data is 0.1 s averaged over I s. These data exhibit three important features. (a)The diagnostic signal amplitude (upper panel of Figs. 10 and i 1) begins to decal as soon as the heater is switched on. It continues to decay, finally reaching a level approximately IO-15 dB below its original amplitude during the heater on period. The diagnostic signal strength begins to recover as soon as the heater turns off. This behaviour is attributed to anomalous absorption due to heater excited small scale plasma irregularities. The decay rime appears to decrease with increasing heater power, as expected from theory (DAS and FEIER. 1979). On the other hand, the anomalous absorption level does not appear to depend strongly on heater power. This would indicate that even the lowest heater powers

signal avoids the heated D-region. A diagnostic frequency of approximately 200-300 kHz above the heater frequency ensures that the diagnostic wave traverses the heated volume and has a reflection point dose to the centre of the heater beam. In addition to the low power diagnostic signal, the ionospherically reflected heater wave was also monitored. A single 20 min heating cycle comprised nine consectuive 90 s periods of heater on separated by 30 s off. with a final off period of 210 s. Within each 90 s on period the effective radiated power (ERP) of the heater was varied according to ERP=(Pj2) x FP for 30 s. P x FP for 30 s, then (P/2) x FP for 30 s. FP represents full power (FP=260 MW. which is equivalent to a power density of 320 PW m-‘). P was varied in consecutive 90 s on periods according to P=O.O5, 0.1, 0.15, 0.2, 0.3, 0.45, 0.6. 0.S and 1.0, respectively. The results of two consecutive 20 min

sEPT.5,1983 HEATER 4.913MHz 0-MDDE ERP=1260.P MWI

1010

1002

1Oi'OIUTl

HEATER POWER CYCLE

P=O& 01

015

02

03

045

06

08

10

Self-action

effects associated with the generation

of plasma

irregularities

1253

SEPT.!i,l983 HEATER 4.913MHz 0-t4XlE ERP426O.P MWI

I-EATER POWER CYCLE

-A

Pd.05 01 Fig.

0.15 0 2

0.3

0.45 0.6

08

1.0

I I. As Fig. IO but from 1021 to 1041 UT.

employed (- 5 PW me2) exceed the required threshold for the excitation of small scale irregularities. (b) The ionosphericall) reflected heater signal (centre panel of Figs. 10 and 11) reaches a peak immediately after heater switch on (i.e. within 0.1 s). Shortly after switch on the reflected heater signal has decayed typically b> 20 dB. This decay time appears to be power dependent and is comparable with the diagnostic decay time m (a). This pump wave overshoot effect is attributed to the anomalous self absorption of the heater wave due to the small scale irregularities, which also cause diagnostic anomalous absorption. The occurrence of pump overshoot each time the pump is turned on is evidence that the small scale irregularities have completely disappeared during the 30 s heater off periods. JONES et al. suggest that this pump depletion effect may well be the stabilising mechanism which terminates irregularity growth. This matter will be dealt with

further in Section 4. (c) When the power exceeds approximately 50 ~IW m- ’ (-0.2 x FP) a sudden increase in the fading rate of both the diagnostic and heater signals in each of the heating cycles (Figs. 10 and 11) is apparent. HF signal fading can be attributed to the presence of plasma irregularities with scale lengths larger than the wavelengths of the radio waves in the vicinity of the radio wave reflection points. It is important to note that the power threshold for the onset of signal fading is well above that for the onset of anomalous absorption effects. Further, the fading in the diagnostic signal persists for almost 2 min after the heater switch off at 10.20 UT. Both the power thresholds for the onset of fading and the long persistence time of the diagnostic amplitude fading after pump switchoff are consistent with the self-focussing irregularities observed by FREY et al. The results above provide important new evidence

T R. RORINSON

1254 for

the creation

both

small

during

of plasma

and

heating

The

reported

that a number

phenomena, excited

which

during

section

recent

of

simultaneously

in this section

7nvolve

non-linear

pump

ionospheric

limit

ment.

STUBBE et al. (1982a)

heating.

In

anomalous

of 0.25.

clearly

However, with

MJOLHUS. that

and

that

are

the

next

these this

saturation

may

absorption

levels of heater

heater

argues

MJOLH~‘S

calculat7ons scale

that

of anomalous

several

irregularities

are not ent7rely

self-consistent

by a

when electroVkstiov

irregularit7es pu7np

absorption

experiments

electromagnetic

these

on

which

relation

new

MJOLHUS

he

heater saturation

A=e-“(I

of pump

relative

relative

scale

energy

(7‘).

-em”): where

ionospheric

scale

to the refracttve the

of the

background

that when applied could

irregularities.

explam

to a the

MJOLHLX’S

with ti77/77o.imply7ng

grow, a decreasing

is available

of the

amphtude to

that R increases

MJOLHUS’S results

irregularity

transmission

is the

L

as the irregularities

growth.

to Budden

(A) coefficients

calculation

of small

model indicates

anoma-

to anomalous

different

typical

MJOLHCS argues this

that

for

amount

maintaining

also imply

that

that A has a71

which

indicate

utilise in

that plasma

of spat7al scales

are

as

to

the

sizes

pump thresholds Both

addition.

at

diagnostic

further heating

agreement

that

occur

occurs is good

FREY

for the self-

et al. observe

irregularities

than longer

the results and

of JONES er o/.

anomalous

simultaneoulsy.

within

a few seconds

Their

of switch

mean that the pump thresholds well

below

without

taking

It is clear, quench

the

value

account

however,

of

absorp-

observation

the self-focussing

time is much

longer

pump atnplitude

well

for self-focussing

are

40 PW m-7

than

pump

estimated

absorption depletion

instability

whose

the time constant

decay which

absorp-

on may

of pump

that

are

scale self-focussing

that the pump wave suffers severe anomalous tion

HF

and those of FREY et

self-focussing

In addition.

self-

of

There

thresholds

Further.

self-focussing

that

pump

fading

:.clf-focussing

these results

irregularities.

of

JOKES er ctl. also

latitudes.

indicate

tion effects

by

evidence

scale

In

FKFY YI al.

of

The

the pump power

shorter

crl. have

of both the

new evidence

heat7ng.

at

high

e7

field in the F-

evidence

observed

more easily produced

the

( -few km) irregularities

first

instability.

the

at low latitudes.

provided

latitudes.

between

focussing that

high

signals

provides during

wave

underdense

the

of and

nature

heating

of large scale

during

focussing

constants

DUNCAN

and the pump

produced

There

the observers

irregularities

BASL cf al. have

at Arecibo

of self-

to exc7te them.

overderlse

the generation have

time

et ul. and

during

the

self-depletion.

the inhomogeneous

plasma

region

as

between

and

required

BASC

heated

and pump

self-focussing

demonstrated

such

of agreement

scale

intermediate

role 7n a number

phenomena.

instabiltty

al. regarding

-em’)‘; Y,,

&7/77” ts the

wave

causes

are analogous

several

irregularities

the

near the upper

demonstrates

N, the first order correction and

from

in that new

I ).

obtains

R=(l

T=e-“;

ionosphere.

that,

then

(R) and absorption

a=2(7rw/c)L@n/r1,1

dispersion

differs relation

in addition

effects

discusses

and

reflection

index

relation

(BUDDEX. 196

an averagIng

waves (HAGFORS. 1984).

&tJOLHCJS

These

tunnelling

been

MJOLHUS has taken

in a modified

wave reflection

absorption.

of

calcu-

recently

of small scale irregularities

lous radio

plasma

accurate

have

AppletonHartree

frequency.

models

More

and cut offs are introduced

the effect

during

waves at the

by utilising

results

dispersion

resonances hybrid

diagnostic

theory

for electromagnetic

well known

height,

point.

this

a step further

technique

form:

HF

by JONES ~7 ul. (1984).

ideas

This

of

that

was due to mode conversion

resonance

based

developed

(independently)

waves into electrostatic

hybrid

4

have been presented

play an important

is large measure

suggested

phenomena

techniques

self-action

focussing

previously

upper

irregularities.

with a wide range

have

lations

induced

to

the observed

excited by powerful HF raio waves in the ionosphere under a variety of circumstances, e.g. at high and low latitudes, during under- and overdense heating. These

and GUREVICH (1976) and GRAHAM and FEJER (1376)

heating

for

paper. The results

irregularities due

excited

wave energy losses are substantial.

anomalous

bound

account

of heating

diagnostic

this summary

PI-PIOUS

absorption

plasma

magnetic

in agreement

5. COM1MENTS A\D C0UCI.I SION

paper

small

suggest,

IS an upper

effects

.ANOMALOL:S .ABSORPTION DL RING HEATING theoretical

well

for

of A in excess of 0.9.

also

there

by experi-

JONES er al.,

aspects

4. THEORETICAL CONSIDERATIONS CONCERNI\C;

to

and

values

authors

Observat7ons

his

is contradIcted

have observed

will be discussed.

In

This

plasma

self-action.

work on important

involving

upper

example,

of different

theoretical

of self-action

irregularities

scales

at high latitudes.

observations

indicate

density

intermediate

occurs

shortly

effects. does

not

growth for the after the

Self-action

effects

associated with the generation of plasma irregularities

pump is turned on. FREY rt al. also observe that there is no tendency for saturation of the self-focussing instability as the pump power is increased. In addition to the experimental observations a theoretical investigation of described above, anomalous absorption and reflection during heating experiments by MJOLHUS was outlined. This theory predicts an upper limit for anomalous absorption and considerable pump reflection at the upper hybrid resonance height. The predictions do not appear to be in quantitative agreement with the experiment, though they may still agree qualitatively. Both MJOLHUS and JONES et (I/. suggest that pump self-

1255

action may be responsible for the amplitude saturation of small scale irregularities produced during heating. In conclusion. the wealth of new data now available from heating experiments typified by the examples in this paper are providing improved insights into the physics of the complicated relationships between the various plasma processes which are excited by powerful radio waves in the ionosphere. There now exists a number of theories which can account for certain isolated phenomena. A comprehensive theory which can explain the whole spectrum of heating observations is still awaited.

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