Enhanced energetic electron intensities at 100 km altitude and a whistler propagating through the plasmasphere

ENHANCED ENERGETIC ELECTRUN I~~NSI~ES AT fOOkm ALTITUDE AND A WHISTLER PROPAGATING THROUGH THE PLASMASPHERE Deparfmenf

M. J. RYCROm of Physics, The University, Southampton,

England

~~~~~~v~d21 Augmf 1972) Abstract-During the flight of a PetreI rocket, instrumented by the SK Radio and Space Research Station with Geiger counters and launched westwards from South Uisf, Outer Hebrides, Scotland (L = 3*38), a transient increase was observed in the intensify of energetic electrons having pitch angles between 60 and 120”. The increase, by a factor of 20 above the quasi-steady intensity observed f~~houf the remainder of the flight, occurred in O-8 set and was simultaneous for both >4f keV and >I10 keV eIe&ons. Recorded --05 set later, on fhe ground, was a two-hop whistIer. During the enhanced electron intensity event, the entire duration of which was ~6 set, the four-, six- and eight-hop whistlers were also received. From an analysis of the whistlers’ spectrogram, it is concluded that the whistlers were ducfed through fbe magnetosphere: along the L = 3.3 f 0.1 field line; the electron density in the equatorial plane is found to be 330 f 10 cm-$, a value characteristic of conditions within the pIasmapause. If is suggested that fhese temporally andjor spatialiy associated phenomena, rafher than arising by a chance coincidence, were the resuh of a ~u~onanf &era&on between energetic electrons and whistIer mode waves moving in opposite directions. For gyroresonance on this field line at the equator, the parallel component of energy of the electrons is 25 keV at 3 kHz in the whistler band, or 100 keV at 1 kHz below it. If is suggested that a magnetospheric event occurred, causing both sudden enhanced electron precipitation and favourable conditions for fhe propagation and/or amplification of whistlers. A possible explanation is that energetic eiecfrons, having a suBciently anisotropic distribution function and associated with those injected during an earlier auroral subsform, become unstable via the fransverse resonance in&ability when they drift into fhe plasmasphere, a region of high density thermal plasma. 1. INTRODUCTION The

gyroresonant interaction between whistler mode radiation and van Allen belt electrons moving in opposite directions along a line of force of the geomagnetic field has been discussed by Dungey,“*el CornwaW~ and &ice. w Dungey’f*8’ has estimated that, for each interaction, with a whistler of amplitude 4.1 nT in the magnetosphere’@ at 3 c L < 4, the electrons’ pitch angle will change by < 0.5”. A gyroresonance instability of the plasma, in which VLF whistler mode emissions grow in energy by taking a very small proportion of that of the electrons, leads to almost pure pitch angle diffusion of the electrons, weak ~~u~on~~~~ This results in the pre~ipi~tion of electrons into the atmosphere, and the ~ab~shment of an anisotropic pitch angle ~s~bu~on that is unstable to further growth of the whistler mode radiation. Consideration of this Ioss-cone instability in the presence of a partially reflecting ionosphere has led Kennel and Petschekcet to explain the observed upper limit to the flux of stably trapped energetic electrons. Further aspects of the transverse resonance instability of magnetospheric plasma have been reviewed recently by GendrW~ and ~y~roft~(g~ In tj$s paper the observation of the almost simultaneous occurrence of whistlers recorded at ground level and a transient increase in the intensity of energetic electrons observed aboard a rocket at an altitude of 100 km is reported. The association of these two phenomena is discussed, and interpreted in terms of a gyroresonant wave-particle interaction. 239

240

M. J. RYCROFT 2. OBSERYATIONS

Magnetic tape recordings of ~nteres~ng VLF radio phenomena, received using conventional equipments are made at the University of South~pton field station, a quiet site some IO km East of the rocket launc~ng range on South Uist, Outer Hebrides, Scotland (I, = 3.38). During the entire five minute flight of P46H, a Petrel rocket launched Westwards at 22.40:55 U T on 30 September 1969 (K,, = 3-), the only received signals that had propagated through the magnetospheric plasma were those whistlers whose spectrograms are shown in Fig, 1. These whistlers are rather weaker than often observed at this site. They are, however, positively identified by the usual methods(~) as being two-, four-, six- and eight-hop, single-path whistlers whose source is the atmospheric occurring at 22.42 :29.4 U.T., at a flight time of 94.4 sec.

I/

0

I[frct. 2. ECKERSLB’ PIXT OF I/d&S.

0.5

f,

set

t, WHERE t IS THE ONE-HOP WHISTLER PH3PAClATK?N TIME.

Shown in Fig, 2 is a composite Eckersley (l/evs_ t) plot. This is obtained by dividing the measured two-hop propagation time from this atmospheric at frequency f by 2, the measured four-hop time by 4, and so on, to obtain the best estimate of the one-hop propagation time t. The line through the origin is drawn by eye through the low frequency points. The reciprocal of the gradient of this line is the dispersion of the whistler; its value is 66.1 secl’s . The higher frequency points depart noticeably from this line, showing that the nose frequency@+) pert~~ng to the path of propagation is only somewhat greater than the highest frequency than can be measured, 6.5 kHz. The Petrel rocket P46H was instrumented with two Geiger counters by Drs D. A. Bryant and 0. M. Courtier, of the S.R.C. Radio and Space Research Station, Slough, U.K.; they most generously provided the data to be discussed. The axis of each Geiger

Causative atmospheric

I

-

I

0 94.4

I

2

I

I

I

?

4

5

set

FIG. 1. SPECTROGRAM OP VLF SIGNAL RECORDED ON SOUTH UIST AT 22.42:29.4 U.T. (t = 0) ON 30 SEPTEMBER 1969 SHOWING RATHER WEAK TWO- AND FOUR-HOP WHISTLERS CAUSED BY THE ATMOSPHERIC OCCURRING AT TIME t = 0. Associated

only with the 2-hop whistler are several fragmentary traces, one of which might he indicative of propagation beyond the plasmapause.

240

ENHANCED ENERGETIC ELECTRONS AND A WHISTLER

241

counter is perpendicular to the rocket spin axis, with an effective fwhm of & 10’. Electrons with pitch angles between 60 and 120” are sampled, that is precipitated, trapped and backscattered electrons are counted. The Geiger counters are basically identical with mica windows which allow a (>50 per cent) response to electrons of >45 keV. One counter has an extra aluminium window and thus responds to electrons of > 110 keV. The number of counts registered by an accumulating staircase counter by each counter each 6-25 msec is telemetred to ground. The counts are summed over one spin period of the rocket, 137 msec. Having subtracted off background counts due to cosmic rays and converted the counts into an electron intensity, an equafly spaced time series of electron intensities is obtained. For low energy (>45 keV) electrons, this is shown in Fig. 3. The

I

95

,

I

,

I

I

,

100

I

j;: 105

Flight time, sets Fra. 3. VARIATTONOP

>45 keV ELECTRONIN~XNSITYOBSERVEDDURINGTHB~~HT~~ P46H, TIMEBRING MJXASURED FROM LAUNCH AT 22.40:SSU.T. ON 30 k'TEMJ3ER 1969. Between 95.8 and 101*4sec, each point represents the average intensity per one rocket spin period, namely 0.137 set; outside this range, where the count rates are smaller, observations are averaged over two spin periods of the rocket in order to reduce the effects of statistical fluctuations. Each error bar is plotted to correspond to rfi the square root of the count accumulated in the internal since the previous reading. Tracings of the spectrogram of the received whistlers are also shown on the same time scale.

feature of this figure is the transient increase, at the time that the whistlers are observed, of electron intensity by a factor of up to and even >20 above the quasi-steady value of ~100 cm-* set-l ster-1 observed throughout the remainder of the flight. A very similar increase of the flux of high energy (> 110 keV) electrons is observed at the same time, to within a fraction of a second. Expressing the intensity I as a function of time t as dominant

I = I, exp (~,t), the rise time T, is found to be O-2 set for electrons recorded by either detector. Tbe fuI1 rise time of the increase is approximately 0.8 sec. The electron intensity reaches a maximum

242

M. J. RYCROFT

value for No.6 see, and then begins to fall. Assuming that, in this regime, I = I, exp (--7,f),

the fall time, T,, is l-3 set for electrons recorded by either detector. The entire duration of the event is 5 or 6 sec. In the flight time interval from 95 to 100 set, the average flux is an order of magnitude greater than typical fluxes observed during other five second intervals during the remainder of the flight. The time profile of the transient increase of the intensity of > 110 keV (1 lo-co kev) electrons has not, at any time, any apparent dissimilarities with that of 45-110 keV electrons, the latter being obtained by subtracting the intensities recorded in the high energy detector from those in the low energy detector. The intensities in these two energy ranges are essentially equal throughout the flight, both during the transient increase and afterwards. These facts are interpreted as showing that the e-folding energy of the electron spectrum is inde~ndent of time and inde~ndent of the intensity of electrons; its value is -90 keV. The electrons are thus seen to exhibit a rather hard spectrum. The number of counts during this transient event is not large; statistical fluctuations thus limit the amount of information that can be deduced. Attention has been focussed on an examination of the intensities of the electrons, rather than on their pitch angle distribution. However it should be mentioned do) that, whatever the electron flux, the observations are consistent with an essentially isotropic pitch angle dist~bu~on within the loss cone, that is for observed electrons with pitch angles between 60 and ~85”. The ratio of the intensities of upgoing electrons (with pitch angles between 90 and 180’) to downcoming ones (with pitch angles between 0 and 90”) is probably between O-1 and O*2.00) 3. FURTHER ANALYSIS

On the basis of the circumstantial evidence presented, it is reasonable to assume that the two phenomena considered are associated. That the association is only a chance coincidence remains a possibility. The probability of this cannot, however, be calculated since the a priori probability of occurrence of whistlers and of occurrence of transient electron intensities is not known. It is suggested that the probability of a chance coincidence is only small. It is possible that the observed variation that is interpreted as being a temporal transient increase is in fact due to the rocket moving through a collimated beam of energetic electrons. Since the longitudinal extent of the beam would then have to be only ~100 gyro-radii, and since the electrons drift longitudinally in the geomagnetic field, the probability of this being the correct explanation is not considered to be large. The association between the two phenomena is now considered further. First an analysis is carried out to determine parameters of the path of propagation of the whistlers. This is done using a method introduced by Dowden and Allcock and extended by Rycroft and Mathur.oal The method, involving the extrapolation of an empirically determined function off and t which varies linearly with f, enables the nose frequency and one-hop propagation time at the nose frequency to be computed. The values found, together with their standard errors, are 9.2 f O-1 kHz and O-88 f @02 set respectively. The L value of the geomagnetic field line along which the whistlers are ducted is thus found(12) (13)to be L = 3-3 & 0.1. The electron density, the density of thermal plasma, on this L shell in the magnetospheric equatorial plane is 330 f 10 cm 5.(13) Such a density is characteristic of conditions existing in the outer plasmasphere, that is just within the plasmapause.(r4) These two numerical

E~~~~

ENERGETIC

ELECTRONS AND A WHISTLER

243

results express concisely and usefully the maximum amount of ma~etosphe~c information that can, at present, be derived from the whistler, a pulse of radiation that is dispersed during its propagation through the magnetospheric plasma. Second the time series of electron intensities is studied in more detail. There is some evidence, from a visual examination of the observations as presented in Fig. 3, of a quasiperiodicity of the electron intensity with a period of ~5 intervals between data points, that is 5 rocket spin periods or 0.685 sec. To investigate this the autocorrelation functiorP of the time series is computed. Shown in Fig. 4(a) is the autocorrelation function of the +1

D z %

HIGH ENERGVPIIO

keVf

ELECTRONS

t 041

0

~EWRI~NED

HIGH

EN

042 1

LAG&x

ii! i: 6 z s? fj -l4 il

+1

PREWHITENEO

LOW ENERGY (r45

ktV)

PREWHITENED,

45-110 kcV ELECTRONS

% F 5 ii! 8 :: ii

0

0

(a) (b) (c) (d) The

FIG. 4. AUT~CXXMLA-IXONPUNCRON OF THE mm SERIESOF: HIGH ENERGY (> 110 keV) 3LBXRON INlXNSlTRX3; %I?-, HIGH ENERGY (>llOkev) ELEClXONSNTENSITIBS; ~~~, LOW BNRRUY (>45 keV) ELECTRONINTENSITIES; ~~~, WELL DRFINRDRN’SRGYW-110 kev) ELECTRONINTENS~?~. si~~~~ and interpretation of the mowed peaks are discussed in the text.

high energy (> 110 kev) electron intensities. Any semblance of a peak at 4 or 5 lags (spin periods) is masked by the tendency for the autocorrelation function to be large at a11lags, and at even much longer lags than shown. This effect is due to the characteristic rise and fall times of the transient increase, and to the fact that all values of the time series of electron intensities are positive. Such features are avoided by creating a new, ‘prewhitened series by subtracting the flux at each successive point from that at the previous point. The operation of prewhitening is thus equivalent to passing the waveform represented by the time series through a high pass filter; low frequency trends are thereby removed. The prewhitened series are shown in Fig. 5; a 4 spin period periodicity is apparent in the high energy series, as is a 5 spin period one in the low energy series. The autocorrelation functions of these prewhitened series are shown in Fig. 4(b) and (c); peaks at the foremention~ periods are apparent and arrowed. Figure 4(d) shows the autocorrelation function of the

244

M. J. RYCROFT

LOW ENERGY

(>&Sk&>

Fro. 5. THE PREWHITENED TIME SERIES OF THE (> 110keV) (UPPER PART), AND Low ENJJRGY

INTENSITIES OF ELECTRONS OF HIOH ENERQY (> 45 keV) (LOWBR PART), FORMED AS DISCUSSED IN THE TEXT.

prewhitened series of the intensity of electrons of well defined energy (45-l 10 keV), which is formed by subtracting the intensities of high energy from low energy electrons. The significance, or otherwise, of these peaks is now considered. The solid line joining the circled points in Fig. 6 is the same as that in Fig. 4(b). At a lag of 4 spin periods, the value of the autocorrelation function is 0+48. The error bar shown is f0*33, & the root mean square error of the autocorrelation function calculated as discussed by Bendat and Piersolo@ It is concluded that this peak is probably significant. Also shown are the autocorrelation functions of two other prewhitened series, + and -. The + series is constructed by taking a first point, as for example, the seventh point shown in Fig. 3, + its shown standard error as given by the square root of the number of counts multiplied by the appropriate counts to intensity conversion factor. The next point is taken - its standard error, the next +, and so on. This series is prewhitened, and its autocorrelation function is computed and plotted in Fig. 6 as a solid line through the + points. The - series is constructed by taking the first point - its standard error, and following a similar procedure. The autocorrelation function is plotted as a dashed line though the - points. Each of these two autocorrelation functions shows a 2 lag periodicity which is spuriously introduced by alternately taking + and - the standard error. It is, however, important to note that

ENHANCED

ENERGETIC

ELECMtONS

AND

PREWHITENED,

ENERGY f*llO

HIGH

A WHISTLER

245

keV)

Fro. 6. AUTOCORRBLATION FUNCTIONOF THE TIME SERIES OF PREWH~D, HIQH PNBRQY (> 110 keV) ELECTRON INTJ3NSITIJB BOTH AS SHOWN IN FIG. 4(b) (SOLIDLINEJOININO CIRCLED POINTS), AND INCORPOBATINO THE EFFECTS OF THE STATISTICAL FLUCTUATIONS OF THB MEASUREMENTS (SOLJD LINE JOINING + POINTS, AND DASHED LINE JOININQ - POINTS).

the 4 lag peak remains, and has not been destroyed by this process; this result lends credence to the view that the 4 lag (055 set) peak is probably a significant one. Returning to Fig. 5(c), at a lag of 5 spin periods (0.69 set), the value of the autocorrelation function is 0.38, with an r.m.s. error of fO*41. It is concluded that this peak is barely sig~fi~nt. In conclusion, it can be stated that the energetic electron intensities seem to exhibit quasi-periodicities, with periods of O-5 to O-7 sec. 4. DISCUSSION

Ruling out the chance coincidence of the tempora1 andfor spatial association between the whistlers and the enhanced electron intensities, two possible explanations exist. These are, firstly, that a wave-particle interaction took place and, secondly, that it did not. Of the first type, the interaction mechanism may be a gyroresonance one, as favoured by Brice,‘*) Kennel and Petschek,(Q Gendrin,“) Kaiserf6) and Rycroft(*) amongst others for the generation of VLF emissions at mid-latitudes (L - 4 f 2), or may be another one, which seems less likely. Of the second type, a ma~etosphe~c event may have occurred which caused favourable conditions for the reception of whistlers and which also-independently-caused a transient increase in the intensity of precipitating energetic electrons. Since, for a process of this second type, no further deductions can be made from the observations presented, attention is concentrated on a ~ro~sonant interaction. Salient features of a single gyroresonant interaction between a #-hop whistler and energetic electrons in-for simplicity-the equatorial plane of the magnetosphere are

246

M. J. RYCRCET

shown diagramatically in Fig. 7. The energetic electrons would arrive in the ionosphere above South Uist at a time 0*25t, after the interaction time, that is earlier than the observed 2-hop whistler by a time 15t - 0*25t , Here t, is the complete periodic bounce time for electrons twice mirrored at ~100 km altitude by the geomagnetic field. At L = 3.38, t, = 0.84 set for 50 keV electrons, and 0.64 set for 100 keV electrons.(16) Thus, for the observed electrons, 0.25tb is small, typically O-2 sec. Because the time of onset of the transient increase of electron intensity cannot be specified to better than f0.14 set [see Fig. 5(a) and (b)] and because its duration is 0.8 set, the interaction time can be deduced from the observations to be at 95.7 set only to within the regrettably Iarge error of &0*5 sec. XlUTHUIST:

ELECTRONS OBSERVED EARLIER THAN 2-HOP WHISTLER

GEOGRAPHIC

LWHISTLER

REFLECTED

FIG. 7. DIAGRAM ILLUSTRATING THE GYRORESOMNT INTERACTION BETWEEN EN~GETZ ELECTRONS ANQ A *-HOP WHISTLER IN THE EQUATORIAL PLANE OF THE MAGNETOSPHERE.

This time has to be compared with the time at which the i-hop whistler goes through the interaction region. Although occurring at a flight time of around 95 set, a more exact estimate of the time is dependent upon which frequency component of the whistler is in gyroresonance with the electrons. At the nose frequency of 9.2 kHz the time is 94.8 set, at 3 kHz 95.0 set, and at 1 kHz (below the observed frequency) it would be 95.4 sec. The observation that a whistler component is not observed at ~1 kHz could be due to little energy being radiated by the causative lightning discharge at this frequency, or to absorption at this frequency, which lies below the low frequency cut-off (~1.8 kHz, as evidenced by the ‘tweek’ characteristic of atmospherics presented in Fig. 1) for propagation in the Earthionosphere waveguide between the lightning flash and the point where VLF radio energy enters the ionosphere. It could also be due to whistler energy having been transferred via waveguide attenuation gyroresonance to energetic electrons, or to Earth-ionosphere between the point where the whistler emerges from the ionosphere and the VLF radio receiver. Returning to a consideration of the quasi-periodic nature of the observed intensities of energetic electrons, with periods of No.55 set at > 110 keV and No.69 set at >45 keV, it is noted that these times are the periodic bounce times of -140 keV and ~80 keV electrons respectively. It is proposed that the periodicities are tentatively identified as being due to the periodic bouncing of ~100 keV electrons between opposite ends of the L = 3.38

ENHANCED

ENERGETIC

ELECTRONS AND A WHISTLER

247

magnetic field line. Now an energetic electron mirroring at 100 km altitude above South Uist is precipitated into the atmosphere at the magnetic conjugate point where, at the same altitude, the geomagnetic field strength is significantly less. Thus, as the flux of backscattered electrons is only ~10 per cent of the incident flux, the periodicity is not due to the bouncing of a well-defined bunch of electrons. It is rather due to the bouncing of electrons whose pitch angles are being reduced by a scattering mechanism. One such mechanism is the loss-cone instability associated with whistler mode noise or microturbulence.(6) This mech~ism is, after all, presumed to be responsible for the quasi-steady intensity of ~100 cm-2 see-1 sterr of precipitating electrons of >45 keV observed throughout the flight. Related to this, it might be expected that echoes of the whistler itself would cause enhanced electron precipitation. Quasi-periodicities corresponding to the echo delays do not seem to be evident in the observations (Figs. 3 and 5). Since the entire event lasts only ~6 set and since the period would depend on th~unknown-resonantly interacting frequency component of the whistler, such perio~cities would be difficult to discover using statistical techniques. To recapitulate, a gyroresonant interaction between a whistler mode signal and >45 keV electrons bouncing along the L - 3.3 geomagnetic field line is proposed to explain the observations. The energetics of this interaction are briefly considered following Brice(4) and Rycroft.(a) The condition for ~roresonance, that the electron is acted on by a wave Doppler-sifted up to its gyrofrequency, relates the component of electron velocity parallel to the geomagnetic field to the phase velocity of the wave. This condition may be rearranged to relate the parallel component of the energy of electrons W,,, resonating with whistler mode radiation of frequencyfat a point where the electron gyrofrequency isfB,*, to the local density of thermal plasma N. Numerically, in the equatorial plane and for a dipole represen~tion of the geoma~etic fieid, and with II’;, in keV, N in cm-s and frequencies in kHz This relation is shown graphically, as a function of L-value, in Fig. 8 for different values of fin the VLF band. At 10 kHz, close to the whistler nose frequency of 9.2 kHz, W,,N = 570 keV cm-s at L = 3.38. At 3 kHz, WIIN = 8200 keV cm-e, and at 1 kHz 33000 keV cm-s. Now N has been derived from analysis of the dispersion of the whistler traces to be 330 f 10 cm-s at L = 3-3 rt 0.1. Inserting this value, WI, is found, for equatorial gyroresonance at 10 kHz, to be 2 keV; such electrons are not recorded by the Geiger counters. At 3 kHz, in the whistler band, W,, = 25 keV, and at 1 kHz, a frequency less than that observed in the whistler, WI1= 100 keV. However, gyroresonance with electrons of a particular energy also changes the characteristics of electrons of other energies. It is clear that electrons having energies comparable with those observed will be in gyroresonance with l-3 kHz whistIer mode radiation. Kennel and Petschek(6) show that the transverse resonance instability occurs only when the anisotropy A of the energetic electron distribution, defined by A = (7’,. - ?‘,,)/T,,, where TL and Tit are the electron temperatures perpendicular and parallel to the geomagnetic field res~tively, is equal to or greater ~~~~~~,~ -f). The critical value of the anisotropy{@, obtained using the equality condition, is plotted as a function of L-value in Fig. 9. At 10 kHz, A = 1.0, a rather large value. However, at 3 kHz A = 0~16, and at 1 kHz A = O-05; such values could well pertain to radiation belt electrons. All conditions for the existence of the gyroresonant whistler-energetic electron instability are seen to be met.

248

M. J. RYCROFT

lklfr

L - VALUE

Flrct.8. VARLATIONOF W/IN, NAMELYTHE PRODUCXOFTHB COMPONRNTOFELRCTRONBNBRGY PARALLSLMTHEGEOMAONBTICFIELD(IN keV) RRQUIRRDFORGYRORBSONANCEINTHEEQUATOR~AL PLANE OF THE MAGNJZOSPHERE WITH WHISTLER MODE RADIATION OF DIFFERENT FREQIIENCIES AND THE NUMBER DENSITYOF THERMAL PLASMA (IN Cl+) IN THE XNTRRACIION REGlON,AS A FUNCTION OF &VALuB. Use of this graph enabies, for example, Wjl for gyroresonance on a particular a certain frequency to be found if N is known.

field line at

The interaction may be initiated by: (a) the passage of a $-hop whistler, or (b) the critical value of the a~sotropy of the energetic electron distribution being exceeded, or (c) an external triggering event or happening, or (d) a combination of (a-c). An observational fact supporting hypothesis (a) is that the &hop whistler goes through the interaction region at a flight time of 95 sec. Energy in the 1-3 kHz band would be transferred to the elctrons. However, then their pitch angles would tend to be increased.@) Another noticeable fact refuting (a) is that during the rocket flight there are many atmospherics, with spectrograms comparable with that of the atmospheric occurring at 94.4 set (see Fig. I), which do not produce whistlers and do not trigger large intensities of precipitating electrons. During this flight, this event is the only enhancement of energetic electron intensities observed. Also during other flights taking place within a few hours of P46H, some not dissimilar transients are observed ;(l”) however, these are not accompanied by any whistlers or, indeed, by any other noticeable VLF phenomena. These features could be explained by the gyroresonance conditions not being met, for example if N is reduced, or by energy from lightning discharges propagating unducted through the magnetosphere.

ENHANCED

ENEZRGETIC ~L~~RONS

AND A WNISTLER

249

?kN%

RO. 9, ~ARXA-ITON OF THE ANl5OTROFY A OF ‘IX3 LXSWmON FWCTION OF BNBROmC BWCTRON5, WHICH MUST BB 3QUAIUD OR 3XC33LED 33FOR.3 THB WROR3SONANC3 lNSTA3W CAN OCCUR AT DW FR3QU3N-, AS A FUNCTrON OF h%U.m.

~b~~tioxal

factssu~~o~ng PD)are those which refute {a).

A rn~h~~

whereby

anisotropy of those eIectr5ns on the L = 3.3 field line at a particular longitude increases and reaches its critical value at a flight time of -95 see is required. The instability then occurs; electrons are put into the loss-cone and precipitated, As the whistler propagates along a pre-existing duct, it is amplified s~me~bat; since the four-hop whistler is weaker than the two-hop one, net ionosphere losses must exceed the magneto~he~~ gain. lt conld also be argned that the precipitating efectrons could create a duct, a condition favourable for whistler propagation. The time scale on which thermal plasma diffuses upwards is much too long for this to be a pos~bi~ity; however a “duct’of energetic secondary electrons might be formed within a few seconds. As an example of (c), the drifting of energetic electrons into the pl~mas~here at this time is suggested, To rear& this point in time and space, the electrons would follow drift paths similar to those proposed by ~~rlw~u~~~~and may be accelerated. They would have to have been injected near the midnight meridian at the time af an aurora1 substorm. As evidence of earlier substorm activity, Fig. 10 shows the H component of the geomagnetic field recorded at Lerwick, the nearest observatory to South Uist and some 200 km to the North. At 18.30 T-XT.,a 1000 y magnetic bay is recorded at Kiruna fL - 5*5), In faot, consi~rable magnetic activity before the laurmhing of P46H is associated with a sudden commexcement magnetic storm occurring at 04.53 U.T. on the previous day. KPvalues of 6 and 7 typify conditions from then until 15&OUT. on 30 September 1969, after which successive three-hourly KPvalues are 45,5+, 3 -. Therefore the time of the observations corresponds to the complex recovery phase of a magnetic storm, when the magnetosphere 7 the

250

M. J. RYCROFT Ii COMPONENT OF GEOMAGNETIC FIELD LERWICK

16

21

30 SEPTEMBER

00

03

06

UT.

1 OCTOBER 1969

FIQ.10. VARIATIONOPTHEHCOMPONENTOPTHEGEOMAGNETIC FIELDRECORDED AT LERWICK DUFUNG THE PERIODOP INTBREST.THERE ISNO GEOMAGNETICACTIVITYDURING THE FLIGHT OF P46H.

is known to be overpopulated by energetic electrons. When these drift through the plasmapause into the plasmasphere, a high electron density region, they become unstable by the transverse resonance instability.(8*1s) In conclusion it can be said that the most satisfactory explanation of the observations may be an example of(d), in which (c) and (b) play prominent roles. At the time (22.42:30 U.T. on 30 September 1969) and place (at the foot of the L = 3.38 field line) of the observations, the consequences of a gyroresonant interaction between ~100 keV energetic eleo trons and a )-hop whistler taking place in the magnetospheric equatorial plane are observed. The loss-cone instability occurs suddenly when energetic electrons appear in the plasmasphere, and grows with a characteristic time of O-2 sec. The intensities of precipitating The electron pitch angle distribution is that electrons are enhanced by a factor of -20. which, if it were changing by a diffusion process, would be expected if strong pitch angle diffusion were taking place rather than weak diffusion; alternatively, it could be indicative of a changing source of magnetospheric electrons. The $-hop whistler is amplified sufficiently to enable the reception of the two-, four-, six- and eight-hop whistlers. Further experiments, with observations made with fine temporal resolution, are desirable in this and related fields. Of particular relevance is the recent report of Rosenberg, Helliwell and ~tsufrak~s(ls) of electron precipitation associated with discrete rising frequency VLF emissions, which is interpreted as representing the first direct experimental evidence for an electron precipitation effect related to gyroresonance on magnetospheric field lines. The observations discussed here provide further such evidence. ~c~~~~~~~ge~e~~~-I am grateful to the S.R.C. for fundingthe South Uist fieldstation and also to cotleagues both at the S.R.C. Radio and Space Research Station, Slough, and at the University of So~t~pton for discussionsconcerning points raised in this paper.

1. 2. 3. 4. 5. 6. 7. 8. 9.

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