Mid-latitude and plasmaspheric hiss: A review

Mid-latitude and plasmaspheric hiss: A review

CQ32-0633192 $5.0+0.00 G 1992 Pergamon Press Ltd Plwwi Spo 2, and another at L c 2, with a minimum at L - 2 (Tsurutani et al., 1975). The emissions o...
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CQ32-0633192 $5.0+0.00 G 1992 Pergamon Press Ltd

Plwwi Spo
MID-LATITUDE

AND PLASMASPHERI~ A REVIEW

HISS :

M. HAYAKAWA

The University of Electra-Communications,

I-5-1 Chofugaoka, Chofu, Tokyo 182, Japan and

S. S. SAZHIN

Department of Physics. Sheffield University, Sheffield S3 7RH, U.K. (Received infinalform

22 June 1992)

Abstract-A review of mid-latitude and plasmaspheric hiss-type emissions observed on ground-based stations and in the Earth’s magnetosphere is presented. Different approaches to modelling of these. emissions are discussed. ft is pointed out that mid-latitude hiss emissions are most likely to be generated in the equatorial magnetosphere where the energy of electrons is transferred to wave energy via the electron cyclotron instability. Some quantitative characteristics of these emissions are explained in terms of a quasilinear model of this instability. Many properties of plasmaspheric hiss emissions are likely to be related to the same electron cyclotron instability, although the contribution of other mechanisms cannot be excluded.

1. INTB~DuCTI~N

This review is complementary to our previous review on chorus emissions (see Sazhin and Hayakawa, 1992). As was the case in that review, this is addressed to those who are already familiar with mid-latitude and plasmaspheric hiss phenomena and wish to update and amend their information. Hence, we have tried to make the reference list as comprehensive as possible, at the expense of illustrative material. In Section 2, we consider the main morphological properties of mid-latitude hiss. then in Section 3, we concentrate on the corresponding morphological properties of plasmaspheric hiss. Generation mechanisms of both these emissions are reviewed in Section 4. Finally, in Section 5 we briefly summarize the general state of the problem.

2. OBSERVATIONS

OF MID-LATITUDE

HISS

2. I. Ground-based observations It was at an early stage of investigation of natural radio waves at mid-latitude stations when electromagnetic emissions at frequencies of a few kilohertz with hiss-type spectral structure were discovered (Watts, 1957; Ellis, 1959, 1960, 1961; Duncan and Ellis, 1959; Dowden, 1961). Jnrrgensen (1966) showed that the amplitude of these emissions decreased with decreasing latitude. The value of this decrease (IO dB per 1000 km) could be explained in terms of wave

attenuation in the Earth-ionosphere waveguide due to wave propagation from the amoral zone to middle and low latitudes. On the basis of this fact, Jorgensen (1966) concluded that hiss-type emissions recorded at low and middle latitudes resulted from the propagation of the hiss-type emissions generated at the aurora1 region (aurora] hiss) to these regions. A similar conclusion was reached later by Rao et al. (1972). When analysing emissions at 8 kHz along the N-S chain of stations covering geomagnetic latitudes 4 = 75-SO”, Harang (1968) suggested dassification of them into two groups : night-time and daytime events. Whereas the first events could be related to the aurora1 hiss, the daytime events seem to be quite independent. Hayakawa et al. (1975a) made an extensive comparison of morphological characteristics of emissions observed at Moshiri (d, = 34.5” ; McIlwain parameter, f. = 1.59) and at Syowa (# = -69.0”; L = 6.1 I) (see also Tanaka et a/., 1976) and found significant differences between them. In particular, the diurnal variation and frequency spectra at Moshiri were quite different from those at Syowa. A well-defined maximum of mid-latitude hiss spectral density in the vicinity of 5 kHz with a bandwidth of a few kilohertz (Outsu and Iwai, 1961; Iwai et al., 1964; Ondoh and Isozaki, 1968) had no direct relationship to the aurora1 hiss spectrum (Hayakawa et al., 1975a). The upper limit of the mid-latitude hiss spectrum could extend up to 8 kHz (Hayakawa et al., 1975a; Khosa et al., 1981). while the spectrum of the aurora1 hiss 1325

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M.

HAYAKAWA

could extend up to 500 kHz and even higher (Jorgensen, 1968; Makita, 1979). Hence it seems justified to consider hiss events recorded at Iow- and midlatitude stations as independent of the aurora1 hiss. In our further analysis we call them mid-latitude hiss, since the maximum of their occurrence corresponded to middle latitudes (55-65”) which are connected with the plasmapause projection (Hayakawa et al., 1975a,b, 1977,1986b, 1988; Dronoveta!., 1984,1985; Kleimenova, 1985). Kleimenova et al. (1990) showed that there is a high latitude boundary of the observation of mid-latitude hiss associated with the main ionospheric trough (decrease of F-region ionization). The ground-based direction finding by Hayakawa et al. (1986b) gave us evidence that the mid-latitude hiss emissions were generated mainly just inside the plasmapause. Furthermore, Tanaka et al. (1974) measured the time delay of the occurrence of mid-latitude emissions after the magnetic storm, which enabled them to estimate the energy of ring current electrons responsible for mid-latitude VLF hiss as being of the order of - 5 keV. Fedyakina and Vershinin (1976) and Fedyakina and Khorosheva (1989) also studied the development of mid-latitude VLF noise bursts (chorus and associated hiss) in relation to the ring current. Hayakawa et al. (198la,b) found one event at 5 kHz with the exit point at the ionospheric level corresponding to the slot region (L = 2.2), but it seems that this event could result from the deflection in the plasmasphere of the emissions which were originally generated at the equatorial plasmapause (cf. Singh et al., 1978). The power flux spectral density of the mid-latitude hiss is lower than that of the amoral hiss, being of the order of IO-“-lo-‘* W rnw2 Hz-’ (Iwai et al., 1964; Harang, 1968 ; Hayakawa et af., 1975a, 1981a,b). According to data obtained at Moshiri station (4 = 34.3”N), the occurrence of mid-latitude hiss increased when CK, increased, where XK, was the daily sum of the K,, index. When ZKp 2 40 the occurrence probability of these emissions reached the value of 0.9 (Hayakawa et al., 1975a). Mid-latitude hiss recorded at Moshiri when CKp -c 30 was called the steady-state hiss to distinguish it from the disturbed hiss recorded when CK,, 2 30. The morphological properties of these two subtypes of mid-latitude hiss appeared, in many cases, to be different from each other. In particular, two maxima in the occurrence rate were observed for steady-state hiss : the main one at OS:00 L.T., and a secondary one at 20:00 L.T. (the corresponding maxima were also observed for midlatitude emissions recorded at Sogra station (d, = 57.5”N) (Raspopov and Kleimenova, 1977). The first maximum corresponds approximately to the

and S. S. SAZHIN

maximum occurrence rate of low-latitude whistlers (Hayakawa and Tanaka, 1978). Two maxima in occurrence were also observed for disturbed hiss, but in the latter case they were approximately equal and corresponded to 20:00 L.T. and 02:OOL.T. (Vershinin, 1970; Hayakawa et al., 1975a). Hayakawa et al. (1977) pointed out that morning-side emissions were typical for the main phase of the magnetic storm, while evening-side emissions appeared mostly during the recovery phase. A maximum of L),, in most cases preceded the emission activity (Ivanova and Kleimenova, 1983; Ben’kova et al., 1984). Like the aurora1 hiss, mid-latitude hiss is more often observed during winter than during summer; the seasonal dependence for steady-state emissions is more pronounced than for disturbed emissions. Duration time of the mid-latitude hiss is usually longer than it is for the aurora1 hiss and often exceeds 5 h (Hayakawa et al., 1975a). Sometimes the frequency of post-midnight and dawn-time emissions exhibited a regular drift, in most cases increasing with time, which could be as much as 1 kHz h- ’ (Vershinin, 1970; Carpenter et a!., 1975 ; Sazhin and Vershinina, 1978 ; Vershinin et al., 1979 ; Hayakawa et al., 1985b, 1986b; Hayakawa, 1989a). A similar frequency drift was also observed for chorus-type emissions (see Sazhin and Hayakawa, 1992). In the pre-midnight hours the temporal variation of the frequency corresponding to maximum emission intensity sometimes indicated an inverted V shape (Hayakawa et al., 1988). The increase of emission frequency often followed a sudden commencement (SC) of a magnetospheric storm (Mullayarov and Yachmenev, 1986). Other peculiarities of SC in VLF emissions were discussed by Mullayarov and Yachmenev (1990). The longitudinal variation of mid-latitude hiss as measured on six balloons at latitudes of 35%55’S, showed a significant minimum at 70”E-80”E, about 1000 km East of the geomagnetic conjugate of the Soviet transmitter, UMS (17.1 kHz) (Dowden and Holzworth, 1990). 2.2. Satellite observations Mid-latitude hiss emissions were observed in the upper ionosphere as well as in the equatorial magnetosphere by many satellites, but they were investigated in detail using data obtained by OVI-14 (Koons and McPherron, 1972), Arie2-3 (Bullough et af., 1969; Hughes et nl., 1971 ; Lefeuvre and Bullough, 1973 ; Hayakawa et al., 1975b, 1977), Arid-4 (Bullough et al., 1975 ; Kaiser and Bullough, 1975 ; Francis, 1979 ; Hayakawa, 1989b), interkosmos-3, -5, -14, -19 (Likhter, 1979; Likhter et al., 1991), IS&l, -2

Mid-latitude

and plasmaspheric

(Ondoh et al., 1980, 1981), DE-1 (Gurnett and Inan, 1988), EXOS-D (Kimura et al., 1990) and Awed-3 (Parrot, 1990). In some cases when the wave spectra were not measured as was the case with Ariel-3, -4 and Int~rkosmo~ observations, we can only assume that they correspond to mid-latitude hiss. The OVZ-14 satellite observed mid-latitude hiss in the slot region (L = 1S-3.6), extending from the magnetospheric equator up to SO”latitude in the frequency range 3.9-10.4 kIIz. The power spectral density of the waves observed by the magnetic antenna attained 1.1 my Hz-“~, and by the electric antenna attained 2.8 @VHz- ‘I* (Koons and McPherron. 1972). The ISIS-l, -2 satellites observed mid-latitude hiss in the upper ionosphere (h < 3500 km) mostly at latitudes 55-63” (although some events were observed at much lower latitudes down to 35”) during the nighttime. The frequency of the emissions was so close to 5 kHz that it was suggested that it be called “narrowband 5 kHz hiss” (Ondoh et al., 1980, 1981) (cf. the corresponding frequency spectra observed on the ground and discussed in Section 2.1). Using Ariel satellite data whose trajectory was within the height range 500600 km, Hayakawa et al. (1977) reported that the most intense waves at frequencies 3.2 and 9.6 kHz were observed at the latitudes S360” which corresponded to L = 2.440. Essentially the same latitude dependence was obtained for the emissions at 2.5 kHz recorded on board Interkosmos-5 (Sazhin et ai., 1980). Hence the latitude variations of mid-latitude hiss amplitude obtained by different satellites are compatible between themselves and with ground-based observations (see Section 2.1). Emissions at 3.2 and 9.6 kHz can be considered as the low and high frequency tails of mid-latitude emission spectra respectively. Identification of the emissions at 2.5 kHz appears to be more ambiguous as they can be confused with chorus type emissions (see Sazhin and Hayakawa, 1992 ; Kulkarni and Das, 1992). In order to make a more detailed identification of the source of mid-latitude hiss recorded on board Arid-3, the occurrence of the emissions at three frequencies of 3.2,9.6 and 16 kHz during three magnetic storms in 1967 was compared with the corresponding plasmapause locations using data from the same satellite (Hayakawa er ai., 1975b, 1977). It was pointed out that dawn (03:00-09:00 L.T.) emissions were observed at all three frequencies during the initial, main and recovery phases of storm development in the region outside the plasmasphere, while dusk (14:0&2 1:00 L.T.) emissions were observed only during the recovery phase. In the latter case the emissions at 3.2 and 9.6 kHz were observed mainly inside

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hiss

the plasmasphere ; no regular dependence between the plasmapause position and the emissions at 16 kHz was discovered. In most cases the emissions were observed in the immediate vicinity of the plasmapause becoming more structured poleward of the plasmapause. However, the plasmapause location determined by Ariel-3 did not necessarily coincide with the plasmapause location in the equatorial plane (Foster ef af., 1976) which brings some ambiguity into physical interpretation of the obtained results. The intensity of the emissions at 3.2 kHz could be influenced by chorus-type emissions in the dawn sector (see Sazhin and Hayakawa, 1992). The relation of 16 kHz emissions to the mid-latitude hiss is not quite clear. We may expect that some of the mid-latitude hiss emissions are related to the smooth (or S-) type hiss reported by Gross and Larocca (1972) as a subtype of LHR emissions. However, further experimental study of both phenomena is necessary in order to make this conclusion more convincing. From the analysis of natural radio noise at the Inrerkos~~s-5 satellite it followed that besides midlatitude hiss another type of emission could be identified at a frequency below 1 kHz (Zakharov et al., 1980). Morphological properties of these emissions are considered in the next section. 3. OBSERVATIONS

OF PLASMASPHERIC

HISS

The term “plasmaspheric hiss” refers to hiss-type ELF emissions observed mostly inside the plasmasphere. The main energy of these emissions is concentrated in the frequency range 100 Hz-l kHz (Thorne et al., 1973), although their upper frequency limit could extend to a few kilohertz (Parady et al., 1975; Ondoh et al., 1982, 1983). It seems possible that the high frequency part of the plasmaspheric hiss spectrum could sometimes result from the influence of mid-latitude hiss considered in the previous section. The plasmaspheric hiss was mainly studied by using magnetic and electric field observations on board the satellites, (X0-3, -4, -5, -6 ; OVl-17 ; Explorer-45 ; GEOS-I, 2; Interkosmos-5, -14; ISIS-l. -2; IRM; S3A ; DE-l ; Awed-3 ; EXOS-D ; ISEE(Thorne et al., 1973 ; Kelley et al., 1975 ; Muzzio and Angerami, 1972 ; Anderson and Gurnett, 1973 ; Tsurutani ef al., 1975 ; Parady et al., 1975 ; CorniIleau-Wehrlin et at., 1978; Kovner et al., 1978; Likhter et al., 1978; Ondoh et al., 1982, 1983; Larkina and Likhter, 1982, 1983; Mikhailova et a/., 1983; LaBelle er al., 1988; Sonwalker and Inan, 1988 ; Rauch et al., 1985 ; Hayakawa et al., 1990; Kokubun et at., 1991 ; Storey et al., 1991; Storey, 1991). Ground-based observations of 400 Hz emissions corresponding to plasmaspheric hiss at

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M.

and

NAYAKAWA

Sogra (L = 3.7) and Dolgoschelie (L = 4) stations during periods of very quiet magnetic activity, K,, = OI, were reported. and these emissions showed poor conjugacy (Kleimenova et al., 1976). Ground-based observations of ELF hiss at 4 = 60” and l9:OO < L.T. < 23:00 reported by Shchekotov and Molchanov (1985) can also be referred to as plasmaspheric hiss. The frequency corresponding to maximum emission intensity increased with increasing local time from 70-100 up to 20%500 Hz. In general, ground-based emissions inside the plasmasphere were in most cases observed at frequencies greater than 2 kHz (I-Iayakawa and Tanaka, 1977). Plasmaspheric ELF hiss in the frequency range l--2 kHz observed at Moshiri (4 = 34.5” ; L = 1.59) had approximately a right-handed circular polarization, which suggested that those emissions must have penetrated the ionosphere close to the station of observation (Hayakawa et al., 1985a). Plasmaspheric hiss in the equatorial magnetosphere was usually observed inside the plasmasphere at all local times (Thorne et al., 1973) ; its intensity in post mid-day hours was higher than in the evening hours (18:00-24:00 L.T.) (Russell et al., 1969 ; Parady et al., 1975). The region of plasmaspheric hiss observation was not always limited by the plasmasphere, and sometimes it was observed about 1.5 RE (R, is the Earth’s radius) outside it. During evening hours (18:0@-24:00 L.T.) the region of plasmaspheric hiss observation contracted when the lu, index increased, which agreed with the corresponding contraction of the plasmasphere. However, at 15:0&l 800 L.T., the relation of this region to the K, index was less evident and the plasmaspheric hiss was often observed at L = 5 even during the periods with large rC, index (Parady et al., 1975). The maximal plasmaspheric hiss spectral density was observed in the frequency range IOf& HZ, being of the order of lo-‘-lo-” y2 Hz- ‘, which indicated the average amplitude 5-50 my (Thorne et al., 1973); maxima1 amplitude was observed in the plasmasphere about I R, inside the plasmapause (Parady ef a!., 1975). An increase of plasmaspheric hiss spectral density Bf, accompanying the increase of thermal (cold) plasma density (n,) was observed (Chan et al., 1974; Cornilleau-Wehrlin et al., 1978). The experimentally obtained relation between these two values was expressed as (Chan and Holzer, 1974; Chan et al., 1974) : In Wn~o) = KB,

(1)

where Ke = 103~-’ Hz”’ and n, is the thermal plasma concentration corresponding to the weakest waves observed by OGO-5; this relation was obtained for

S. S. SAZHIN

values of Br in the range (0.5-l .5) x tom3 y Hz-‘j2. An increase of Br with increasing n, for ELF hiss in detached plasma regions outside the plasmasphere was also reported by Hayakawa et al. (1986a) and Hayakawa (1987). Parrot et al. (1991) showed that the intensity of plasmaspheric hiss as observed by ARCAD(apogee 2012 km) was greatest on Monday and decreased until Saturday. The measurements of wave normal angles B of plasmaspheric hiss led to different results. Sometimes the observed values of 0 were less than 20”, with polarization corresponding to whistler-mode waves (Thorne et al., 1973 ; Hayakawa et ai., 1986c, 1987; Parrot and Lefeuvre, 1986), but in other cases larger values of 0 were predominant (Hayakawa et al., 1986c, 1987 ; Parrot and Lefeuvre, 1986 ; Storey et al., 1991 ; Hayakawa, 1991a). Even wave normal angles close to 90” for plasmasphere hiss were reported. Anisotropy of hiss propagation has been pointed out (Parady et al., 197.5). Using plasmaspheric hiss observations on board GEOS-I, Lefeuvre et at. (1981) and Buchalet and Lefeuvre (1981) pointed out that its energy was often contained within two different wave packets whose wave normal angles were concentrated in the same off-meridian plane. The wave normal angle of the most intense emissions was close to 6o (Gendrin angle) defined by the relation f?o = arccos (2flfH) wheref is the wave frequency and fH is the electron gyrofrequency. When analysing three equatorial events recorded on GEUS-I by methods which included that of the maximum entropy, Hayakawa et al. (l986c, 1987) and Hayakawa (199la) found that just inside the plasmapause the wave normal direction B of the plasmaspheric hiss was nearly aligned with the magnetic field, and when the observing position was 0.3-0.5 RE inside the plasmapause there were two different groups of wave normal angles; one was a medium wave normal angle ranging from 20 to 60” and the other was a large wave normal angle in a range from 70 to 80”, slightly smaller than 0,. Parrot and Lefeuvre (1986) carried out the statistical study of wave normal angles of the plasmaspheric hiss observed on GEOS-1, which indicated a wide scatter of wave normal angles from 0” to Q,,, (oblique resonance angle). However, they did not consider the relation of the wave normal angle of each event to the corresponding plasmapause location. A recent study by Storey et al. (1991) for the ISEE data indicated that wave normal angles for a majority of plasmaspheric hiss events near the equatorial plane within the plasmasphere were in the range between -t& and

e 03.

Direction finding for the plasmaspheric

ELF hiss

Mid-latitude

and plasmaspheric

in the upper ionosphere was carried out for the first time onboard the Awed-3 satellite (Lefeuvre et al., 1985; Hayakawa et al., 1990) in the height range of 400-2000 km. Hayakawa et al. (1990) found from direction finding at frequencies of 375, 450, 525 and 675 Hz the following properties of the plasmaspheric hiss. Firstly, the waves at all these frequencies were right-handed, except for one rare occurrence of lefthanded waves, and the percentage of cases with two propagation directions increased at lower frequencies and also at lower geomagnetic latitudes. Secondly, the wave normal angles of the secondary waves when they were observed, were very close to the wave normal angles of the most intense waves and they had a tendency to decrease with latitude. The most intense waves had wave normal angles in the range from 40 to 70”. Inverse ray tracing computations (Lefeuvre and Helliwell, 1985 ; Cairo and Lefeuvre, 1986; Muto et al., 1987; Hattori et al., 1990) of plasmaspheric hiss emissions toward higher altitudes would be very useful and should be compared with the corresponding direction finding results in the equatorial source region mentioned above. In some cases the activity of the plasmaspheric hiss was accompanied by an increase in electron fluxes in the energy range 20-l 00 keV. Sometimes its maximal amplitude was observed in the region where the plasmasphere intersected with the inner edge of the ring current (Parady and Cahill, 1973 ; Parady et al., 1975). In general, plasmaspheric hiss amplitude had a tendency to increase when the D,, index increased (Larkina and Likhter, 1982). The role of plasmaspheric hiss in electron precipitation from the magnetosphere was discussed by Imhof et al. (1982, 1986). For plasmaspheric hiss emissions recorded on board OGO-4 (at the heights 430-900 km) both the lower and upper cut-offs were observed in their spectra. The former could be approximately identified with the local lower hybrid resonance frequency, fLH, while the latter remained most of the time at the level of 600 Hz. This sort of plasmaspheric hiss was called the band-limited hiss (Muzzio and Angerami, 1972). These emissions were observed at the latitudes 10-55” (at latitudes close to 10” a decrease of the emission upper cut-off was observed which preceded their disappearance), mainly in the period 06:00-22:00 L.T. The maximal energy spectral density of these emissions attained the value of 2 x 10e4 y* Hz-’ at 10:00 L.T. After long periods of low magnetic activity, the upper cut-off frequency of these emissions decreased to 420 Hz, but returned to its usual value within 1 h after the sudden commencement of the magnetic storm. At latitudes close to 58” the lower cut-off fre-

hiss

1329

quency decreased and became more diffuse (Muzzio and Angerami, 1972). For the hiss-type emissions recorded on board In terkosmos- 14, the upper cut-off of their spectrum was identified with the local proton gyrofrequency (Vzrnova et al., 1978 ; Likhter et al., 1978). A more detailed analysis of the problem by Mikhailova et al. (1983) showed that the central frequency of these emissions was also related to the proton gyrofrequency. Rauch et af. (1985) discussed the attenuation bands and cut-off frequencies of plasmaspheric ELF hiss observed on the Aureol-3 satellite (400-2000 km). At L values less than four, a wide band emission was seen at and above the local proton gyrofrequency. The lower cut-off was slightly below the proton gyrofrequency and the upper cut-off frequency varied from 100 to 600 Hz. A significant difference from the previous observations was the presence of an attenuation band above the proton gyrofrequency. At L values between four and six a band below the proton gyrofrequency was detected. Ducted waves with small wave normal angles were also often observed at these L values, and they were clearly associated with fieldaligned irregularities just inside the plasmapause. The energy spectral density of plasmaspheric hiss emissions recorded on board OGO-6 (at heights 4001100 km) had a well pronounced dependence on the level of magnetic activity (Smith et al., 1974). In particular, plasmaspheric hiss emissions at L = 2 with energy spectral density 10m6-4.5 x lop4 y* Hz-’ were observed at the beginning of the storm recovery phase, while the emissions with energy spectral density less than lo-’ y* Hz- ’ were observed during quiet periods (Tsurutani el at., 1975; Thorne er al., 197’7). The occurrence of plasmaspheric hiss observed on OGO-6 was largest at 06:00-18:00 L.T., which was compatible with the results obtained from OGO-4 (Muzzio and Angerami, 1972) and interkosmos-5 (Kovner et al., 1978) data. The latitude dependence of the occurrence of emissions at frequencies below 1 kHz recorded on a magnetic antenna on board OGO-6 showed two maxima : one at L > 2, and another at L c 2, with a minimum at L - 2 (Tsurutani et al., 1975). The emissions observed at L > 2 were identified with the conventional plasmaspheric hiss, while the emissions at L < 2 were called the inner zone hiss (Tsurutani et al., 1975). The latter were not observed in the region close to the equatorial plane at L < 1.1, which was compatible with the earlier mentioned OGO-4 observations. The maximal energy spectral density of the inner zone hiss was observed at frequencies close to 500 Hz and was equal to about 4 x lo-’ y2 Hz-‘, which corresponded to 10 my as the bandwidth of

1330

M.

HAYAKAWA and

the hiss under consideration was approximately equal to 300 Hz (Tsurutani et al., 1975). Different results were obtained based on wave electric field measurements on board ISIS-1 (at heights 575-3512 km) and ISIS-2 (at the height 1400 km) satellites, wave spectra from which were received at Kashima Station (Geomagnetic latitude = 25.8”N) (Ondoh er al., 1982, 1983). In particular, the OGO-6 minimum (magnetic) at L = 2 was displaced to L x 3. Hence, the distinction between the inner zone hiss and the plasmaspheric hiss appears to be not at all obvious. Moreover, it was pointed out that the greatest plasmaspheric hiss occurrence rate took place when Kp = 2-3, which also seems to differ from the results of the previous observations. Based on the behaviour of the upper frequency cut-off of the plasmaspheric hiss emissions Ondoh er al. suggested two subtypes: a steady hiss whose upper frequency cut-off was approximately independent of latitude, and a hiss whose upper frequency cut-off increased with increasing latitude. The emission frequency in the latter case could be as high as 5 kHz, and these emissions could be associated with the mid-latitude hiss. Emissions observed on board ISIS-l, -2 at midand low latitudes, but not near the geomagnetic equator, often had a strong spin effect so that their electric field intensity was weak in the direction of geomagnetic field. The occurrence of emissions observed on ISIS-1 and -2 was largest at 08:00-19:00 L.T. which is compatible with the above-mentioned results obtained from OGO-4 and -6 (Ondoh et al., 1982, 1983). During the observation of 0.5 kHz emissions on board the Interkosmos-5 satellite (at heights of 200-1200 km), a relation between wave amplitude and the local magnetic field amplitude (B,) was pointed out. At a fixed L shell and at a fixed height this amplitude was higher in regions with smaller values of BO. A maximal amplitude was observed at heights of 300-400 km; when the altitude increased to 1000 km this amplitude decreased by approximately an order of magnitude (Kovner et al., 1978). It is not clear whether the emissions at frequencies below 1 kHz recorded by Interkosmos-14 at postmidnight aurora1 latitudes ($I > 58.4”N) (Mikhailova et al., 1986) should be referred to as plasmaspheric hiss or considered as a separate phenomenon, presumably related to polar chorus.

4. GENERATION

MECHANISMS

The limitations of the frequency bandwidth of midlatitude and plasmaspheric hiss emissions, as well as the latitudes of their observation, allow us to expect

S. S. SAZHIN

that mid-latitude hiss emissions and some of the plasmaspheric hiss emissions are generated mainly in the vicinity of the equatorial plane of the magnetosphere by the electron cyclotron instability ofwhistlermode waves. Without discussing details of the theory (see e.g. Sazhin, 1992) we only mention that in magnetospheric conditions the growth rate of this instability is largest for propagation parallel to the Earth’s magnetic field in the equatorial magnetosphere. In this case waves grow when the following inequality is satisfied :

(24 where A, is the anisotropy of the electron distribution function in the equatorial magnetosphere (the ratio of the electron temperature in the direction perpendicular to the magnetic field, T,, to the electron temperature in the direction parallel to this field, T,,), fH is the electron gyrofrequency and f is the wave frequency. Condition (2a) can be rewritten as f < (A,-- l)fulA,

(2b)

which imposes an upper limit on the frequency of unstable waves for a given A,. Remembering that f< fn for whistler-mode waves, we can see that condition (2a) can be satisfied only when A, > I. When deriving equation (2a) we neglected the contribution of relativistic effects. These effects are important mainly in a rarefied plasma when the electron plasma frequency,f,, is of the order of or belowf,, and they lead to a slight decrease of the frequency range of unstable waves (see Sazhin and Temme, 1991a,b; Sazhin et al., 1992; Sazhin, 1992). This effect has never been taken into account when interpreting the observed mid-latitude and plasmaspheric hiss emissions, although it cannot always be neglected. It is not uncommon to observe electrons with A, > 1 in the equatorial magnetosphere (see e.g. Frank, 1966, 1968, 1971; Owens and Frank, 1968; Russell and Thorne, 1970 ; Akasofu and Chapman, 1972 ; Burton, 1976 ; Ronnmark et al., 1978; Maeda and Lin, 1981 ; Bahnsen et al., 1985). Waves generated in the equatorial magnetosphere increase their wave normal angle when propagating away from the magnetospheric equator unless there are cross-field gradients in electron density (which can occur, for example, in the vicinity of the plasmapause). When the wave frequency becomes equal to the local lower hybrid frequency the waves are reflected back to the magnetospheric equator and thus become trapped within it. It seems that some of the mid-latitude hiss trapped in the magnetosphere can

Mid-latitude and plasmaspheric hiss

be observed in the upper ionosphere as a smooth type hiss (Gross and Larocca, 1972), with a well-defined lower cut-off at the lower hybrid frequency. Most plasmaspheric ELF hiss emissions seem to be trapped in the magnetosphere. The computation of ray paths of plasmaspheric hiss emissions recorded at L < 3 within the plasmasphere suggests that these emissions are initially generated at large wave normal angles (close to the resonance cone angle 6,) in the equatorial plane of the magnetosphere in the vicinity of the plasmapause (Ondoh et al., 1982, 1983). This conclusion is compatible with the relative independence of the plasmaspheric hiss spectrum from the local L value. The hypothesis about a latitudinally localized source of the plasmaspheric hiss emission is also compatible with the observation of increases in plasmaspheric hiss which are unaccompani~ by any corresponding increases in local energetic electron fluxes. The preferential mid-latitude VLF hiss observation on the ground and in the topside ionosphere near the plasmapause projection can be explained, not only by the effect of local wave generation, but also by the effect of wave guidance in the vicinity of the plasmapause (Inan and Bell, 1978 ; Thorne et al., 1979 ; Semenova and Trakhtengertz, 1980 ; Hattori et al., 1991). It is worth pointing out that waves can be observed on the ground only when they exit from the ionosphere with almost vertical wave normals (Iwai et al., 1974). The connection between the early morning maximal occurrence rate of mid-latitude hiss and whistlers at 05:OOL.T. can be related to the decrease of ionospheric absorption at that time (Hayakawa et al., 1975a). An alternative explanation of this phenomenon can be based on the fact that, at about 06:OO L.T., the thermal plasma concentration increases in the equatorial magnetosphere, due to ionospheric heating by sunlight. Thus f, increases, and this leads to an increase of the instability growth rate (see e.g. Sazhin, 1992) [cf. equation (l)]. An absence or low level of mid-latitude hiss at evening hours during initial and main phases of the substorm can be explained by the fact that most energetic electrons, responsible for wave generation, drift from the plasma sheet to the inner magnetosphere, precipitate in the morning sector and do not reach the evening sector. However, during the recovery phase and decay of the ring current, the plasmasphere expands, and so less energetic electrons can participate in wave generation; the latter can drift around the Earth several times. This assumption is compatible with the observation of mid-latitude hiss inside the plasmasphere in the evening hours during the recovery ^. phase ot the substorm, as the plasmasphe~c radius at

1331

this time increases (Hayakawa et al., 1977). The linear theory of whistler-mode cyclotron instability in the piasmasphere in the presence of electron transport across L shells was considered by Bespalov et al. (1983). Detailed analysis of plasmaspheric ELF hiss ampiification during the process of its propagation was undertaken by many authors, in particular by Church and Thorne (1983), Huang and Goertz (1983), Huang et al. (1983) and Storey et al. (1991). The results of these papers were notably different from each other, due mainly to the use of different models of electron distribution in the magnetosphere. However, the general character of wave propagation predicted essentially the same results: waves were launched at the equatorial plane with small wave normal angles which eventually increased (cf. also Lefeuvre and Helliwell, 1985) so that Landau damping could no longer be neglected (Sazhin, 1992). As a result the total amplification of plasmaspheric hiss during the process of its propagation from one hemisphere to another, predicted by Huang et al. (1983) and Church and Thome (1983), appeared insu~cient to expiain the actual amplitude of these emissions. However, a direct comparison between electron and wave data obtained on board GEOS-1 and -2 led Cornilleau-Wehrlin et al. (1985) and Solomon et al. (1988) to a positive conclusion : cyclotron amplification of plasmaspheric hiss was sufficient to explain its actual amplitude. The results of a similar comparison for hiss immediately outside the plasmasphere were not so obvious and additional mechanisms for its generation (e.g. a contribution due to amplification of hiss leaking from the plasmasphere at high latitudes) seem to contribute. Wave amplification due to cyclotron instability was especially well pronounced for the dayside magnetosphere during the storm sudden commencement (SSC) (Cornilieau-Wehrlin et al., 1988). The linear theory of whistler-mode instability considered so far predicts only the tendency for the waves to be amplified or to be damped. However, it seems to be an oversimplification to use it for the estimation of wave amplitude in the case of steady-state midlatitude or plasmaspheric hiss emission which can interact with the bouncing electrons during a long period of time such that the deformation of the electron distribution function cannot be neglected. If there were no wave energy and particle losses in the magnetosphere, then when estimating the spectral density of these emissions we could use the quasilinear model based on the assumption of a homogeneous plasma (Roux and Solomon, 1970, 1971). However, again, this would be an oversimplification in typical magnetospheric conditions. When constructing a

1332

M. HAYAKAWA and S. S. SAZHIN

quantitative model for mid-latitude and plasmaspheric hiss emissions we need to take into account particle sources and losses, wave sources (instability) and losses, as we!! as the deformation of the particle distribution function in the wave field and the actual inhomogeneity of magnetospheric plasma. The first model which took into account some of these processes was suggested by Etcheto et al. (1973) on the assumption of longitudinal wave propagation. The essential restriction of this mode! was that the instability and diffusion processes were assumed to take place only in a localized region along the field line, of an a priori defined width in the vicinity of the equatorial plane, where the plasma and magnetic field were assumed homogeneous. This latter assumption is not obviously acceptable. In particular, the value of this width, strictly speaking, must be frequency dependent, a fact which was not taken into account in Etcheto et al.‘s (1973) mode!. Bespalov and Trakhtengertz (1980) made an attempt to construct a more realistic model for hisstype steady-state emission with cyclotron generation. They considered a realistic magnetic field tube of the magnetosphere, but averaged the diffusion equation over a bounce period. Wave energy density was also assumed to be averaged aiong the field line. The corresponding system of equations appeared to be much more complicated than in the case of Etcheto et al. (1973), and in order to obtain its analytical solution certain restrictions were imposed on the source function and wave parameters. As a result, the wave spectrum appeared to be less realistic than in the case of Etcheto et al.‘s (1973) mode!. Hence, it seemed necessary to look for an alternative solution to the problem and to construct a mode! which would be more realistic than Etcheto et al.‘s (1973), both from the point of view of assumptions and correspondence to the observed wave spectra. Such a mode1 was suggested by Sazhin (1977) and improved by Sazhin (1984, 1989). Many assumptions of Sazhin’s model are identical to those adopted by Etcheto et al. (1973). !n particular, it was assumed that whistler-mode waves, in the mode of which the emissions propagate, had zero wave normal angle (0 = 0), low frequencies (J’<<,fn) and were generated by the electron cyclotron instability (the generalization of these models to 0 << 1 could be achieved by replacing f by fcos 0 in the final expressions; see Sazhin, 1987). In contrast to Etcheto et at. (!973), Sazhin considered a realistic magnetic fie!d and eiectron dist~bution in the equatorial magnetosphere. Without giving any details of the analysis of the model by Sazhin, we just present and discuss some

final expressions. This mode! predicts that the frequency at which the wave spectral density is maximal is determined by the following expression : (3) where M’,,is the electron thermal velocity in the direction parallel to the magnetic field, c is the velocity of light, and the subscript eq indicates that the corresponding parameters are taken in the equatorial plane of the magnetosphere. The maxima! wave spectral density within this mode! is determined by the following expression : (B;)

x 5 x ,0-27AW;5L~'~,

(4)

where A = nJ.‘.j, neq is the plasma density at the equatorial magnetosphere (in reciprocal cubic centimetres), W, is the electron energy (in electronvolts) at L = L,, (B:),,, is measured in square gauss per hertz, and dn,/dt is the rate at which the hot electrons from the source are supplied (in reciprocal cubic centimetres per second). When deriving equation (4) we assumed that the first adiabatic invariant of the source electrons drifting from L, (where their energy was WJ to a given L was conserved. The model is a quasistationary one, i.e. the number of electrons penetrating the field line under consideration is equal to the number of electrons precipitating into a loss cone Assuming that dn,/dt is a priori known and does not depend on L, it follows from equation (4) that the dependence of (L$ >,,, on L is controlled by the dependence of A on L. When considering the latter dependence we must first choose the mode! for n(L). The mode! based on the experimental results of Chappeil et al. (1970) seems to be the most convenient. Taking appropriate values of n,, presented in Fig. 7 of Chappell et af. (1970) for K,, = 3, we calculated the function h(L) which had a sharp maximum at L N 3.6 corresponding to the internal boundary of the plasmapause. Hence, we can conclude that the most intense waves are most probably generated in this region of the magnetosphere which is consistent with the experimental results referring to both mid-latitude and plasmaspheric hiss emissions. Propagation of the waves from the vicinity of the magnetospheric equator to the Earth’s surface can be understood in terms of wave trapping in the vicinity of the plasmapause (Inan and Beti, 1978; Semenova and Trakhtengertz, 1980; Hattori et al., 1991 ; Molchanov and Shkulanov, 1991).

Mid-latitude and plasmaspheric hiss Let us now consider the frequency f,,* in more detail. We first restrict ourselves to considering the average value of the magnetic disturbance (K,, = 3). When taking L = 3.6 corresponding to the region near the inner boundary of the plasmapause where the most intense waves are generated, and neq = 2 x IO3 cm- 3 (Chappell et al., 1970) we obtain from equation

1333

waves generated in the vicinity of the plasmapause strongly influence the intensity of the waves in the region away from it. As follows from equation (3), the value off,,, is controlled by fHeq, fpeq and )Y,,,so that the change of any of these parameters would cause a corresponding change infmax. In particular, an increase infmar can be due to the inward E x B drift of the electrons which is (3) : accompanied by the corresponding increase of fHes fmax(kHz) = 8.3/W(keV), (5) (see Sazhin and Vershinina, 1978). It can also be due where W = m,wf/2 and m, is the electron mass. to the inward motion of the plasmapause which is From this equation it follows that the frequency of accompanied by a decrease offpeq in a fixed magnetic generated waves decreases when W increases ; for an field tube in the vicinity of the plasmapause (see Vera priori given value of the magnetic disturbance the shinin et al., 1979). Finally, it can be due to the velocity energy of the incoming electrons is the only parameter dispersion of the electrons drifting from midnight to which determines the value off,,,,. dawn sector which is accompanied by a decrease of w For other values of K,, equation (5) takes a form in a given meridional sector (see Foster et al., 1976). which differs by a numerical coefficient. In particular, The first process can take place during 0O:OO< L.T. < for K,, = 2 (L = 4; neq = 0.5 x IO3 cm-‘); ,fmaX 06:00, the second during periods of increasing mag(kHz) = 12.9/W (keV); for K,, = 4-5 (L = 3.3, netic activity ; the third can follow the periods of n = 2x lo3 cmM3);f,,, (kHz) = 18.2/W (keV). intense electron injection, following, for example, the As follows from these estimates, the mid-latitude onset of a magnetospheric substorm. As was shown hiss at frequencies of 4-5 kHz can be generated by by Hayakawa et al. (1985b, 1986b) the third process electrons with energies in the range of a few kilowas the one which was mainly responsible for the electronvolts, which is consistent with the experobserved increase of frequency of post-midnight emisimental estimate by Tanaka et al. (1974). Electrons sions discussed in Section 2.1. The same process is with higher energies (> 20 keV) seem to be responsible also responsible for the initial frequency increase in for the generation of the plasmaspheric hiss. pre-midnight emissions. The subsequent frequency Thus this model predicts the generation of both decrease of these emissions was interpreted in terms mid-latitude and plasmaspheric hiss emissions in the of the effect of additional electron injection into the same region of the magnetosphere (plasmapause) but inner magnetosphere due to the development of the by electrons of different energies. Using numerical following substorm (Hayakawa et al., 1988). As follows from the previous analyses, the quasivalues corresponding to mid-latitude hiss observed whenKP=3(A=2.3x106;L,=3.6; W,=2keV) linear model based on the electron cyclotron whistlerand taking dn,/dt = 3 x 1O- 4 cm- 3 s- ’ (see Etcheto et mode instability in the equatorial magnetosphere al., 1973) we have (Bt),,, ;t: 1 my2 Hz-‘, which seems to be compatible with most of the observations agrees with experimental results. Considering the elecof mid-latitude VLF hiss emissions. Some of the trons responsible for the generation of plasmaspheric plasmaspheric hiss emissions are likely to be due to hiss emissions (W,, = 20 keV; dnJdt = 10d4 cme3 the same quasilinear electron cyclotron whistler-mode s-‘) we have (Bf),,, = IO my2 Hz-‘, which also instability whenever the necessary conditions exist. agrees with the experimental data. The considered However, this model cannot explain several expernumerical values are taken only to show the conimental results mentioned in Section 2. Namely : sistency of the model. Its quantitative verification (1) the observation of large wave normal angles of would follow from the simultaneous observations of dni/dt and (Bf ),,, for electrons of different energies. plasmaspheric ELF hiss emissions in the equatorial Sazhin et al. (1980) compared predictions of the plane ; (2) nearly stationary observation of plasmaspheric models by Etcheto et al. (1973) and Sazhin (1977) for the latitude dependence of the emissions at 0.5 and hiss on low-altitude satellites down to the equatorial 2.5 kHz observed on board Interkosmos-5 and showed latitude ; (3) its occasional penetration through the ionothat the model by Sazhin (1977) was most consistent with their results. However, the analysis of these sphere down to the ground even at low latitudes authors seems to be oversimplified when they assumed (4 - 35”); (4) the relation of plasmaspheric hiss emissions to that the waves were generated at the same field line where they were observed. We can expect that the the inner edge of the proton ring current ; and

1334

M.

HAYAKAWA

(5) the relation of plasmaspheric hiss energy density to the local value of magnetospheric field induction (B,) and its decrease with an increase in the height from 400 to 1000 km. These facts make it necessary to look for alternative mechanisms for plasmaspheric hiss emission generation which are briefly discussed below. The relation (4) between plasmaspheric hiss and ring current protons can be understood within the model by Parady (1974), who assumed that this hiss is generated due to an anisotropic proton instability (see also Wang and Goldstein, 1988). Wave normal angles in the latter model were assumed to be close to 90” and one could expect these emissions to be trapped within the equatorial region of the magnetosphere. On the other hand, the relation (5) of plasmaspheric hiss with B, as well as height dependence of its intensity can be understood within the model of ionospheric wave generation suggested by Kovner et al. (1978). This model is also compatible with the observation of more intense waves at Kerguelen where B0 is smaller (Kleimenova et al., 1976). A similar mechanism of ELF hiss generation in the ionosphere in terms of the proton instability was suggested by Bud’ko (1984). The daily variations in intensity of plasmaspheric hiss were attributed by Molchanov et al. (1991) to the contribution of PLHR emissions related to human activity. Similarly, the longitudinal minimum in midlatitude hiss intensity was related to electron pitchangle diffusion induced by the Soviet transmitter (Dowden and Holzworth, 1990). When modeling mid-latitude and plasmaspheric hiss emissions we should in general take into account the contribution of distant sferics (see the discussion by Gusev and Chernysh, 1985; Druzhin et ai., 1986, 1990; Druzhin and Shapaev, 1988; Hayakawa 1989b). Based on the observation of whistler-triggered hiss in the frequency range 1.5-6.0 kHz, Sonwalker and Inan (1989) concluded that lightning can be considered as an embryonic source of this hiss, which is further amplified due to the development of the electron-cyclotron instability [cf. also whistlertriggered emissions observed by the ISIS satellite (Nakamura and Ondoh, 1989) and on the ground (Hayakawa, I991 b)].

5. C~NCLU~~ION As has been shown in this review, mid-latitude hiss emissions are likely to be generated in the equatorial plane of the magnetosphere near the inner boundary of the plasmapause due to the development of the

and S. S. SAZHIN

electron-cyclotron whistler-mode instability in the outer radiation belt. Both linear and quasilinear theories of this instability have been developed and their predictions are in good agreement with experimental data. In contrast to mid-latitude hiss, the origin of plasmaspheric hiss is more obscure. On one hand some of its properties are compatible with the hypothesis that it is generated by the same electron instability and in the same region of the magnetosphere as the mid-latitude hiss, but with the more energetic electrons. However, other properties can be explained in terms of the proton instability of the waves propagating almost perpendicular to the magnetic field in the equatorial magnetosphere, or in terms of an ionospheric origin of the waves. Further experiments including the direction finding measurements and simultaneous wave and particle distribution function measurements are needed before we can derive a quantitative model of the plasmaspheric hiss. ~c~~o~~e~~e~e~z~-The authors are grateful to Dr K. Bullough of Sheffield University (UK) for-very useful comments on the draft version of this paper and MS M. Sasaki for careful typing. One of the authors (S. Sazhin) would like to thank NERC (UK) for financial support.

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Mid-latitude and r )lasmaspheric hiss sions observed on Ariel4. Proc. Roy. .Soc. A343, 207. Bullough, K., Hughes, A. R. W. and Kaiser, T. R. (1969) VLF observations on Arief 3. Proc. Roy. Sot. A311,569: Burton, R. K. (1976) Critical electron oitch anele anisotroov necessary fo; choius generation. J. &ophys.-Res. 81,47%. Cairb, L. and Lefeuvre. F. (1986) Localization of sources of ELF/VLF hiss observed in the magnetosphere : threedimensional ray tracing. J. geophys. Res. 91,4352. Carpenter, D. L., Foster, J. C., Rosenberg, T. J. and Lanzerotti, L. J. (1975) A subauroral and mid-latitude view of substorm activity. J. geophys. Res. 80,4279. Chan, K. W. and Holzer, R. E. (1974) ELF hiss associated with plasma density enhancements in the outer magnetosphere. J. geophys. Res. 81,2267. Chan, K. W., Holzer, R. E. and Smith, E. 3. (1974) A relation between ELF hiss and plasma density in the outer plasmasphere. J. geophys. Rex 79, 1989. Chappell, C. R., Harris, K. K. and Sharp, G. W. (1970) A study of the influence of magnetic activity on the location of the plasmapause as measured by OGO-5. .I. geophys. Res. 75, 50.

Church, S. R. and Thorne, R. M. (1983) On the origin of plasmaspheric hiss : ray path integrated amplification. J. geophys. Res. 88,794l.

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Dowden, R. L. and Holzworth, R. H. (1990) Longitudinal variation of mid-latitude hiss from six long duration balloon flights. J. geophys. Res. 95, 10,599. Dronov, A. V., Fedyakina, N. I. and Khorosheva, 0. V. (1984) On the interrelationship of subauroral bursts of VLF hiss, precipitating electrons and the plasmapause. Phys. Solar i terr. 35,61 (in Russian). Dronov, A. V., Fedyakina, N. I. and Khorosheva, 0. V. (1985) The interrelationship of subauroral bursts of VLF hiss, precipitating electrons and the plasmapause. Geomagn. Aeronomy 25, 87 (English translations. Druzhin, G. I. and Shapaev, V. 1. (1988) A role of global thunderstorm activity in the generation of the regular noise background. Geontagn. Aeronomy 27, 81 (in Russian). Druzhin, G. I., Toropchinova, T. V. and Shapaev, V. I. (1986) The regular noise background in VLF radiation and global thunderstorm foci. Geomagn. Aeronomy 26, 211 (English translation). Druzhin, G. I., Trakhtengertz, V. Yu. and Shapaev, V. I. (I 990) Longitudinal drift of the sources of VLF emissions in regions of thunderstorm concentration. Geomagn. Aeronomy 30,328 (in Russian). Duncan, R. A. and Ellis, G. R. A. (1959) Simuitaneous occurrence of subvisual aurora1 and radio noise bursts on 4.6 kc/s. Nature 183, 1618.

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Ellis, G. R. A. (1959) Low frequency el~tromagnetic radiation associated with magnetic disturbances. Planet. space Sci. 1, 253. Ellis, G. R. A. (1960) Geomagnetic disturbances and 5 kilocycles per second electromagnetic radiation. J. geophys. Res. 65, 1705. Ellis, G. R. A. (1961) Spaced observations of the lowfrequency radiation from the Earth’s upper atmosphere. J. geophys. Rex 66, 19. Etcheto, J., Gendrin, R., Solomon, J. and Roux, A. (1973) A self-consistent theory of magnetospheric ELF hiss. J. geophys. Res. 78,8 f 50. Fedyakina, N. I. and Khorosheva, 0. V. (1989) A develop ment of a noise storm in VLF emissions and its relation to magnetospheric dynamics during geomagnetic storms. Geomagn. A&onomy-29, 253 (in Russian). Fedvakina. N. I. and Vershinin. E. F. (19761 Relation of VLF noise storms to the magnetospheric ‘ring current. Geomagn. Aeronomy 16, 462 (English translation). Foster, J. C., Rosenberg, I. J. and Lanzerotti, L. J. (1976) Magnetospheric conditions at the time of enhanced waveparticle interactions near the plasmapause. J. geophys. Res. 81, 2175.

Francis, C. R. (1979) Satellite and ground-based studies of ELF/VLF wave-particle interactions. Ph.D. thesis, University of Sheffield. Frank, L. A. (1966) Explorer-I 2 observations of the temporal variations of low-energy electron intensities in the outer radiation zone during geomagnetic storms. J. geophys. Res. 71, 4631. Frank, L. A. (1968) Recent observations of low-energy charged particles in the Earth’s magnetosphere, in Phy& of the Maqnetosphere (Edited by Carovillano. R. L.. I&Clay, J. P. and_Radoski, H. R.),bp. 271-289. D. Reidel, Dordrecht. Frank, L. A. (1971) Relationship of the plasma sheet, ring current, trapping boundary and the plasmapause near the magnetic equator at local midnight. J. geophys. Res. 76, 2265. Gross, S. H. and Larocca, N. (1972)Phenomenologicai study of LHR hiss. J. geophys. Res. 77, 1146.

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Hattori, K., Ishikawa, K. and Hayakawa, M. (1991) Raytracing study of the plasmapause effect on non-ducted whistler-mode wave propagation. Pkmet. Space Sci. 39, 425.

Hattori, K., Ishikawa, K. and Hayakawa, M. (1990) Raytracing interpretation of wave normal directions of chorus emissions observed in the off-equatorial region of the outer magnetosphere. Proc. natn Insr. Polar Res., Symp. Upper Atmos. Phys. 3, 70.

Hayakawa, M. (1987) The generation mechanism of ELF hiss in detached plasma regions of the magnetosphere, as based on the direction finding results. Mem. nam Inst. Polar Res. 47, 173.

Hayakawa, M. (I989a) Further study of the frequency drift of dawnside mid-latitude VLF emissions associated with magnetic disturbances. Planet. Space Sci. 37,269.

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