The propagation of auroral hiss observed on the ground as deduced from direction-finding measurements

Planer. Printed

Space Sci., Vol. 36, No. 3, pp. 259-269, in Great Britain.

00324633/88 $3.00+0.00 Pergamon Press plc

I988

THE PROPAGATION OF AURORAL HISS OBSERVED ON THE GROUND AS DEDUCED FROM DIRECTIONFINDING MEASUREMENTS YOSHIHITO

Research

Institute

TANAKA

of Atmospherics,

and MASANORI

Nagoya

(Received injnalform

NISHINO

University,

Toyokawa

23 September

1981)

Aichi 442, Japan

Abstract-Based on the direction-finding results of aurora1 VLF hiss at Syowa (geomag. lat. -7O”), an extensive ray tracing analysis, and an estimation of transmission loss in the simulated amoral ionosphere, the propagation characteristics of ground-based aurora1 hiss in the magnetosphere and ionosphere are deduced ; incoherent transmission of impulsive hiss with a wide frequency range (2 100 kHz) from localized exit regions at the ionospheric level almost coincident with some localized regions of bright electron auroras; non-ducted propagation of continuous hiss with a narrow frequency range (<20 kHz) after emerging from a duct exit at higher altitudes (3000-5000 km) ; and coherent transmission from an exit region at the ionospheric level between Syowa and the location of a quiet aurora1 arc appearing poleward far from Syowa. Coupled with previous spacebased observations and model calculations of the aurora1 hiss power flux spectrum, we see that impulsive hiss emissions generated in a wide range of higher to low altitudes by the beam amplification are trapped in irregularities of decreased electron density outside the aurora1 arc at lower altitudes, propagate down to the ionosphere and are transmitted to the ground, and that after emerging from a duct exit at altitudes of 3OOl%5000 km, continuous hiss emissions are propagated in a non-ducted mode down to the ionosphere at different latitudes, corresponding to the initial wave normals at the duct exit, and are transmitted to the ground.

1. INTRODUCTION

A broadband hiss is believed

electromagnetic to be generated

emission

called

in the whistler

geomagnetic field line by an aurora1 observatory have shown that the continuous hiss is not generated in the immediate vicinity of a quiet aurora1 arc (Mosier and Gurnett, 1972). The coordinated observation of auroral VLF hiss between the ISIS satellite and the ground (Syowa) suggested that the continuous hiss emerges from a field-aligned duct at an altitude higher than the satellite level (1400 km) and is propagated down to the ionosphere in a non-ducted mode of propagation (Makita, 1979). Observations of lowenergy aurora1 electrons onboard polar orbit satellites have established electron precipitation of inverted Vshape in the aurora1 regions (Frank and Ackerson, 1971), a good correlation between the regions of the inverted V electron precipitation and the aurora1 arc (Meng, 1976), and the occurrence of inverted Vshaped aurora1 hiss associated with the electron precipitation (Gurnett and Frank, 1972). Based on the characteristic V-shaped low-frequency cutoff of the radiation observed at low altitudes, ray path studies have indicated that the aurora1 hiss is generated in a spatially localized source at altitudes from 5000 to 10,000 km and is propagated down to the satellite level at wave normal angles very close to the resonance cone (in the quasielectrostatic whistler mode) (Gurnett and Frank, 1972 ; Gurnett et al., 1983). As mentioned above, the generation and propagation of auroral hiss observed onboard satellites are rather well

aurora1 mode

by

(100 eV-10 keV) aurora1 electrons (Martin et al., 1960; Jorgensen, 1968; Gurnett and Frank, 1972). Aurora1 hiss emissions are observed on the ground (Syowa, geomag. lat. -7O”, L = 6.1) during the period of evening to night, predominantly before the local magnetic midnight. From its appearance on frequency-time spectrograms, the hiss can be categorized into two types of impulsive and continuous emissions (Tanaka, 1972; Kokubun et al., 1972; Tanaka et al., 1976; Makita, 1979). The impulsive hiss has a wide frequency range of more than 100 kHz with a spectral peak frequency of about 10 kHz, and its duration is usually no more than 10 min. The hiss usually occurs in association with moderate aurora1 (N 5 kR) and geomagnetic [ - 200 nT (AH)] activities, and the intensity varies almost simultaneously with the intensity of the aurora (Srivastava, 1976 ; Nishino et al., 1981). The continuous hiss has a narrow frequency range of less than - 20 .kHz and a long time duration of more than a few tens of minutes. The hiss is observed at weak geomagnetic activities less than -50 nT (AH) in association with a quiet aurora1 arc. Simultaneous observations of aurora1 hiss by the Znjun 5 satellite and of visual aurora along the same low-energy

259

260

Y.

TANAKA and M. NISHINO

understood, but those of aurora1 hiss observed on the ground are scarcely comprehended. So, in order to elucidate the propagation of aurora1 hiss observed on the ground and the transmission out of the ionosphere, the newly developed direction findings (DFs) were carried out in 1978 at Syowa station, Antarctica (Nishino et al., 1981). The purpose of this paper is to deduce the propagation of aurora1 hiss in the magnetosphere and the ionosphere to satisfy the results of the DF measurements, and then to understand the generation and propagation of ground-based aurora1 hiss of the two (impulsive and continuous) types. 2. RESULTS FROM DIRECTION FINDINGS

VLF emissions usually exhibit a temporally incoherent phase pattern, resulting in a temporally changing wave polarization and waveform. Based on the measurement of time differences of wave arrival, a DF technique for aurora1 VLF hiss has been developed, which is independent of wave polarization as well as of waveform. The arrival time differences among the signals received at three spaced observing points are determined by computing the cross-correlation among the received wide-band (558 kHz) signals (Nishino et al., 1981). Results obtained from the DF observations are discussed below, referring to the location of the aurora. 2.1. Impulsive auroral hiss From a comparison of the DF results with the ground-based aurora data (all-sky photographs of auroras), it has been found that impulsive hiss with a wide frequency range does not emerge from the whole region of a bright aurora but from some localized regions of bright electron auroras at the ionospheric level, and that the arrival directions of aurora1 hiss change rapidly in accordance with the aurora1 movements (Nishino et al., 1981). Figure 1 shows a typical example from the results of aurora1 hiss observations in 1978 at Syowa, where an impulsive-type aurora1 hiss started simultaneously with a sharp negative decrease of the geomagnetic disturbance. Foldingform amoral arcs which initially extended above the horizon suddenly expand towards the zenith. The arrival directions of the hiss are well in accord with the folding areas in the active auroras, and subsequently the hiss intensity decreases with the wide expansion of a bright aurora around the zenith, due to the heavy absorption of the hiss in the lower ionosphere ionized by precipitating aurora1 particles. However, a comparison of the centre of the hiss exit region with the active region of the aurora projected

to the ionospheric level of 100 km has shown that both locations are not always coincident. The distance between the centre of the exit region of the impulsive hiss propagated approximately along the geomagnetic meridian plane (within f 15” off the meridian plane), and the location of the active region of the aurora along the meridian plane, has been investigated. Figure 2 shows these distances at the ionospheric level obtained from the DF observations. 2.2. Continuous aurora1 hiss Continuous aurora1 hiss with a narrow frequency range less than -20 kHz usually occurs at a weak geomagnetic activity in association with a quiet auroral arc. The exit region of the hiss is located at a higher latitude than Syowa and also at a lower latitude than the aurora, which usually appears poleward of Mizuho (geomag. lat. -72.3”, L = 7.1) (Nishino et al., 1982). Figure 3 shows a typical example from the results of observations of continuous hiss emissions. Figure 4 shows the distance of the centre of the exit region from Syowa at the ionospheric level of 100 km for continuous hiss propagated approximately along the geomagnetic meridian plane (within + 15” off the meridian plane), and the distance of the quiet aurora1 arc from Syowa along the meridian plane. It should be noted that the continuous and impulsive hiss are of the same order of amplitude of 10m’5lOPI4 W mm2 Hz-’ at VLF, and that each of them exhibits similar temporal evolution in the intensity variations at Syowa and Mizuho (about 250 km distant from Syowa to the geomagnetic South direction), although both types of hiss emissions exhibit different characteristics in the distance between the exit region of the hiss and the aurora. 3. DISCUSSION

Below we discuss the propagation and generation of aurora1 hiss observed on the ground, based on the DF results. 3.1. Transmission of auroral hiss out of the ionosphere The impulsive hiss emerges from a localized exit region which is somewhat distant from the active region of the aurora. The exit measured from the DF at Syowa changes rapidly in accordance with aurora1 movements, but nevertheless the hiss emissions exhibit similar temporal evolution at Syowa and Mizuho. These facts suggest a ducted propagation of the hiss down to the ionosphere along a geomagnetic field line somewhat distant from the field line intersecting the active region of the aurora, and incoherent (spherical) transmission out of a localized exit region at the lower

JUNE

26

261

1978

a)

21:10

21:oo

UT

05,00, MIRIMP~~I~A~~~~L~AT 21U.T.,26Jw~ 1978 (AFTERNISHINOer al., 1981). (a) Intensity of 8 kHz aurora1 hiss. (b) Estimated azimuthal and incident angles of aurora1 hiss at 5-8 kHz. (c) All-sky photographs of aurora.

~G.~.T~MPO~LEVOL~ION~FO~ER~D~SUL~ AT SYOWA

262

Y. TANAKA and

M. NISHINO

263

Aurora1 hiss propagation , Higher

Lower

latitude

latitude

sidetexit)

-3

-2

-1

0

1

f x 100 km Distance

between

aurora

2

3

1

and hiss

exit

FIG. 2. DISTANCE BETWEEN THE CENTRE OF THE EXIT REGION AT Tik lONOSPHERIC LEVEL (100 km) OF IMPULSIVE AURGRAL

1978AT

HISS EMISSIONS OBSERVED

IN

APPROXIMATELY

THE GEOMAGNETIC

(WITHIN

+

ALONG

15” OFF

OF THE ACTIVE

THE MERIDIAN

AURORAL

SYOWA

PLANE),

REGION ALONG

AND PROPAGATED MERIDIAN

PLANE

AND THE LOCATION

THE MERIDIAN

PLANE.

Positive distance indicates a hiss exit at a lower latitude than

the location of the aurora (after Nishino ef al., 1982).

ionospheric level, onto the ground. On the other hand, simultaneous observations of aurora1 hiss by satellites with aurora on the ground have shown that the continuous hiss is not propagated down to the satellite level along the geomagnetic field line of aurora1 precipitation (Mosier and Gurnett, 1972; Makita, 1979). Moreover, the DF results indicate that the exit region of the hiss is located at a higher latitude than Syowa and also at a lower latitude than the aurora which appears polewards of Mizuho. However, the hiss emissions exhibit a similar temporal evolution at Syowa and Mizuho. These facts may suggest that a duct of the hiss approximately along the geomagnetic field line of aurora1 precipitation terminates at altitudes from 3000 to 5000 km and the hiss emissions are propagated in a non-ducted mode down to the ionosphere at different latitudes, corresponding to the initial wave normal angles at the duct exit, and are coherently transmitted onto the ground. 3.2. Abjuration of aurora1 hiss in the lower ionosphere The aurora1 ionosphere is simulated as follows. The total electron density (N) in the ionosphere and the magnetosphere is expressed as a product of three factors with different contributions : N

=

ND,

X &SON‘i

X NAURORA.

indicates a contribution factor of the enhanced ionization of the aurora1 ionosphere at lower altitudes. Details of the simulation of the aurora1 ionosphere and adequacy of the simulation have been introduced by Nishino and Tanaka (1987). The expression and parameters for the present simulation are all the same as in Nishino and Tanaka (1987). The diffusive librium model (ND,) for the magnetosphere is acterized at a reference level of 500 km by an composition of 99% O+, 1% H+, an isothermal

equicharionic

heavily

“tem-

electron and ion temperature of 2500 K, and an electron density of 1.Ox 1O4cne3. Above altitudes of the auroral ionosphere, the simulated profile of electron density would also be realistic; for example, at 1000 km altitude above which the density profile is independent of the contribution factor (NAURORA),the electron density is 2.5 x lo3 cm-’ and the ionic composition is 85% O+ and 15% H+, the values of which are reasonable for the electron density (Maeda, 1975 ; ISIS-2 data over Syowa, 1985) and the ionic composition (Maeda, 1975; Iwamoto ef al., 1982) in the night-time ionosphere in the winter season over Syowa. Wave attenuation is calculated for 30 MHz and 8 kHz waves incident vertically in the ionosphere models for which a dip angle of - 64.5” and an electron gyrofrequency of 1.6 MHz for the true geomagnetic field (IGRF 75) at Syowa, and a realistic altitude profile of collision frequency with neutral particles (Thrane and Piggott, 1966), are assumed. The attenuation values for the vertical incidence of 30 MHz waves may correspond to 30 MHz cosmic noise absorption (CNA) levels expected in the simulated ionosphere, being coincident with the observed CNA levels related to the electron densities measured by rockets in the lower ionosphere (< 90 km) over Syowa (Miyazaki et al., 1981). By means of the relation between 30 MHz CNA and geomagnetic activity observed in 1971-1978 at Syowa (Miyazaki et al., 19811, the variation (AH) of the geomagnetic horizontal component is estimated in the simulated ionosphere. Characteristics of the simulated models are shown in Table 1. The wave attenuation at VLFs such as 8 kHz indicates a comparatively gradual increase with increasing wave normal angles to the geomagnetic field, so that the field intensity of aurora1 VLF hiss attenuated through the lower ionosphere could be evaluated, representatively, by the attenuation value of 8 kHz waves incident vertically in the aurora1 ionosphere. In the ionosphere model (4) during a severe aurora1 disturbance, 8 kHz waves are attenuated.

Such heavy

attenuation

ND, is a diffusive equilibrium model for the magporally” induces the depressed intensity of aurora1 netosphere. AiLIoN is an appropriate reduction factor hiss below a threshold level of detection on the expressing the effect of the lower ionosphere. NAUaoRA ground, and “spatially” induces the emergence of hiss

Y. TANAKA

264

from a localized exit region somewhat the centre of the aurora1 active region.

distant

and M. NISHINO

from

3.3. Inverse ray tracing As shown in Fig. 4, the exit region of continuoustype aurora1 hiss is located at a lower latitude than a quiet aurora1 arc, and the distance between the centre of the exit region and the aurora is more than 200 km. Ray and wave normal directions have been computed in a centred dipole magnetic model by using a twodimensional (geomagnetic latitude and altitude) ray tracing computer programme (Walter, 1969). For continuous aurora1 hiss emissions, ray tracings have been made inversely for 8 kHz waves incident from free space into the model (1) of the quiet ionosphere sharply bounded at 100 km altitude until the rays encountered the geomagnetic field lines of aurora1 precipitation, based on the locations of the centre of the hiss exit region and the aurora represented in Fig. 4. The altitudes at which the rays intersect the geomagnetic field lines of aurora1 precipitation with wave normal angles directed almost vertically upwards are represented in Fig. 5. Based on these inverse ray tracings, the propagation of continuoustype aurora1 hiss is schematically shown in Fig. 6.

3.4. Generation of aurora1 hiss Observed power fluxes of the VLF hiss by satellites are often as high as 10-l’ W m-* Hz-’ (Gurnett and Frank, 1972; Mosier and Gurnett, 1972). The computations of incoherent Cerenkov radiation including propagational spreading (James, 1973), collisional damping (Taylor and Shawhan, 1974) and collisionless damping (Maggs, 1976), indicate that the incoherent mechanism cannot produce the observed power fluxes. It has been proposed that the observed bandwidth and power fluxes result from convective beam amplification of incoherent Cerenkov whistler radiation by the beam of precipitating aurora1 electrons (Maggs, 1976). Based on the model calculation of the aurora1 hiss power flux spectrum resulting from the beam amplification mechanism developed by Maggs (1976), Yamamoto (1979) suggested generation mechanisms of the continuous hiss with a “narrow” frequency range and the impulsive hiss with a “wide” frequency range as follows. The noise emitted at an altitude much higher than an observation point in the beam precipitation region (the aurora1 arc) is eventually absorbed by the beam electrons even if it is amplified sufficiently, so that high power flux levels of aurora1 VLF hiss can be observed at low altitudes outside the aurora1 arc rather than inside,

South

. . l. . l. l .

. .. . . . .

_ -200

South

,0

200

400

600

km

syowa Distance

of

aurora

from

Syowa

FIG. 4. RELATIONSHIP BETWEEN THE DISTANCE OF THE CENTRE OF THE EXIT REGION FROM SYOWA AT THE IONOSPHERIC LEVEL (100 km) FOR CONTINUOUS HISS EMISSIONS PROPAGATED APPROXIMATELY ALONG THE GEOMAGNETIC MERIDIAN PLANE (WITHIN+~~~ OFF THE MERIDIAN PLANE) AND TfrIz DISTANCE OF THE QUIET AURORALARCALONGTHEMERIDIANPLANEFROMSYOWA.

265

Aurora1 hiss propagation TABLE

1. CHARACTERISTICS OF THE SIMULATED AURORAL IONOSPHERE WHICH ARE CLASSIFIED INTO FIVE MODELSACCO~INGTO~AURO~LACTIVIT~. Atten~~onis~alculated for 30 MHzand 8 kHz wavesincidentverti~liyinto ~eionospheremodels.The variationofthe geomagnetic ho~zont~component (~ isestimated by the relationwith 30 MHzCNA

at Syowa (Miyazaki et al., 1981). Ionosphere models

Aurora1 disturbance

I:;

Weak Quiet

;;

Severe Moderate

(5)

Severe

Luminosity (OH 5577 8) (kR)

30 MHz CNA (dB)

No visible 0.5--laurora

0.15 0.07

50 0

14.3 8.7

IO 5

0.65 2.66

440 200

65.2 30.2

> 10

4.03

600

82. I

. l

. . l

.

* .

.’ :

SyOWa

Oaomagnetic

Latitude

FIG. 5. RELATION BETWEEN THE GEOMAGNETIC LATITIJDE OF AURORAL PRECIPITATION AT THE IONOSPHERICLEVEL (100 km) AND THE ALTITUDE AT WHICH A 8 kHz RAY INTERSECTS THE CORRESPONDING FIELD LINE OF AUROP.AL PRECIPITATION, OBTAINED FROM INVERSE RAY TRACINGS OF 8 kHz WAVES INCIDE~~OM~F~ESPACEI~OT~~~RBO~ARY (100 km) oF~QU~ION~P~~[MODEL (l)]BY MEANSOF T~~LA~ONSHIPBE~ENT~~SSEXITAND~AURO~L PRECIPITATION INDICATED IN FIG. 4.

and significant magnetic components of the VLF hiss measured by satellites (Mosier and Gurnett, 1972) can be explained by propagation of waves amplified with wave normal angles close to the resonance cone angle at much higher altitudes. When the beam is weak, the VLF waves cannot be amplified enough except at high altitudes and so the hiss at the ground levels will have a comparatively “narrow” frequency spectrum. On the other hand, in the strong beam case the hiss with a “wideband” spectrum could be observed at ground levels since it can be sufficiently amplified even at low altitudes. A rocket experiment has shown that the intensity of aurora1 hiss was greater outside the auroral arc than within the arc and a maximum intensity

pn;

8 kHz attenuation (de)

of IO-* V’ my2 Hz“ at 30 kHz was seen in the dark region outside the arc, and that the correlation between 0.147 keV electron flux and hiss intensity was not better than the correlation with the flux of 5 keV electrons (Bering et al., 1987). Aurora1 electrons precipitating from the boundary plasma sheet in the aurora1 plasma between altitudes of 3000 and 10,000 km are accelerated downward by a l-10 kV potential drop along the geomagnetic field lines (Mozer et al., 1979) intersecting a localized latitudinal region (1” N 2” width) in the aurora1 oval. The accelerated aurora1 electron fluxes with a sharp energetic spectra1 peak in l-10 keV and with an inverse V-shaped latitudinal dist~bution of the peak energy (Gumett and Frank, 1972) are closely related to the aurora1 arcs (Meng, 1976), and the electron beam amplification mechanism mentioned above can account for the observed power fluxes of aurora1 hiss. The boundary plasma sheet which is connected with invariant latitudes of 70”-75” for the quiet time of a substorm is shifted -5” equatorward of quiet times as the substorm develops, and the average energy and energy flux of the electron flux increase (Winningham et al., 1975). The above facts lead us to understand comprehensively that the continuous hiss with a narrow frequency range observed on the ground is generated at altitudes higher than -3000 km by the amplification mechanism of the electron beam associated with a quiet aurora1 arc appearing poleward of Syowa (A = 66. I “) and Mizuho (68.0”), and that the impulsive hiss with a wide frequency range is generated in a wide range of higher to low altitudes by the amplification mechanism of the strong beam associated with a bright aurora expanding over the ground stations and changing rapidly in intensity. 3.5. Propagation Figure 7 shows and lower-hybrid the geomagnetic

of aurora1 hiss

electron plasma (f,) and gyro (fuJ resonance (fLHR)frequencies along field line of a centred dipole model

266

Y. TANAKA

Propagation

M.

and

NISHINO

of continuous

hiss

Mizuho

Geomagnetic

Latitude

FIG. 6. SCKEMATIC DIAGRAM

SHOWING THE PROPAGATION OF CONTINUOUS P~CIPITATINGELE~RONSOFQU~TAuRORA~~~.

at 70” on the Earth’s surface in the magnetosphere which is represented by a diffusive equilibrium model indicated in Section 3.2. Due to the small ratio oft’,, /c for low-energy aurora1 electrons, Cerenkov emissions in the whistler mode occur at large indices of refraction. Thus, the whistler radiation is propagated at a wave normal very close to the resonance cone, At altitudes of 300~10,000 km where aurora1 VLF hiss could be amplified by the beam of aurora1 electrons, the ray direction of the VLF hiss is almost parallel to the magnetic field, due to the resonance cone angle [O,, = cos-‘(f/~ne)] being close to 90”. So, the VLF hiss can be propagated down to the ionosphere level, but the waves with large wave normal angles cannot be transmitted out of the ionosphere onto the ground. For the transmission of aurora1 hiss emissions with

AURORAL

HISS AND

THE

sufficient intensities onto the ground, a ducted propagation of the waves in a field-aligned irregularity of electron density would be required. For simplicity we discuss trapping of waves infieldaligned irregularities of increased density (crests) and decreased density (troughs) for which the magnetic field is constant in magnitude and direction and the electron density varies only in the direction normal to the field. The magnitude of the irregularity is given by an enhancement factor defined for troughs as &=$#and crests as

1,

Aurora1 hiss propagation kHz

r

103

f Pe 10*

10

I

103

!

104

Altitude(km) FIG. 7. ELECTRON PLASMA (f,), GYRO (fHs) AND LOWERHYBRID RESONANCE (fLHR) FREQUENCIES IN THE MAGNETOSPHEREMODEL. The diffusive equilibrium model along a dipole field line at

70” on the gro&d is characterized at a reference level of 500 km bv an ionic composition of 99% O+, 1% H+. an isothennai electron and ion temperature of 2560 K, and an electron density of 1.0 x lo4 cmm3. where N(0) is the electron density on the axis of the irregularity, and N(p) is the electron density at the outermost excursion of the ray path from the axis (Helliwell, 1965). For the ratio (f/f& < 0.5, the crest trapping occurs only when the initial wave normal angles (0,) with the axis are smaller than O2 [Q, = cos-‘(2f/f,&]. At altitudes of the hiss generation region in the magnetosphere model, the crest trapping cannot occur for initial wave normals very close to the resonance cone. However, as the initial wave normals deviate from the resonance cone, the crest trapping will become more feasible because the enhancement factor required for trapping decreases. For example, for 8 kHz waves with initial wave normal angles deviating more than 30” from the resonance cone angle, the enhancement factors decrease down to realistic values less than 1.0 (100%). For initial wave normals close to the resonance cone, the trough trapping is more plausible. The enhancement factor (ET) for troughs is given by

cos26, ET = 4Cf/.f”e)bs

00 - (.f/ffie)l

- I

(Helliwell,

1965).

267

The enhancement factor is calculated as a parameter of an angle (A@) deviating from the resonance cone, defined as A0 = &-BO. Figure 8 represents the calculated enhancement factor as a function of the wave frequency normalized by the gyrofrequency, corresponding to the altitude at each of two representative frequencies, 8 and 100 kHz. If the initial wave normals could deviate slightly (lo-Z’) from the resonance cone, the waves would be trapped in irregularities of decreased density with realistic values (< -0.5) of ET, and would be propagated down to the ionosphere level. However, a deviation such as lo-2” is impossible for Cerenkov whistler radiation generated and propagated in the magnetosphere model except for waves satisfying the following specific conditions : (1) waves at VLF generated above - 10,000 km by precipitating electrons of more than - 10 keV, (2) waves propagated below - 1500 km and refracted towards the vertical direction, due to a vertically large increase of electron density with decreasing altitude in this region, and (3) waves scattered from small-scale field-aligned irregularities. At all events, impulsive aurora1 hiss emissions can be trapped, in most cases, in irregularities of trough type outside the aurora1 arc at lower altitudes, and are propagated down to the ionosphere level and transmitted onto the ground. If VLF waves were scattered to some degree (5”-10” from the resonance cone), the scattered energies would be trapped in irregularities of the trough type, and would be propagated down to altitudes of 300&5000 km. As below these altitudes the enhancement factors for trough trapping increase rapidly, the ducts may terminate effectively at these altitudes, so that aurora1 VLF hiss of continuous type observed on the ground would be propagated down to the ionosphere in a non-ducted mode after emerging from the ducts. Finally, we deduce the intensities of aurora1 VLF hiss emissions of both types in the generation regions. Impulsive hiss usually occurs in association with moderate aurora1 (- 5 kR) and geomagnetic [ - 200 nT (AH)] activities, and continuous hiss usually occurs at weak geomagnetic activities [ <50 nT (AH)] in association with a quiet aurora1 arc appearing poleward far from Syowa. From the attenuation of 8 kHz waves through the aurora1 ionosphere indicated in Table 1, we would, therefore, adopt a wave attenuation of 30.2 dB in the ionosphere model (3), and an average value of 11.5 dB in models (1) and (2), for aurora1 VLF hiss emissions of impulsive and continuous types, respectively. For impulsive hiss emissions radiated incoherently (spherically downward) from a localized exit region at the boundary of the ionosphere onto the ground, we assume a defocussing

Y. TANAKA

268

and

M. NISHINO

Altitude(km) for 8 kHz ,I I

(

.

.I-...1

f/f,,)

FIG. 8. VARIATIONSOF ENHANCEMENT FACTORS(ET) FOR TROUGHTRAPPINGWITH THE WAVE FREQUENCY NORMALIZED BY THE GYROFREQUBNCY (f/fHe) AS A PARAMETER OF AN ANGLE (A@) DEFINED AS & = &$-At’, WHERE&_ IS THE RESONANCE CONE ANGLE AND o. IS THE INITIAL WAVE NORMAL ANGLE. At the top, altitudes corresponding to (f/f& along the geomagnetic field line at 70” on the ground are represented for 8 and 100 kHz.

loss of 20 dB. We also assume a loss of 20 dB for continuous hiss emissions scattered so as to have the wave normals deviated slightly (-5”) from the resonance cone. Based on the intensities of 10-‘5-10-‘4 W m-’ Hz-’ for aurora1 hiss emissions of both types on the ground, are estimated

the intensities in the generation to have plausible

values of

regions

10P’o-10~9

and 10~‘2-10~1~ W m-* Hz-’ for impulsive tinuous hiss emissions, respectively.

and con-

REFERENCES Bering, E. A., Maggs, J. E. and Anderson, H. R. (1987) The plasma wave environment of an aurora1 arc, 3. VLF hiss. J. geophys. Rex 92, 758 1.

Amoral

hiss propagation

Frank, L. A. and Ackerson, K. L. (1971) Observations of charged particle precipitation into the amoral zone. J. geophys. Res. 76, 3612. Gumett, D. A. and Frank, L. A. (1972) VLF hiss and related plasma observations in the polar magnetosphere. J. geophys. Res. 77, 172. Gurnett, D. A., Shawhan, S. D. and Shaw, R. R. (1983) Aurora1 hiss, Z mode radiation, and aurora1 kilometric radiation in the polar magnetosphere. J. geophys. Res. 88, 329. Helliwell, R. A. (1965) Whistlers and Related Ionospheric Phenomena, p. 45. Stanford University Press, Palo Alto, California. ISIS-2 data over Syowa (1985) N-h profile at night, January and June, 1977 over Syowa. Sci. Antarctica 9, p. 133. Data Compil., Natn. Inst. Polar Res., Tokyo. Iwamoto, I., Sagawa, E. and Suitz, T. (1982) Observation of the ion composition by ionosphere sounding satellite (ZSS-b). Rev. Radio Res. Labs Tokyo 28, No. 146,457. James, H. G. (1973) Whistler mode hiss at low and medium frequencies in the day side cusp ionosphere. J. geophys. Res. 78, 4578. Jorgensen, T. S. (1968) Interpretation of aurora1 hiss measured on Ogo 2 and at Byrd Station in terms of incoherent Cerenkov radiation. J. geophys. Res. 73, 1055. Kokubun, S., Makita, K. and Hirasawa, T. (1972) VLF-LF hiss during polar substorm. Rep. Zonosph. Space Res. Japan 26, 138. Maeda, K. (1975) A calculation of aurora1 hiss with improved models for geoplasma and magnetic field. Planet. Space Sci. 23, 843. Maggs, J. E. (1976) Coherent generation of VLF hiss. J. geophys. Res. 81, 1707. Makita, K. (1979) VLF-LF hiss emissions associated with aurora. Mem. natn. Inst. Polar Res. Japan Series A, 16, 90. Martin, L. H., Helliwell, R. A. and Marks, K. R. (1960) Association between aurorae and very-low-frequency hiss observed at Byrd Station, Antarctica. Nature 187, 751. Meng, C. (1976) Simultaneous observations of low-energy electron precipitation and optical aurora1 arcs in the evening sector by the DMSP 32 satellite. J. geophys. Res. 81, 2771. Miyazaki, S., Ogawa, T. and Mori, H. (1981) Some features of nighttime D and E region electron density profiles in

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the polar ionosphere. Mem. natn. Inst. Polar Rex Japan Special Issue 18, 304. Mosier, S. R. and Gurnett, D. A. (1972) Observed correlations between aurora1 and VLF emissions. J. geophys. Res. 77, 1137. Mozer, F. S., Cattell, C. A., Temerin, M., Torbert, R. B., Von Glinski, S., Woldorff, M. and Wygant, J. (1979) The dc and ac electric field, plasma density, plasma temperature, and field-aligned current experiments on the S3-3 satellite. J. geophys. Res. 84, 5875. Nishino, M. and Tanaka, Y. (1987) Observations of aurora1 LHR noise by the sounding rocket S-310JA-6. Planet. Space Sci. 35, 127. Nishino, M., Tanaka, Y., Iwai, A. and Hirasawa, T. (1981) A new direction finding technique for aurora1 VLF hiss based on the measurement of time differences of arrival at three spaced observing points. Planet. Space Sci. 29, 365. Nishino, M., Tanaka, Y., Iwai, A., Kamada, T. and Hirasawa, T. (1982) Comparison between the arrival direction of aurora1 hiss and the location of aurora observed at Syowa Station. Mem. natn. Inst. Polar Res. Japan Special Issue 22, 35. Srivastava, R. N. (1976) VLF hiss, visual aurora and the geomagnetic activity. Planet. Space Sci. 24, 375. Tanaka, Y. (1972) VLF hiss observed at Syowa Station, Antarctica. II-Occurrence and polarization of VLF hiss during disturbances. Proc. Res. Inst. Atmos. Nagoya Univ. 19, 63. Tanaka, Y., Hayakawa, M. and Nishino, M. (1976) Study of aurora1 VLF hiss observed at Syowa Station, Antarctica. Mem. natn. Inst. Polar Res. Japan Series A, 13,40. Taylor, W. W. L. and Shawhan, S. D. (1974) A test of incoherent Cerenkov radiation for VLF hiss and other magnetospheric emissions. J. geophys. Res. 79, 105. Thrane, E. V. and Piggott, W. R. (1966) The collision frequency in the E- and D-regions of the ionosphere. J. atmos. terr. Phys. 28, 721. Walter, F. (1969) Non-ducted VLF propagation in the magnetosphere. Tech. Rep. SEL-69-061, Radiosci. Lab., Stanford University. Winningham, J. D., Yasuhara, F. and Akasofu, S.-I. (1975) The latitudinal morphology of 10 eV to 10 keV electron fluxes during magnetically quiet and disturbed times in the 2100-0300 MLT sector. J. geophys. Res. 80, 3148. Yamamoto, T. (1979) On the amplification of VLF hiss. Planet. Space Sci. 27, 273.