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The morphologies of low-latitude and auroral VLF ‘hiss’
inNorthern Ireland Pergamon Press.Printed Journal ofAtmospheric andTerrestrial Physics, 1975, Vol.37,pp.517-529.
The morphologies of low-latitude and aurora1VLF %iss’ MASASHI HAYAKAWA, The Research Institute
YOSHIHITO TANAKA and JINSUKE
of Atmospherics,
Nagoya University,
Toyokawa,
OHTSU Aichi, 442, Japan
(Received 11 March 1974; in revised form 1 June 1974) Abstract-The purpose of this paper is to make the extensive comparison of features between the aurora1 and low-latitude VLF hiss from the morphological standpoint. Then it is found that the low-latitude hiss is not considered to be the consequence of the Earth-ionosphere waveguide mode propagation of the aurora1 hiss and it is totally different from the aurora1 hiss. Further, the low-latitude VLF hiss is found to be divided into two types. One is the quiet-time hiss and the other the storm-time one. The diurnal variation of the occurrence rate of quiet-time hiss shows a quite similar shape with that of the occurrence rate of lowlatitude whistlers, which may be resulted from the diurnal variation of the ionospheric absorption of plasmaspheric VLF hiss. Then the diurnal variation of the occurrence rate of the storm-time hiss consists of two peaks in the evening and in the morning. The characteristics of the evening and morning hiss are different from each other, which suggests that the asymmetric structure of the plasmasphere has an essential influence on the generation of the storm-time VLF hiss. Lastly, we discuss the association of aurora1 and stormassociated low-latitude hiss with the magnetospheric substorms and storms.
1. INTRODUCTION
EMISSIONS known as ‘hiss’ are the least investigated phenomena of various kinds of VLF emissions (JORGENSEN, 1966 ; KIMURA, 1967; HELLIWELL, 1969). One reason of this is that it is difficult to distinguish hiss from other noises owing to the white noise character of hiss. Moreover, on frequency-time spectrogram of VLF noise, it is difficult to identify hiss due to the lack of structure in hiss. JORGENSEN (1966) have summarized the observations of hiss in the frequency range 4-9 kHz at thirteen stations in the world and found that the intensity of hiss is relatively constant in the aurora1 zone, while it decreases with decreasing latitude outside the aurora1 zone. The obtained rate of decrease of maximum spectral density with latitude of about 10 dB/lOOO km and a satellite evidence of correlation of aurora1 hiss with particle precipitation have led him to conclude that VLF hiss is exclusively generated in the aurora1 zone and it propagates to lower latitude via waveguide mode propagation below the ionosphere, resulting in the low-latitude hiss. Since his work, the low-latitude hiss observed on the ground is thought to be the consequence of waveguide mode propagation of aurora1 hiss (RAO et al., 1972). In addition to the aurora1 hiss, HARANG (1968) has found a new type of VLF hiss named ‘low-latitude’ type hiss which show features significantly different from aurora1 hiss. This low-latitude hiss is characterized by the fact that it appears at daytime with considerable intensity at stations of lower latitudes than the aurora1 zone during and after great magnetic disturbances and it is usually not recorded close to the aurora1 zone. VERSHININ (1970) has also observed hiss belonging to the category of low-latitude hiss. The hiss observed by him occurred during evening to midnight hours and it differs, in time of occurrence, from the daytime hiss found by
VLF
HARANG (1968). 517
518
MA~ASHI HAYAKAWA, YOSHIHITO TANAKAand JINSWEOHTSU
However, the clear understanding of the distinction in characteristics between aurora1 and low-latitude VLF hiss has not been obtained up to date. The purpose of this paper is, therefore, to make the extensive comparison of features between aurora1 and low-latitude VLF hiss, from the morphological standpoint. In Section 3 the detailed description of the morphology of aurora1 hiss is made and then lowlatitude hiss observed at Moshiri in Japan is discussed morphologically with special reference to its comparison with aurora1 hiss. The study of low-latitude hiss has yielded that there are two types in low-latitude hiss, one is not associated with magnetic storms (quiet-time hiss) and the other closely correlated with magnetic storms (storm-time hiss). Of the two types, the latter seems to be of greater importance in relation with magnetospheric storms. Storm-time low-latitude hiss is found to be essentially different from aurora1 hiss. In addition, such storm-time hiss can be divided into evening and morning hiss and the difference in nature between them seems to be closely related with asymmetric magnetospheric structure. In Section 4 we discuss the characteristics of aurora1 as well as low-latitude hiss with special reference to their relationship with magnetospheric substorms or storms.
2. DATA USED FOR ANALYSES The data of aurora1 and low-latitude VLF hiss are, respectively, based on the observations at Syowa Stat,ion in Antarctica in t’he aurora1 zone (NISHINOand
TANAKA, 1969; TANAKAet al., 1970; TANAKA, 1972) and at Moshiri in Japan (IWAI et al., 1964; TANAKAand KASHIWAGI,1968). The period of observation at Syowa Station is 1967-1969 and the observation of low-latitude hiss has been continued at Moshiri since 1964. The VLF hiss at Syowa Station was recorded on several frequencies of O-75, 2, 5, 8, 12, 25, 40 and 70 kHz, while the observing frequencies are 0.3, 2, 5 and 8 kHz at Moshiri. The geomagnetic latitude of Syowa Station is -69.6” and Moshiri is situated at a lower latitude than any other stations where hiss is observed (34.3”). Further details of the equipment at Syowa Station and Moshiri have been described in previous papers (IWAI et al., 1964; TANAKA,1972). 3. MORPHOLOQY OP AURORAI,AND LOW-LATITUDEVLF HISS 3.1. Morphology of aurora1 hiss Aurora1 hiss is named on account of its close association with aurora1 display (M&TIN et al., 1961; MOROZUMI and HELLIWELL,1966). The diurnal variation of percentage occurrence of such an aurora1 hiss at Syowa Station at four specific frequencies of 5, 8, 12 and 25 kHz is shown in Fig. 1. An arrow in the top figure in Fig. 1 indicates the time of magnetic midnight. It is found from Fig. 1 that aurora1 hiss is predominantly observed during a limited local time period around midnight, i.e. 2000 h to 0400 h LT for all the relevant frequencies. These results are in good agreement with those of earlier works by HARANGand LARSEN(1965), JOROENSEN (1966) and HARANQ(1968). In addition to the occurrence of hiss around midnight, the satellite observations have demonstrated an additional maximum in hiss intensity in the afternoon approximately 1400 h LT (GURNETT,1966; MCEWENand BARRINGTON, 1967; BULLOUOH et al., 1969; LAASPEREet al., 1971; HUGHESand KAISER,1971; KAISER,1972). The disappearance of afternoon hiss on the ground is probably due to the enhanced absorption of VLF waves in the afternoon or due to the
The morphologies of low-latitude and aurora1 VLF ‘hiss’
519
5 kHz
Fig. 1. Diurnal variation of percentage occurrence of auroral hiss observed at Syowa Station at four specific frequencies. The arrow in the top figure indicates the time of magnetic midnight.
difference in S/N ratio. The peak intensity of aurora1 hiss is found to follow the aurora1 oval (RUSSELL, 1972; KAISER, 1972). The zone of hiss appears to coincide with the precipitation zone of soft electrons (~1 keV) (HARTZ and BRICE, 1967; PAULIKAS, 1971), which has led JORQENSEN (1968) to present the reasonable hypothesis that aurora1 hiss may be generated by incoherent Cerenkov radiation from incoming aurora1 electrons with energies of the order of 1 keV. The weaker hiss in the afternoon may be due to this incoherent Cerenkov mechanism. But recent studies by LIM and LAASPERE ( 1972)) GURNETTand FRANK ( 1972) and TAYLOR and SHAWHAN (1974) have shown that the particle fluxes at night are certainly too low to explain the observed intensity and then they have suggested that a coherent plasma instability mechanism is involved in aurora1 hiss generation. The seasonal variation of occurrence rate of hiss at 12 kHz observed at Syowa Station is shown in Fig. 2. It is found that aurora1 hiss tends to appear predominantly in winter season. This seasonal dependence can be attributed to the seasonal effect of ionospheric absorption, as suggested by HARAN~ and HAUGE (1965). It is interesting to investigate the association of aurora1 hiss with the geomagnetic activity. Figure 3 shows the occurrence probability of aurora1 hiss at Syowa Station at 12 kHz versus K-index at Syowa. From this figure it is seen that the occurrence probability of aurora1 hiss has a positive correlation with geomagnetic activity during slightly and moderately disturbed periods and it decreases with further
520
MASASHIHAYAKAWA, YOSHIHITOTANAKA and JINSUKEOHTSU
0
4
“‘OB”‘.’
6
8
10
12
Month Fig. 2. Seasonal variation of occurrenceof aurora1hiss.
The index of magnetic activity has a good enhancement of geomagnetic activity. correlation with precipitating electrons in the aurora1 zone (KUCK, 1970). So the ionization in the lower ionosphere is greatly enhanced during severe magnetic activity, which then results in the depleted occurrence of VLF emissions (ECKLUND et al., 1965), being consistent with the result of Fig. 3. Figure 4 shows the occurrence histogram of the duration of amoral hiss at 12 kHz. It is clear that the duration of the aurora1 hiss is predominantly of the order of less than an hour. This duration is of the same order with the duration of polar substorms (AEASOFU, 1968) and hence the aurora1 hiss can be considered to be a temporThe hiss with the duration more than a few hours is ally localized phenomenon. seldom observed. Of course, the duration of amoral hiss shows slight dependence on local time. The intensity of aurora1 hiss observed on the ground is typically of the order of m1O-14 W/m2/Hz (JORGENSEN, 1966 ; TANAKA, 1972). While, the maximum hiss
Fig. 3. Dependence of occurrenceprobability of aurora1hiss on magnetic activity.
The morphologies of low-latitude and aurora1VLF ‘hiss’
521
Fig. 4. Occurrence histogram of duration of aurora1hiss. intensity measured on board the satellites is about lo-l2 W/m2/Hz (GURNETT,1966; RUSSELL,1972), being higher by two orders of magnitude than that observed on the ground. This may be resulted from the total as well as partial reflections in the ionosphere (THOMASand SMEATHERS, 1971), the ionospheric absorption (ONDOH, 1963 ; HAYAKAWAand OHTSU,1972 ; HAYAKAWA,1974a, b) and divergence in the
space after penetration through the ionosphere. The frequency spectrum of hiss intensity is illustrated in Fig. 5 which is based on seven hiss events. Figure 5 indicates that the maximum intensity takes place at frequency around 10 kHz, being in good agreement with the previous result of JORGENSEN (1968). The intensity of hiss at a specific frequency increases as the magnetic
activity
increases to a critical value and it decreases when the activity
1
100
10
Frequency,
kHz
Fig. 5. Frequency spectra of aurora1hiss based on seven hiss events.
622
MA~ASHIHAYAKAWA, YOSEIIHITO TANAKA and JINSUKEOHTSU
exceeds such a critical value (TANAKA, 1972). This tendency seems to be identical to that for the association of occurrence rate with magnetic activity. The most interesting characteristic of the frequency spectrum is the relation of hiss band with magnetic activity. When the magnetic activity is enhanced during moderately disturbed conditions, the frequency spectrum of hiss becomes very wide, that is, the upper cutoff frequency extends up to more than 100 kHz (DOWDEN, 1960; JORGENSEN, 1968;
TANAKA, 1972).
3.2. Morphology
of low-latitude VLF hiss
The morphological features of low-latitude VLF hiss observed at Moshiri in Japan (geomag. lat. 34.3”N) are discussed in comparison with those of aurora1 hiss. The association of low-latitude VLF hiss with magnetic activity seems to be distinctly different from that of aurora1 hiss. Figure 6 illustrates the occurrence probability of low-latitude hiss at 5 kHz versus magnetic activity. The daily sum 1.0
r 13
29
54 45
0
r
13
1
10
I
I
20
30
I
40
240
KP Fig. 6. 2 Kp dependence of low-latitude VLF hiss observed at Moshiri Station. The value on the rectangle means the number of events for each 2 Kp range. I:
of ?cp index is used as the measure of geomagnetic activity. It is clear from the figure that the occurrence probability of low-latitude hiss increases with increasing C kp. We define tentatively the boundary between quiet- and storm-time as 2 kp = 30. Then about three fourths of the total hiss events occur during relatively quiet periods. We call these ‘quiet-time hiss’. While, the remaining onefourthisclosely correlated with severe magnetic storms and is named ‘storm-time hiss’. Especially, we can expect such a surprisingly high value of NO.9 in occurrence probability for great magnetic storms whose 2 kp exceed 40. The hiss appearing in a high probability during severe magnetic storms differs essentially from aurora1 hiss which occur during moderately disturbed conditions. Figure 7 shows the temporal variation and duration of the storm-time hiss. For each particular magnetic storm the duration of each of the initial, main and recovery phases is normalized to the corresponding mean duration (all events) depicted in the graph. It is seen from Fig. 7 that most of storm-time VLF hisses tend to occur
The morphologies
i? ‘,o
f
of low-latitude
and aurora1 VLF
623
2.0 --
;
= I
‘hiss’
1.5 I
-
0.5
L -
-
0
I-
\-Initial
-l---
-
_
Yz=..
-p
-
I
phase-]-Main
__
_ ,
I
Last
phase-1
phase
t EndIng
Beginning
Fig. 7. The relationship of occurrence of storm-time low-latitude VLF hiss with the phase of magnetic storm. The mean duration of initial phase is 10 hours, and then the ration of the duration of main and recovery phases to that of initial phase is, respectively, 1.0 and 2.5. The intensity is also illustrated.
during the latter part of main phase to last or recovery phase and this coincides with the result by HARANG (1968). The intensity of storm-time hiss is of the order 10-18 W/m-2 Hz-l (I WA1 et al., 1964) as seen from Fig. 7. This value holds for quiettime hiss. The duration of low-latitude VLF hiss during quiet- as well as storm-time is shown in Fig. 8 which illustrates the occurrence histogram of duration. In the case of aurora1 hiss in Fig. 4, the maximum duration is about 3 hours at longest. In contrast, a lot of low-latitude VLF hiss have long-lasting nature and their duration is, on some occasions, as long as over five hours. As the result of appearance of low-latitude VLF hiss during main and last phase, it occurs at any time of the day. Figure 9 shows the diurnal variations of
Duration,
hour
Fig. 8. Occurrence histogram of duration of low-latitude
hiss.
624
MASASHIHAYAKAWA,YOSHIHITOTANAKAand JINSWE OHTSU
,/’
.-a,
0
A’.
1200
”
1600
2000
”
’
0000 0400
s
If
’
0800 1200 . _ LI
Fig. 9. Diurnal variation of occurrence rate of low-latitude VLF hiss associated with magnetic storms (full line) and not associated with storms (broken line). hiss associated as well as not associated with magnetic storms. Quiet-time or the term ‘not associated with magnetic storms’ roughly means the magnetic activity of local k-index less than four. In the figure the maximum occurrence rate at a certain time is normalized to unity. The hiss not associated with magnetic storms corresponds to quiet-time hiss and its diurnal variation is consisted of a pronounced peak at ~5 h MLT and a,subsidiary one at 20 h MLT. The remarkable peak at ~5 h MLT is found to coincide with the peak in occurrence rate of lowlatitude whistlers (OUTSUet al., 1963) and this suggests that the diurnal variation in Fig. 9 may be, in most parts, resulted from the ionospheric absorption. The present diurnal pattern of quiet-time hiss based on ground observations is found to be in good accord with the satellite result (KAISER, 1972). But the origin of quiet-time hiss is not well known and needs further investigations. We turn to the discussion of storm-time low-latitude hiss which is of greater significance in relation with magnetospheric consequences. It is reasonable to regard the diurnal variation of storm-time hiss as being composed of an evening peak around 20 h MLT (we call ‘evening hiss’) and a post-midnight one around 2 h MLT (‘morning hiss’). The earlier work by ELLIS (1959) has given a strong support, to such two peak structure in diurnal variation of midle-latitude VLF hiss as mentioned above. In his paper such evening and morning peaks in occurrence pattern are much more well-isolated and the occurrence peak of evening hiss occurs around 18 h LT. The evening occurrence of hiss is of the same kind with the result by VERSHININ (1970) who has found that the generation region of evening hiss is located inside the plasmapause. While, the appearance of hiss in the morning is closely correlated with D(day)-type hiss observed at lower latitudes by HARANG (1968). B’igure 10 illustrates the seasonal variation of quiet-time and storm-time hiss. The broken line refers to the case of quiet-time hiss, while the full line storm-time hiss. This figure shows that the seasonal variation of quiet-time hiss is wholly controlled by the difference of ionospheric absorption with season. On the other
occurrence rate of low-latitude
The morphologies of low-latitude and aurora1 VLF ‘hiss’
Fig. 10. Seasonal variation of occurrence of storm-time hiss (broken line).
525
(full line) and quiet-time
hand, the storm-time hiss shows less seasonal dependence, which indicates the predominant effect of generation source. The frequency spectra of quiet- as well as storm-time low-latitude hiss show distinct nature from those of aurora1 hiss. That is to say, the low-latitude VLF hiss Its center frequency is found to is always relatively of narrow band in frequency. lie in the frequency range b-5 kHz and its bandwidth is about 1 kHz. On some occasions the upper frequency limit extends up to as high as 8 kHz. According to the recent work by LIKHTER et al. (1973), the VLF emission spectrum during geomagnetic storms has two maximum, one is about 6 kHz and the other below 1 kHz. This satellite result, is likely to be in good agreement with our ground observations. 4. CORRELATION OF AURORAL AND LOW-LATITUDE VLF HISS WITH MAGNETOSPHERICSUBSTORMSAND STORMS Aurora1 hiss appears predominantly during evening to post-midnight, as is seen in Fig. 1. The close relationship of this occurrence of hiss zone with the diurnal pattern of aurora1 precipitating electrons (JORGENSEN, 1968; RUSSELL et al., 1972) has led JORGENSEN (1968) and LIM and LAASPERE (1972) to hypothesize that aurora1 hiss may be generated by incoming or precipitating aurora1 electrons during polar substorms. This idea is evidenced by the correlated observation of aurora1 hiss and soft electrons on board satellites (FRITZ and GURNETT, 1965 ; JORGENSEN, 1968). Then aurora1 hiss can be considered to be only a manifestation of polar substorms which occur intermittently and impulsively with the life time of order l-3 hours (AKASOFU, 1968). The nature of short duration of aurora1 hiss is probably resulted from the short life time of polar substorm. MOROZUMI and HELLIWELL (1966) have classified the nighttime sequence of VLF hiss occurrence into three phases Nl, N2 and N3 phases from comparisons of VLF emissions with other geophysical phenomena. The Nl phase before midnight is characterized by the appearance of intense hiss and diffuse arc-like auroras. While, the N2 phase, when impulsive hiss bursts occur almost simultaneously with a sudden increase in brightness of aurora, pi bursts and a sharp onset of cosmic noise absorption, is interpreted as the expansion phase onset of substorm in accordance with the recent. concept of magnetospheric substorm (AKASOFU, 1968). Recently, KOKUBUN et al. (1972) have examined
526
MASASHIHAYAKAWA,YOSHIHITOTANAICAand JINSUKEOHTSU
in detail the properties of hiss in iV1 and N2 phases. Generally speaking, aurora1 hiss tends to occur during moderate magnetic activity, not during severe magnetic activity. This is likely to be due to the absorption of VLF emissions by the increase in ionization of the lower ionosphere during severe magnetic activity. Aurora1 hiss sometimes propagates toward lower latitudes and observed at Actually, some of VLF hiss observed at middle latitudes are middle latitudes. found, by means of direction finding or intensity comparison at multi-stations, to be fed into the Earth-ionosphere waveguide from the ionosphere in the aurora1 zone (ELLIS, 1960, 1961; DOWDEN, 1961). As is clearly seen from Fig. 6, low-latitude VLF hiss can be divided into two types. About three fourths of total hiss events observed at Moshiri take place during relatively quiet periods (quiet-time hiss). We believe that the present paper is the first reliable report on the ground reception of low-latitude VLF hiss during The origin or mechanism of quiet-time low-latitude magnetically quiet conditions. hiss is not well understood. However, we can give the following plausible possibility. The observation by Injun III has demonstrated, for the first time, the occurrence of wide band VLF hiss at low L values less than 1.2 (GURNETT, 1968). Later observations by the satellite-Alouette 2 has shown that VLF hiss at the frequencies of 5 and 7 kHz is frequently, although less frequently than aurora1 hiss, observed in the range of invariant latitude 303 to 40” (ONDOH et al., 1972). It is significant to note that Ondoh’s latitude range of hiss is around the latitude of Moshiri. The frequency spectrum of ground-based VLF hiss is peaked at ~5 kHz which is in agreement with the satellite observation. So it is probable that the hiss reported by ONDOH et al. (1972) propagates along the field-aligned ducts and then penetrates through the ionosphere. Provided that VLF hiss is generated at low L values in relatively steady state, we can interpret satisfactorily the diurnal pattern of quiet-time hiss in Fig. 9. The pronounced peak at ~5 h MLT in Fig. 9 is found to coincide with the peak in diurnal variation of occurrence rate of low latitude whistlers given by OUTSU et al. (1963) and then Fig. 9 seems to be mainly attributed to the ionospheric absorption. The less remarkable peak at ~20 h MLT in Fig. 9 may include the effect of aurora1 hiss. This is due to the following two reasons. One is that aurora1 hiss is a phenomenon from evening to midnight during slightly and moderately disturbed conditions. The other is that aurora1 hiss sometimes propagates toward lower latitudes via Earth-ionosphere waveguide as is found by ELLIS (1960, 1961) and DOWDEN (1961) and the direction of propagation of hiss observed at Moshiri is approximately along the magnetic meridian plane, based on the goniometric measurement (IWAI and TANAKA, 1968). Except for the possibility that the quiet-time hiss at ~20 h MLT may include the propagated aurora1 hiss, we can consider the quiet-time low-latitude VLF hiss as being originated in the plasmasphere and we can call these quiet-time low-latitude hiss ‘plasmaspheric VLF hiss’ in analogy to plasmaspheric ELF hiss by THORNE et al. (1973). About one fourth of total hiss events at Moshiri are found to be closely correlated with magnetic storms, as is seen from Fig. 6. This storm-time low-latitude hiss differs essentially from aurora1 hiss. For magnetic storms whose daily sum of kp index exceed 40, we can expect the occurrence of VLF hiss in a high probability of -0.9. The storm-time hiss occurs during the latter part of main phase to last or
527
The morphologies of low-latitude and aurora1VLF ‘hiss’
recovery phase, which suggests that storm-time hiss may be closely associated with the ring current resulting from magnetic storms. And the diurnal variation of occurrence of storm-time low-latitude VLF hiss is composed of two peaks in the evening (evening hiss) and at post-midnight (morning hiss), as is seen from Fig. 9. The two peaks in Fig. 9 are not so well-isolated, but such tl structure with two peaks has been evidenced by the earlier work of ELLJS (1959). In his paper, such evening and morning peaks in occurrence pattern are well-isolated and the occurrence peak of evening hiss occurs around 18 h MLT. Furthermore, the evening hiss is thought to correspond to the hiss observed by VERSHININ (1970)) who has also found that the generation region of evening hiss is located inside the plasmapause. While, the appearance of hiss in the morning is closely correlated with D(day)-type hiss observed at lower latitudes by HARANG (1968). Taking into account the close association of storm-time hiss with the phase of storms, TANAKA et al. (1974) have examined the time lag of occurrence of hiss behind the time of maximum depression in geomagnetic field for all storm-time hiss events. Then they have statistically found that storm-time hiss may be generated by soft electrons of the order 5-10 keV drifting eastward in the ring current. The relative location of ring current with the plasmapause seems to be very important in solving the generation mechanism of storm-time hiss. Satellite observation of hiss during a severe magnetic storm (BULLOU~H et ccl., 1969; TULUNAY and HTJ~HES, 1973) has shown that the VLF hiss in the morning side is generated outside the plasmapause, while the hiss in the evening sector is observed inside the plasmapause or in the plasmasphere. Also, the occurrence of evening hiss is found to lag significantly behind the storm sudden commencement. Such asymmetric appearances of hiss with respect to the position of plasmapause is apparently owing to the asymmetric structure of the plasmasphere (CARPENTER, 1970) having the bulge region in the evening sector. Above facts suggest that the plasmapause has an essential role on the generation of VLF hiss (RUSSEL and THORNE, 1970; THORNE, 1972). Tulunay and Hughes’s result that evening hiss is located inside the plasmapause coincides well with the result obtained by ground-based observations by VERSHININ (1970). The relative location of D-type low-latitude hiss by H~RANG (1968) referred to as morning hiss in this paper with the position of plasmapause is not investigated. However, taking account of the satellite observation by TULUNAY and HUGHES (1973),it is likely that Harang’s D-type low-latitude hiss may be originated outside the plasmapause. Shove-mentioned problem will be discussed in detail in our following paper by making use of the correlated study of magnetospheric hiss during a severe magnetic storm observed on board the satellite and at many ground stations (HAYAKAWA etal. 1974). Furthermore, we will investigate the detailed characteristics of morningandeveninghiss with special reference to the comparison with the morphological study presented in this paper. Acknowledgemelzts-The of our institute.
IWAI
authors deeply appreciate the continuing encouragement of Prof. A. Thanks are also due to the referee for his useful comments and suggestion.
REFERENCES .&CASOFUs-1
1968
Polar and Magnetospheric Reidel.
Dordrecht,
Holland.
Substom8.
528
MASASHI HAYAKAWA,
BTJLLOUQH K., HUGHES A. R. W. and KAISER T. R. CARPENTER D. L. DOWDEN R. L. DOWDEN R. L. ECELUND W. L., HAROEAVES J. K. and POPE J. H. ELLIS G. R. A. ELLIS G. R. A. ELLIS G. R. A. FRITZ T. A. and GIJRNETT D. A. GURNETT D. A. GURNETT D. A. HARANU L. HARANU L. and LARSEN R. HARANU L. and HAUOE K. N. HARTZ T. R. and BRICE N. M. HAYAKAWA M. HAYAKAWA HAYAKAWA HAYAKAWA HELLIWELL HUGHES A.
YOSHIHITO TANAKA
and JINSUI(E
OHTSU
1969
Proc. R. Sot. A311, 563.
1970 1960
An& Gdophys. 26, 363. Nature, Lond. 187, 677.
1961 1965
J. geophys. J. geophys.
1959 1960 1961 1965 1966 1968 1968 1965 1965 1967 1974a
Planet. Sci. 1, J. geophys. Res. 65, 839. J. geophys. Res. 66, 19. J. geophys. Res. 70, 2485. J. geophys. Res. 71, 5699. J. geophys. Res. 73, 1096. J. atmos. terr. Phys. 30, 1143. J. atmos. terr. Phys. 27, 481. J. atmos. terr. Phys. 27, 499. Planet. Space Sci. 15, 301. Trans. Inst. Elect. Comm. Eng., Japan
1974b 1972 1974 1969 1971
Res. 66, 1587. Res. 70, 4285.
57B, 177.
IWAI A., Oasu J. and TANAKA Y. IWAI A. and TANAKA Y. JORGENSEN T. S. JORGENSEN T. S. KAISER T. R.
1964 1968 1966 1968 1972
KIMIJRA I. KOKIJB~ S., MAEITA K. and HIRASAWA T. KUCK G. A. LAASPERE T., JOHNSON W. C. and SEMPREBON L. C. LIM T. L. and LAASPERE T. MARTIN L. H., HELLIWELL R. A. and MARKS K. R. MCEWEN D. J. and BARRINUTON R. E. NISHINO M. and TANAJZA Y. ONDOH T. ONDOH T., AIKYO K. and NAUAYAMA M. Oasu J., TANAKA Y. and IWAI A. PAULIKAS G. A. RAO L. M., SOMAYAJ~LU V. V. and TANTRY B. A. P. RUSSELL C. T. and THORNE R. M. RUSSELL C. T., MCPHERRON R. L. and COLEMAN P. J. TANAKA Y.
1967 1972
Pure Appl. Geophys. (in press). Planet. Space Sci. 20, 1895. J. geophys. Res. (inpress). Rev. Geophys. Space Phya. 7, 281. The Radiating Atmosphere (Edited by B. M. MCCORMAC), p. 336. D. Reidel Dordrecht, Holland. Proc. Res. Inst. Atmos. 11, 29. Proc. Res. Inst. Atmos. 15, 1. J. geophya. Res. 71, 1367. J. geophys. Res. 73, 1055. Earth’8 Magnetospheric Process (Edited by B. M. MCCORMAC), p. 340. D. Reidel, Dordrecht, Holland. Planet. Space Sci. 15, 1427. Rep. Ionoaph. Space Res., Japan 26, 138.
1970 1971
An& Ge’ophya. 26, 689. J. geophya. Res. 76, 4477.
1972 1960
J. geophya. Res. 77, 4145. Nature, Lond. 187, 751.
1967 1960 1963 1972
Can. J. Phya. 45, 13. Rep. Jap. Ant. Res. Exp. 35, 37. J. Geomagn. Geoelect. 15, 90. J. Radio Res. Labs. Japan 19, 23.
1963 1971 1972
Bull. Proc. Res. Inst. Atmos. 13, 11. Rev. Geophya. Space Phya. 9, 659. J. Geomagn. Geoelect. 24, 261.
1970 1972
Cosmic Electrodym. 1, 67. Space Sci. Rev. 12, 810.
1972
Proc.
M. M. and OHTSU J. M., TANAKA Y. and OHTSU J. R. A. R. W. and KAISER T. R.
Res. Inst.
Atmos.
19, 33.
The morphologies
of low-latitude
TANY. and KAEETIWAQI M. Tmm Y., NISHINO M. and IWAI A. THOMAS L. and SMEATHERS J. R. THORNE R. M., SMITH E. J., BURTON R. K. and HOLZER R. E. TULUNAY Y. K. and HUUHES A. R. W. VERSHININ E. F. Reference
is also made to the following
‘hiss’
1968 1970 1971 1973
Proc. Re8. Inst. Atmos. 15, 67. Proc. Res. Inst. Atmos. 17, 43. J. atmos. terr. Phys. 33, 959. J. geophys. Res. 78, 1581.
1973 1970
J. atmos. terr. Phys. 35, 153. An& Qdophys. 26, 703.
unpublished
LIKHTER YA. I., LARKINA V. I. and MIKHAILOV Yu. M. MOROZUMI H. M. and HELLIWELL R. A.
1973
TANY., HAYAKAWA M. and OHTSU J. THORNE R. M.
1974
10
and aurora1 VLF
1966
1972
529
material: Presented at COSPAR meeting, Konstantz, FRG. Tech. Rep. Radioscience Lab. p. 66-124. Stanford Electronics Labs. Stanford Univ. SU-SEL-, U.S.A. in preparation. Proc. COSPAR/IAGA/URSI Madrid, Spain.
Symp-
The morphologies of low-latitude and aurora1VLF %iss’ MASASHI HAYAKAWA, The Research Institute
YOSHIHITO TANAKA and JINSUKE
of Atmospherics,
Nagoya University,
Toyokawa,
OHTSU Aichi, 442, Japan
(Received 11 March 1974; in revised form 1 June 1974) Abstract-The purpose of this paper is to make the extensive comparison of features between the aurora1 and low-latitude VLF hiss from the morphological standpoint. Then it is found that the low-latitude hiss is not considered to be the consequence of the Earth-ionosphere waveguide mode propagation of the aurora1 hiss and it is totally different from the aurora1 hiss. Further, the low-latitude VLF hiss is found to be divided into two types. One is the quiet-time hiss and the other the storm-time one. The diurnal variation of the occurrence rate of quiet-time hiss shows a quite similar shape with that of the occurrence rate of lowlatitude whistlers, which may be resulted from the diurnal variation of the ionospheric absorption of plasmaspheric VLF hiss. Then the diurnal variation of the occurrence rate of the storm-time hiss consists of two peaks in the evening and in the morning. The characteristics of the evening and morning hiss are different from each other, which suggests that the asymmetric structure of the plasmasphere has an essential influence on the generation of the storm-time VLF hiss. Lastly, we discuss the association of aurora1 and stormassociated low-latitude hiss with the magnetospheric substorms and storms.
1. INTRODUCTION
EMISSIONS known as ‘hiss’ are the least investigated phenomena of various kinds of VLF emissions (JORGENSEN, 1966 ; KIMURA, 1967; HELLIWELL, 1969). One reason of this is that it is difficult to distinguish hiss from other noises owing to the white noise character of hiss. Moreover, on frequency-time spectrogram of VLF noise, it is difficult to identify hiss due to the lack of structure in hiss. JORGENSEN (1966) have summarized the observations of hiss in the frequency range 4-9 kHz at thirteen stations in the world and found that the intensity of hiss is relatively constant in the aurora1 zone, while it decreases with decreasing latitude outside the aurora1 zone. The obtained rate of decrease of maximum spectral density with latitude of about 10 dB/lOOO km and a satellite evidence of correlation of aurora1 hiss with particle precipitation have led him to conclude that VLF hiss is exclusively generated in the aurora1 zone and it propagates to lower latitude via waveguide mode propagation below the ionosphere, resulting in the low-latitude hiss. Since his work, the low-latitude hiss observed on the ground is thought to be the consequence of waveguide mode propagation of aurora1 hiss (RAO et al., 1972). In addition to the aurora1 hiss, HARANG (1968) has found a new type of VLF hiss named ‘low-latitude’ type hiss which show features significantly different from aurora1 hiss. This low-latitude hiss is characterized by the fact that it appears at daytime with considerable intensity at stations of lower latitudes than the aurora1 zone during and after great magnetic disturbances and it is usually not recorded close to the aurora1 zone. VERSHININ (1970) has also observed hiss belonging to the category of low-latitude hiss. The hiss observed by him occurred during evening to midnight hours and it differs, in time of occurrence, from the daytime hiss found by
VLF
HARANG (1968). 517
518
MA~ASHI HAYAKAWA, YOSHIHITO TANAKAand JINSWEOHTSU
However, the clear understanding of the distinction in characteristics between aurora1 and low-latitude VLF hiss has not been obtained up to date. The purpose of this paper is, therefore, to make the extensive comparison of features between aurora1 and low-latitude VLF hiss, from the morphological standpoint. In Section 3 the detailed description of the morphology of aurora1 hiss is made and then lowlatitude hiss observed at Moshiri in Japan is discussed morphologically with special reference to its comparison with aurora1 hiss. The study of low-latitude hiss has yielded that there are two types in low-latitude hiss, one is not associated with magnetic storms (quiet-time hiss) and the other closely correlated with magnetic storms (storm-time hiss). Of the two types, the latter seems to be of greater importance in relation with magnetospheric storms. Storm-time low-latitude hiss is found to be essentially different from aurora1 hiss. In addition, such storm-time hiss can be divided into evening and morning hiss and the difference in nature between them seems to be closely related with asymmetric magnetospheric structure. In Section 4 we discuss the characteristics of aurora1 as well as low-latitude hiss with special reference to their relationship with magnetospheric substorms or storms.
2. DATA USED FOR ANALYSES The data of aurora1 and low-latitude VLF hiss are, respectively, based on the observations at Syowa Stat,ion in Antarctica in t’he aurora1 zone (NISHINOand
TANAKA, 1969; TANAKAet al., 1970; TANAKA, 1972) and at Moshiri in Japan (IWAI et al., 1964; TANAKAand KASHIWAGI,1968). The period of observation at Syowa Station is 1967-1969 and the observation of low-latitude hiss has been continued at Moshiri since 1964. The VLF hiss at Syowa Station was recorded on several frequencies of O-75, 2, 5, 8, 12, 25, 40 and 70 kHz, while the observing frequencies are 0.3, 2, 5 and 8 kHz at Moshiri. The geomagnetic latitude of Syowa Station is -69.6” and Moshiri is situated at a lower latitude than any other stations where hiss is observed (34.3”). Further details of the equipment at Syowa Station and Moshiri have been described in previous papers (IWAI et al., 1964; TANAKA,1972). 3. MORPHOLOQY OP AURORAI,AND LOW-LATITUDEVLF HISS 3.1. Morphology of aurora1 hiss Aurora1 hiss is named on account of its close association with aurora1 display (M&TIN et al., 1961; MOROZUMI and HELLIWELL,1966). The diurnal variation of percentage occurrence of such an aurora1 hiss at Syowa Station at four specific frequencies of 5, 8, 12 and 25 kHz is shown in Fig. 1. An arrow in the top figure in Fig. 1 indicates the time of magnetic midnight. It is found from Fig. 1 that aurora1 hiss is predominantly observed during a limited local time period around midnight, i.e. 2000 h to 0400 h LT for all the relevant frequencies. These results are in good agreement with those of earlier works by HARANGand LARSEN(1965), JOROENSEN (1966) and HARANQ(1968). In addition to the occurrence of hiss around midnight, the satellite observations have demonstrated an additional maximum in hiss intensity in the afternoon approximately 1400 h LT (GURNETT,1966; MCEWENand BARRINGTON, 1967; BULLOUOH et al., 1969; LAASPEREet al., 1971; HUGHESand KAISER,1971; KAISER,1972). The disappearance of afternoon hiss on the ground is probably due to the enhanced absorption of VLF waves in the afternoon or due to the
The morphologies of low-latitude and aurora1 VLF ‘hiss’
519
5 kHz
Fig. 1. Diurnal variation of percentage occurrence of auroral hiss observed at Syowa Station at four specific frequencies. The arrow in the top figure indicates the time of magnetic midnight.
difference in S/N ratio. The peak intensity of aurora1 hiss is found to follow the aurora1 oval (RUSSELL, 1972; KAISER, 1972). The zone of hiss appears to coincide with the precipitation zone of soft electrons (~1 keV) (HARTZ and BRICE, 1967; PAULIKAS, 1971), which has led JORQENSEN (1968) to present the reasonable hypothesis that aurora1 hiss may be generated by incoherent Cerenkov radiation from incoming aurora1 electrons with energies of the order of 1 keV. The weaker hiss in the afternoon may be due to this incoherent Cerenkov mechanism. But recent studies by LIM and LAASPERE ( 1972)) GURNETTand FRANK ( 1972) and TAYLOR and SHAWHAN (1974) have shown that the particle fluxes at night are certainly too low to explain the observed intensity and then they have suggested that a coherent plasma instability mechanism is involved in aurora1 hiss generation. The seasonal variation of occurrence rate of hiss at 12 kHz observed at Syowa Station is shown in Fig. 2. It is found that aurora1 hiss tends to appear predominantly in winter season. This seasonal dependence can be attributed to the seasonal effect of ionospheric absorption, as suggested by HARAN~ and HAUGE (1965). It is interesting to investigate the association of aurora1 hiss with the geomagnetic activity. Figure 3 shows the occurrence probability of aurora1 hiss at Syowa Station at 12 kHz versus K-index at Syowa. From this figure it is seen that the occurrence probability of aurora1 hiss has a positive correlation with geomagnetic activity during slightly and moderately disturbed periods and it decreases with further
520
MASASHIHAYAKAWA, YOSHIHITOTANAKA and JINSUKEOHTSU
0
4
“‘OB”‘.’
6
8
10
12
Month Fig. 2. Seasonal variation of occurrenceof aurora1hiss.
The index of magnetic activity has a good enhancement of geomagnetic activity. correlation with precipitating electrons in the aurora1 zone (KUCK, 1970). So the ionization in the lower ionosphere is greatly enhanced during severe magnetic activity, which then results in the depleted occurrence of VLF emissions (ECKLUND et al., 1965), being consistent with the result of Fig. 3. Figure 4 shows the occurrence histogram of the duration of amoral hiss at 12 kHz. It is clear that the duration of the aurora1 hiss is predominantly of the order of less than an hour. This duration is of the same order with the duration of polar substorms (AEASOFU, 1968) and hence the aurora1 hiss can be considered to be a temporThe hiss with the duration more than a few hours is ally localized phenomenon. seldom observed. Of course, the duration of amoral hiss shows slight dependence on local time. The intensity of aurora1 hiss observed on the ground is typically of the order of m1O-14 W/m2/Hz (JORGENSEN, 1966 ; TANAKA, 1972). While, the maximum hiss
Fig. 3. Dependence of occurrenceprobability of aurora1hiss on magnetic activity.
The morphologies of low-latitude and aurora1VLF ‘hiss’
521
Fig. 4. Occurrence histogram of duration of aurora1hiss. intensity measured on board the satellites is about lo-l2 W/m2/Hz (GURNETT,1966; RUSSELL,1972), being higher by two orders of magnitude than that observed on the ground. This may be resulted from the total as well as partial reflections in the ionosphere (THOMASand SMEATHERS, 1971), the ionospheric absorption (ONDOH, 1963 ; HAYAKAWAand OHTSU,1972 ; HAYAKAWA,1974a, b) and divergence in the
space after penetration through the ionosphere. The frequency spectrum of hiss intensity is illustrated in Fig. 5 which is based on seven hiss events. Figure 5 indicates that the maximum intensity takes place at frequency around 10 kHz, being in good agreement with the previous result of JORGENSEN (1968). The intensity of hiss at a specific frequency increases as the magnetic
activity
increases to a critical value and it decreases when the activity
1
100
10
Frequency,
kHz
Fig. 5. Frequency spectra of aurora1hiss based on seven hiss events.
622
MA~ASHIHAYAKAWA, YOSEIIHITO TANAKA and JINSUKEOHTSU
exceeds such a critical value (TANAKA, 1972). This tendency seems to be identical to that for the association of occurrence rate with magnetic activity. The most interesting characteristic of the frequency spectrum is the relation of hiss band with magnetic activity. When the magnetic activity is enhanced during moderately disturbed conditions, the frequency spectrum of hiss becomes very wide, that is, the upper cutoff frequency extends up to more than 100 kHz (DOWDEN, 1960; JORGENSEN, 1968;
TANAKA, 1972).
3.2. Morphology
of low-latitude VLF hiss
The morphological features of low-latitude VLF hiss observed at Moshiri in Japan (geomag. lat. 34.3”N) are discussed in comparison with those of aurora1 hiss. The association of low-latitude VLF hiss with magnetic activity seems to be distinctly different from that of aurora1 hiss. Figure 6 illustrates the occurrence probability of low-latitude hiss at 5 kHz versus magnetic activity. The daily sum 1.0
r 13
29
54 45
0
r
13
1
10
I
I
20
30
I
40
240
KP Fig. 6. 2 Kp dependence of low-latitude VLF hiss observed at Moshiri Station. The value on the rectangle means the number of events for each 2 Kp range. I:
of ?cp index is used as the measure of geomagnetic activity. It is clear from the figure that the occurrence probability of low-latitude hiss increases with increasing C kp. We define tentatively the boundary between quiet- and storm-time as 2 kp = 30. Then about three fourths of the total hiss events occur during relatively quiet periods. We call these ‘quiet-time hiss’. While, the remaining onefourthisclosely correlated with severe magnetic storms and is named ‘storm-time hiss’. Especially, we can expect such a surprisingly high value of NO.9 in occurrence probability for great magnetic storms whose 2 kp exceed 40. The hiss appearing in a high probability during severe magnetic storms differs essentially from aurora1 hiss which occur during moderately disturbed conditions. Figure 7 shows the temporal variation and duration of the storm-time hiss. For each particular magnetic storm the duration of each of the initial, main and recovery phases is normalized to the corresponding mean duration (all events) depicted in the graph. It is seen from Fig. 7 that most of storm-time VLF hisses tend to occur
The morphologies
i? ‘,o
f
of low-latitude
and aurora1 VLF
623
2.0 --
;
= I
‘hiss’
1.5 I
-
0.5
L -
-
0
I-
\-Initial
-l---
-
_
Yz=..
-p
-
I
phase-]-Main
__
_ ,
I
Last
phase-1
phase
t EndIng
Beginning
Fig. 7. The relationship of occurrence of storm-time low-latitude VLF hiss with the phase of magnetic storm. The mean duration of initial phase is 10 hours, and then the ration of the duration of main and recovery phases to that of initial phase is, respectively, 1.0 and 2.5. The intensity is also illustrated.
during the latter part of main phase to last or recovery phase and this coincides with the result by HARANG (1968). The intensity of storm-time hiss is of the order 10-18 W/m-2 Hz-l (I WA1 et al., 1964) as seen from Fig. 7. This value holds for quiettime hiss. The duration of low-latitude VLF hiss during quiet- as well as storm-time is shown in Fig. 8 which illustrates the occurrence histogram of duration. In the case of aurora1 hiss in Fig. 4, the maximum duration is about 3 hours at longest. In contrast, a lot of low-latitude VLF hiss have long-lasting nature and their duration is, on some occasions, as long as over five hours. As the result of appearance of low-latitude VLF hiss during main and last phase, it occurs at any time of the day. Figure 9 shows the diurnal variations of
Duration,
hour
Fig. 8. Occurrence histogram of duration of low-latitude
hiss.
624
MASASHIHAYAKAWA,YOSHIHITOTANAKAand JINSWE OHTSU
,/’
.-a,
0
A’.
1200
”
1600
2000
”
’
0000 0400
s
If
’
0800 1200 . _ LI
Fig. 9. Diurnal variation of occurrence rate of low-latitude VLF hiss associated with magnetic storms (full line) and not associated with storms (broken line). hiss associated as well as not associated with magnetic storms. Quiet-time or the term ‘not associated with magnetic storms’ roughly means the magnetic activity of local k-index less than four. In the figure the maximum occurrence rate at a certain time is normalized to unity. The hiss not associated with magnetic storms corresponds to quiet-time hiss and its diurnal variation is consisted of a pronounced peak at ~5 h MLT and a,subsidiary one at 20 h MLT. The remarkable peak at ~5 h MLT is found to coincide with the peak in occurrence rate of lowlatitude whistlers (OUTSUet al., 1963) and this suggests that the diurnal variation in Fig. 9 may be, in most parts, resulted from the ionospheric absorption. The present diurnal pattern of quiet-time hiss based on ground observations is found to be in good accord with the satellite result (KAISER, 1972). But the origin of quiet-time hiss is not well known and needs further investigations. We turn to the discussion of storm-time low-latitude hiss which is of greater significance in relation with magnetospheric consequences. It is reasonable to regard the diurnal variation of storm-time hiss as being composed of an evening peak around 20 h MLT (we call ‘evening hiss’) and a post-midnight one around 2 h MLT (‘morning hiss’). The earlier work by ELLIS (1959) has given a strong support, to such two peak structure in diurnal variation of midle-latitude VLF hiss as mentioned above. In his paper such evening and morning peaks in occurrence pattern are much more well-isolated and the occurrence peak of evening hiss occurs around 18 h LT. The evening occurrence of hiss is of the same kind with the result by VERSHININ (1970) who has found that the generation region of evening hiss is located inside the plasmapause. While, the appearance of hiss in the morning is closely correlated with D(day)-type hiss observed at lower latitudes by HARANG (1968). B’igure 10 illustrates the seasonal variation of quiet-time and storm-time hiss. The broken line refers to the case of quiet-time hiss, while the full line storm-time hiss. This figure shows that the seasonal variation of quiet-time hiss is wholly controlled by the difference of ionospheric absorption with season. On the other
occurrence rate of low-latitude
The morphologies of low-latitude and aurora1 VLF ‘hiss’
Fig. 10. Seasonal variation of occurrence of storm-time hiss (broken line).
525
(full line) and quiet-time
hand, the storm-time hiss shows less seasonal dependence, which indicates the predominant effect of generation source. The frequency spectra of quiet- as well as storm-time low-latitude hiss show distinct nature from those of aurora1 hiss. That is to say, the low-latitude VLF hiss Its center frequency is found to is always relatively of narrow band in frequency. lie in the frequency range b-5 kHz and its bandwidth is about 1 kHz. On some occasions the upper frequency limit extends up to as high as 8 kHz. According to the recent work by LIKHTER et al. (1973), the VLF emission spectrum during geomagnetic storms has two maximum, one is about 6 kHz and the other below 1 kHz. This satellite result, is likely to be in good agreement with our ground observations. 4. CORRELATION OF AURORAL AND LOW-LATITUDE VLF HISS WITH MAGNETOSPHERICSUBSTORMSAND STORMS Aurora1 hiss appears predominantly during evening to post-midnight, as is seen in Fig. 1. The close relationship of this occurrence of hiss zone with the diurnal pattern of aurora1 precipitating electrons (JORGENSEN, 1968; RUSSELL et al., 1972) has led JORGENSEN (1968) and LIM and LAASPERE (1972) to hypothesize that aurora1 hiss may be generated by incoming or precipitating aurora1 electrons during polar substorms. This idea is evidenced by the correlated observation of aurora1 hiss and soft electrons on board satellites (FRITZ and GURNETT, 1965 ; JORGENSEN, 1968). Then aurora1 hiss can be considered to be only a manifestation of polar substorms which occur intermittently and impulsively with the life time of order l-3 hours (AKASOFU, 1968). The nature of short duration of aurora1 hiss is probably resulted from the short life time of polar substorm. MOROZUMI and HELLIWELL (1966) have classified the nighttime sequence of VLF hiss occurrence into three phases Nl, N2 and N3 phases from comparisons of VLF emissions with other geophysical phenomena. The Nl phase before midnight is characterized by the appearance of intense hiss and diffuse arc-like auroras. While, the N2 phase, when impulsive hiss bursts occur almost simultaneously with a sudden increase in brightness of aurora, pi bursts and a sharp onset of cosmic noise absorption, is interpreted as the expansion phase onset of substorm in accordance with the recent. concept of magnetospheric substorm (AKASOFU, 1968). Recently, KOKUBUN et al. (1972) have examined
526
MASASHIHAYAKAWA,YOSHIHITOTANAICAand JINSUKEOHTSU
in detail the properties of hiss in iV1 and N2 phases. Generally speaking, aurora1 hiss tends to occur during moderate magnetic activity, not during severe magnetic activity. This is likely to be due to the absorption of VLF emissions by the increase in ionization of the lower ionosphere during severe magnetic activity. Aurora1 hiss sometimes propagates toward lower latitudes and observed at Actually, some of VLF hiss observed at middle latitudes are middle latitudes. found, by means of direction finding or intensity comparison at multi-stations, to be fed into the Earth-ionosphere waveguide from the ionosphere in the aurora1 zone (ELLIS, 1960, 1961; DOWDEN, 1961). As is clearly seen from Fig. 6, low-latitude VLF hiss can be divided into two types. About three fourths of total hiss events observed at Moshiri take place during relatively quiet periods (quiet-time hiss). We believe that the present paper is the first reliable report on the ground reception of low-latitude VLF hiss during The origin or mechanism of quiet-time low-latitude magnetically quiet conditions. hiss is not well understood. However, we can give the following plausible possibility. The observation by Injun III has demonstrated, for the first time, the occurrence of wide band VLF hiss at low L values less than 1.2 (GURNETT, 1968). Later observations by the satellite-Alouette 2 has shown that VLF hiss at the frequencies of 5 and 7 kHz is frequently, although less frequently than aurora1 hiss, observed in the range of invariant latitude 303 to 40” (ONDOH et al., 1972). It is significant to note that Ondoh’s latitude range of hiss is around the latitude of Moshiri. The frequency spectrum of ground-based VLF hiss is peaked at ~5 kHz which is in agreement with the satellite observation. So it is probable that the hiss reported by ONDOH et al. (1972) propagates along the field-aligned ducts and then penetrates through the ionosphere. Provided that VLF hiss is generated at low L values in relatively steady state, we can interpret satisfactorily the diurnal pattern of quiet-time hiss in Fig. 9. The pronounced peak at ~5 h MLT in Fig. 9 is found to coincide with the peak in diurnal variation of occurrence rate of low latitude whistlers given by OUTSU et al. (1963) and then Fig. 9 seems to be mainly attributed to the ionospheric absorption. The less remarkable peak at ~20 h MLT in Fig. 9 may include the effect of aurora1 hiss. This is due to the following two reasons. One is that aurora1 hiss is a phenomenon from evening to midnight during slightly and moderately disturbed conditions. The other is that aurora1 hiss sometimes propagates toward lower latitudes via Earth-ionosphere waveguide as is found by ELLIS (1960, 1961) and DOWDEN (1961) and the direction of propagation of hiss observed at Moshiri is approximately along the magnetic meridian plane, based on the goniometric measurement (IWAI and TANAKA, 1968). Except for the possibility that the quiet-time hiss at ~20 h MLT may include the propagated aurora1 hiss, we can consider the quiet-time low-latitude VLF hiss as being originated in the plasmasphere and we can call these quiet-time low-latitude hiss ‘plasmaspheric VLF hiss’ in analogy to plasmaspheric ELF hiss by THORNE et al. (1973). About one fourth of total hiss events at Moshiri are found to be closely correlated with magnetic storms, as is seen from Fig. 6. This storm-time low-latitude hiss differs essentially from aurora1 hiss. For magnetic storms whose daily sum of kp index exceed 40, we can expect the occurrence of VLF hiss in a high probability of -0.9. The storm-time hiss occurs during the latter part of main phase to last or
527
The morphologies of low-latitude and aurora1VLF ‘hiss’
recovery phase, which suggests that storm-time hiss may be closely associated with the ring current resulting from magnetic storms. And the diurnal variation of occurrence of storm-time low-latitude VLF hiss is composed of two peaks in the evening (evening hiss) and at post-midnight (morning hiss), as is seen from Fig. 9. The two peaks in Fig. 9 are not so well-isolated, but such tl structure with two peaks has been evidenced by the earlier work of ELLJS (1959). In his paper, such evening and morning peaks in occurrence pattern are well-isolated and the occurrence peak of evening hiss occurs around 18 h MLT. Furthermore, the evening hiss is thought to correspond to the hiss observed by VERSHININ (1970)) who has also found that the generation region of evening hiss is located inside the plasmapause. While, the appearance of hiss in the morning is closely correlated with D(day)-type hiss observed at lower latitudes by HARANG (1968). Taking into account the close association of storm-time hiss with the phase of storms, TANAKA et al. (1974) have examined the time lag of occurrence of hiss behind the time of maximum depression in geomagnetic field for all storm-time hiss events. Then they have statistically found that storm-time hiss may be generated by soft electrons of the order 5-10 keV drifting eastward in the ring current. The relative location of ring current with the plasmapause seems to be very important in solving the generation mechanism of storm-time hiss. Satellite observation of hiss during a severe magnetic storm (BULLOU~H et ccl., 1969; TULUNAY and HTJ~HES, 1973) has shown that the VLF hiss in the morning side is generated outside the plasmapause, while the hiss in the evening sector is observed inside the plasmapause or in the plasmasphere. Also, the occurrence of evening hiss is found to lag significantly behind the storm sudden commencement. Such asymmetric appearances of hiss with respect to the position of plasmapause is apparently owing to the asymmetric structure of the plasmasphere (CARPENTER, 1970) having the bulge region in the evening sector. Above facts suggest that the plasmapause has an essential role on the generation of VLF hiss (RUSSEL and THORNE, 1970; THORNE, 1972). Tulunay and Hughes’s result that evening hiss is located inside the plasmapause coincides well with the result obtained by ground-based observations by VERSHININ (1970). The relative location of D-type low-latitude hiss by H~RANG (1968) referred to as morning hiss in this paper with the position of plasmapause is not investigated. However, taking account of the satellite observation by TULUNAY and HUGHES (1973),it is likely that Harang’s D-type low-latitude hiss may be originated outside the plasmapause. Shove-mentioned problem will be discussed in detail in our following paper by making use of the correlated study of magnetospheric hiss during a severe magnetic storm observed on board the satellite and at many ground stations (HAYAKAWA etal. 1974). Furthermore, we will investigate the detailed characteristics of morningandeveninghiss with special reference to the comparison with the morphological study presented in this paper. Acknowledgemelzts-The of our institute.
IWAI
authors deeply appreciate the continuing encouragement of Prof. A. Thanks are also due to the referee for his useful comments and suggestion.
REFERENCES .&CASOFUs-1
1968
Polar and Magnetospheric Reidel.
Dordrecht,
Holland.
Substom8.
528
MASASHI HAYAKAWA,
BTJLLOUQH K., HUGHES A. R. W. and KAISER T. R. CARPENTER D. L. DOWDEN R. L. DOWDEN R. L. ECELUND W. L., HAROEAVES J. K. and POPE J. H. ELLIS G. R. A. ELLIS G. R. A. ELLIS G. R. A. FRITZ T. A. and GIJRNETT D. A. GURNETT D. A. GURNETT D. A. HARANU L. HARANU L. and LARSEN R. HARANU L. and HAUOE K. N. HARTZ T. R. and BRICE N. M. HAYAKAWA M. HAYAKAWA HAYAKAWA HAYAKAWA HELLIWELL HUGHES A.
YOSHIHITO TANAKA
and JINSUI(E
OHTSU
1969
Proc. R. Sot. A311, 563.
1970 1960
An& Gdophys. 26, 363. Nature, Lond. 187, 677.
1961 1965
J. geophys. J. geophys.
1959 1960 1961 1965 1966 1968 1968 1965 1965 1967 1974a
Planet. Sci. 1, J. geophys. Res. 65, 839. J. geophys. Res. 66, 19. J. geophys. Res. 70, 2485. J. geophys. Res. 71, 5699. J. geophys. Res. 73, 1096. J. atmos. terr. Phys. 30, 1143. J. atmos. terr. Phys. 27, 481. J. atmos. terr. Phys. 27, 499. Planet. Space Sci. 15, 301. Trans. Inst. Elect. Comm. Eng., Japan
1974b 1972 1974 1969 1971
Res. 66, 1587. Res. 70, 4285.
57B, 177.
IWAI A., Oasu J. and TANAKA Y. IWAI A. and TANAKA Y. JORGENSEN T. S. JORGENSEN T. S. KAISER T. R.
1964 1968 1966 1968 1972
KIMIJRA I. KOKIJB~ S., MAEITA K. and HIRASAWA T. KUCK G. A. LAASPERE T., JOHNSON W. C. and SEMPREBON L. C. LIM T. L. and LAASPERE T. MARTIN L. H., HELLIWELL R. A. and MARKS K. R. MCEWEN D. J. and BARRINUTON R. E. NISHINO M. and TANAJZA Y. ONDOH T. ONDOH T., AIKYO K. and NAUAYAMA M. Oasu J., TANAKA Y. and IWAI A. PAULIKAS G. A. RAO L. M., SOMAYAJ~LU V. V. and TANTRY B. A. P. RUSSELL C. T. and THORNE R. M. RUSSELL C. T., MCPHERRON R. L. and COLEMAN P. J. TANAKA Y.
1967 1972
Pure Appl. Geophys. (in press). Planet. Space Sci. 20, 1895. J. geophys. Res. (inpress). Rev. Geophys. Space Phya. 7, 281. The Radiating Atmosphere (Edited by B. M. MCCORMAC), p. 336. D. Reidel Dordrecht, Holland. Proc. Res. Inst. Atmos. 11, 29. Proc. Res. Inst. Atmos. 15, 1. J. geophya. Res. 71, 1367. J. geophys. Res. 73, 1055. Earth’8 Magnetospheric Process (Edited by B. M. MCCORMAC), p. 340. D. Reidel, Dordrecht, Holland. Planet. Space Sci. 15, 1427. Rep. Ionoaph. Space Res., Japan 26, 138.
1970 1971
An& Ge’ophya. 26, 689. J. geophya. Res. 76, 4477.
1972 1960
J. geophya. Res. 77, 4145. Nature, Lond. 187, 751.
1967 1960 1963 1972
Can. J. Phya. 45, 13. Rep. Jap. Ant. Res. Exp. 35, 37. J. Geomagn. Geoelect. 15, 90. J. Radio Res. Labs. Japan 19, 23.
1963 1971 1972
Bull. Proc. Res. Inst. Atmos. 13, 11. Rev. Geophya. Space Phya. 9, 659. J. Geomagn. Geoelect. 24, 261.
1970 1972
Cosmic Electrodym. 1, 67. Space Sci. Rev. 12, 810.
1972
Proc.
M. M. and OHTSU J. M., TANAKA Y. and OHTSU J. R. A. R. W. and KAISER T. R.
Res. Inst.
Atmos.
19, 33.
The morphologies
of low-latitude
TANY. and KAEETIWAQI M. Tmm Y., NISHINO M. and IWAI A. THOMAS L. and SMEATHERS J. R. THORNE R. M., SMITH E. J., BURTON R. K. and HOLZER R. E. TULUNAY Y. K. and HUUHES A. R. W. VERSHININ E. F. Reference
is also made to the following
‘hiss’
1968 1970 1971 1973
Proc. Re8. Inst. Atmos. 15, 67. Proc. Res. Inst. Atmos. 17, 43. J. atmos. terr. Phys. 33, 959. J. geophys. Res. 78, 1581.
1973 1970
J. atmos. terr. Phys. 35, 153. An& Qdophys. 26, 703.
unpublished
LIKHTER YA. I., LARKINA V. I. and MIKHAILOV Yu. M. MOROZUMI H. M. and HELLIWELL R. A.
1973
TANY., HAYAKAWA M. and OHTSU J. THORNE R. M.
1974
10
and aurora1 VLF
1966
1972
529
material: Presented at COSPAR meeting, Konstantz, FRG. Tech. Rep. Radioscience Lab. p. 66-124. Stanford Electronics Labs. Stanford Univ. SU-SEL-, U.S.A. in preparation. Proc. COSPAR/IAGA/URSI Madrid, Spain.
Symp-