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Hiss emitting auroral activity
Perpamon Press.Printed inNorthern Irelaxld Journal ofAtmospheric andTerrestrial Physics, 1975, Vol.37,pp.761-768.
Hiss emitting
aurora1 activity
TAKASI OGUTI Geophysics Research Laboratory, University of Tokyo, Tokyo 113, Japan (Received12 August
1974; in revisedform
17 October 1974)
Abstract-Real-time auroral records on a video tape obtained by use of a highly sensitive TV camera, and simultaneous records of VLF waves on the sound track of the same video tape were analyzed and specificsmall scale aurora1activities were found to be associatedwith specific burst-like hiss enhancements with durations of 0.1-l s. The hiss emitting aurora1 activities were identified by cross correlation analysis between the temporal variations in luminosity of aurora1 structures and the temporal variations of hiss intensities. The hiss emitting aurora were long-rayed sheet fragments or folds of a few km to several tens of km in horizontal scale which rapidly changed in luminosity, and displayed rapid motions such as splittings and rotations. The enhancements of local aurora1electron precipitations responsible for the local aurora1 activity and of hiss emissions are concluded to be simultaneous within a time accuracy of 200 ms at worst and within a few tens of ms at best.
A CLOSE relation
between aurora1 activity and VLF hiss emission was first found by BURTON and BOARDMAN (1933). Since then, many efforts have been made to clarify the relation not only between aurora and hiss observed on the ground (MOROZUMI, 1965; HELUWELL, 1965; HARANG and LARSEN, 1965; JORGENSEN, 1966; ROSENBERG, 1968; HIRASAWA and NAGATA, 1972; KOKTJBUN et al., 1972) but also between aurora1 particles and hiss at satellite altitudes (e.g. GURNETT and FRY, 1972). The relation so far reported, however, is somewhat statistical in nature due mainly to the limit in temporal and spatial resolutions of the observations of auroras or aurora1 particles. Generally speaking, the characteristic scale of aurora1 activity, as we shall call it, can be as small as a few km in horizontal scale during the breakup phase, while the characteristic time of the temporal variation can be as brief as 0.1-10 s. Many active arc fragments brighten, rotate and decay from place to place in the fragments and from time to time. Corresponding to the rapid variations of small-scale fragments, the aurora1 hiss shows burst-like enhancements in intensity consisting of many spikes with duration from 0.1-l s. The relation between aurora1 activity and hiss enhancement with duration of a few minutes so far reported (ROSENBERG, 1968; HIRASAWA and NAGATA, 1972) is thus understood to be a relation between a group of these hiss spikes and a group of aurora1 activities, consisting of many ‘elementary’ activities, occurring over an extended area of at least several tenshundreds of km. A recently developed technique for observing aurora, a highly sensitive TV camera system has allowed us to record aurora with a high resolution both in time (60 frames/s) and in space ( ~1 km). VLF emissions can simultaneously be recorded on the sound track of the same video-tape. A direct comparison between hiss spikes and related ‘elementary’ aurora1 activities has thus become possible. 6
761
Observations of this type have been carried out at Syowa Station (Geographm lat. 69’00’~, Xong, 39O35’E; Geomagnetic Iat;. -6&7”, long. 72-5“) since April 1971, and many examples of extremely good correlation between specific auroral a~ti~ty and hiss emission have been obtained. This paper desoribes the identi6cation of hiss emitting aurora1 activities, the characteristics of hiss emitting aurora, and the simultaneity of the brightening of aurora1 arc fragments (or folds) and hiss spikes.
The examples used in this paper are those obtained from 00~~005~ WT on 46 June 1971. The general magnetict activity was modera~ t~o~gho~t the night as shown in Fig. 1. A small but sharp negative bay started at 0047, with the maximum depression of horizontal component (- 135 y) at 0051. The ~o~es~o~di~
aword breaknp started at O&Min a region. about 300 km north ~~~~~~~~~d~ of Syowa station. The aurora then expanded pofeward, passing the magnetic zenith of Syowcb about 0048, TV images indicate that the front of the aurora1 expansion consisted of arc slatting or smaff-scale up-turning. The split sheets rotated elockwisely viewed &om below, and the original src gradually decayed into arc fragments. As the expansion and the fragmentation proceeded, many fragments split within the moving front and eventually were left behind. Sometimes the fragments abruptly brightened and rotated. The region of strong hydrogen (Ham emission (over 10 R/A) appeared behind the po~eward expanding front and it also expanded in a fashion similar to that of a sheet aurora just followiug an electron aurora1 front. AU sky camera photo~~phs of aurora during the breakup are shown in Fig. 2. The hiss comity began to rise at abont the same instant as the onset of aurora1 focal
4 3
Pig. 4. Sequential photographs roprodu~od ovcry & s from the video-tape. The viewing angle is 60° and the center of the frame is nearly coincident with the magnetic zenith. The rapidly varying portions designated il sntl 73are the specific hiss emitting am*oras.
26 June 00:50?27-3lslQ?l *.
26
June
00:49~2?-325
Hiss emitting amoral a&ivity
763
Fig. 3. General hiss activity during the IocaJ.aurora1breakup shown in Fig. 2. Arrows desigmtsd at II and III at the bottom of the 32 kHz channel indicate the TV obae~ation periods given in Figs. 4 and 0.
breakup. The intensity increased with a maximum power spectral density of IO-*6watt/msHz in the 8-16 kHz frequency range at 0048 and decayed at 0052. The average intensity of hiss was maximum within the time interval when the poleward expanding front passed over the magnetic zenith. The general hiss enhancement for the period is shown in Fig. 3. OBSERVATIONAL RESULTS In order to identify a specific aurora1 activity which corresponds with a specific spike-like enhancement of hiss emission, the time variations of the aurora1 luminosities of small areas of the observed auroras and narrow band, hiss densities were compared. Figure 4 shows sequential pho~~aphs of auroras with a viewing angle of GO” with its center nearly pointing to the magnetic zenith. The pictures are samples at a rate of 21 frames/s selected from the original video-tape records of 60 frames/s, during the period from 0050 : 2’7-3 1. It is clearly seen in the figure that a sheet fragment designated as A, abruptly brightened at 0050: 28.2. The luminosity attained a maximum level of about 10 kR for the 4278 A line at 28.4 s, then faded out at 28.9 s. The portion A at its maximum Iuminosity was the brightest part of the aurora in the field of view at that moment. The portion B appeared at nearIy the same times as A and its luminosity rapidly increased over the luminosities of surrounding anroras at 29.1 s with the maximum at 29.5 s; then, it decayed to the surrounding Iuminosity level at 29.8 s. The time variations of the lu~o~~es at A and B, are reproduced at the top of Fig. 5 along with those of the narrow band hiss intensities in the frequency ranges
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Fig. 5. l’ime variations of luminosity of A and B (at the top) in Fig. 4, and those of hiss intensity in frequency bands of 24-3~4 kHz, 6*8-S-8 kHz and M-13.6 kHz (at the bottom). 2.4-3~4 kHz, 6+--9*S kRz and 93-13.7 kHz in arbitary scales. The brightening of small aurora1 fragments A and B are clearly found to be simultaneou~y associated with the hiss enhancements. Similar examples of hiss emitting aurora1 activities are seen in Fig. 6 designated as C, D and E. These examples, in contrast to A and B, however, were charatlterized not only by rapid brightening but also by expansion or movement of the bright portions along a thin arc. Although the characteristic motion is not clear in the pictures because their size is too small, the motion of the bright portions is found to consist of splitting of the arc and clockwise unfolding or rotation as the luminosity increased. The time variations of luminosities of these moving portions are illustrated in Fig. 7 in a manner similar to those in Fig. 5. These examples are typical hiss emitting aurora1 activities. The ~haracter~tic feature of these activities is the rapid brigh~n~g associated with fast motions. The local increasing rate of luminosities exceeds 10 kR/s, and the expanding (or rotational) speed is over 30 km/s. The characteristic scale of the hiss emitting aurora is in a range from a few km up to several tens of km in horizontal length and from 1 to several tens of km in horizontal width. With regard to the field-aligned extent of the luminous region, examples A and B are long rayed sheet fragments and C, D and E are also quite long. Hence, a highly extended vertical structure seems likely to be one of the required conditions for hiss emitting aurora. The simultaneity of the aurora1brightening and hiss enhancement is summarized in Fig. 8 in terms of time-shifted cross-correlations. Solid curves with triangles, crosses and circles indicate the time-s~f~d correlation between aurora1 luminosities and the hiss intensities in the frequency bands 2-4-3-4 kHz, 6+--9-S kHz and
765
Hiss emitting auroral aotivity
D
.9.6-13.6
_Khz E
_ _ __ .-_._. 1.7-2.4
-. _______--
Khz
I.__..
26 iune 00 49:UT 1971 Fig. 7. Time variations of luminosity of C, D and E (at the top) in Fig. 6 and corresponding hiss enhancements (at the bottom).
9%13.6 kHz, respectively. Dashed curves with crosses in Figs. S(a,b) are those from smoothed hiss intensities. What is meant by smoothing here is the elimination of the steep spikes with period shorter than 250 ms, several of which are seen in the hiss intensities of Fig. 5, especially on the curve of the 64-94 kHz band. The adoption of smoothed hiss intensity is based on an assumption that the overlapping shorter spikes of hiss intensity are due to the cont~butio~s from portions of aurora. other than A. and 23, which are most likely to be outside the field of view. The appreciable improvement of the correlation coefficient by the smoothing suggests that the assumption is correct. One remarkable point in these figures is the rather high correlation coefficient, in almost all oases exceeding 0% This indicates that the association of the local aurora1 activities with hiss enhancements is real. Moreover, it may be somewhat surprising to learn that these examples with high correlations are not special eases but rather are very common along the discrete aurora1 arc near the high latitude boundary of the aurora1 activity in the dusk to midnight sector. As a matter of fact, more than 70 examples of this type of aurora1 activity (small sheet fragment with luminosity variation > 10 kRfs or expansion speed > 30 km/s with luminosity > 5 kR) were readily discernible in 55 h of VTR records and of these greeter than 80 per cent were associated with simultaneous spike-like enhancements of hiss emission. High correlation, however, is rarely obtained unless the field of view of the TV covers the zenith and then only when there is general aurorsl activity near the zenith. Also, the correlation must be made within a few seconds, because otherwise the spatial resolutions of both the aurora1 structure and the related hiss enhancement region become signScantly reduced leading to a reduction in the correlation due to the contamination of hiss emissions from other activities. In other words, the high correlation is observable only when the aurora1 activity near the zenith
I
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HISS delay
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Fig. 8. Time-shifted cross correlation between the aurora1luminosities and hiss enhancements for auroral regions A, 23, 0, D and E. Triangles, crosses and circles indicate the correlation is taken between aurora1 luminosities and narrow band hiss intensity in the frequency bands N-3+4 kHz, 6.8-S-8 kI& and S-813.0 kH2, respectively.
0
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HISS advance -4-
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767
is at a much higher level than surrounding regions and when there are only a few characteristic hiss emitting activities on the field of view of the TV system. It is clear that the examples of this paper fall, for the most part, under the above conditions, That the peak to peak relation between them has not been clearly seen previously and accordingly that the correlation heretofore reported has not been very high may be due to the observational limitations on the spatial and time resolutions of the aurora1 structure and its time variations. Generally many such hiss emitting activities can exist simultaneously in widespread aurora1 forms. A second point is the slight (above 50 ms) advancement in time of hiss enhancement over aurora1 brigh~~g on the average, although scattered from -80 to + 160 ms in these examples. The 50 ms advancement of hiss, however, is not all real. This is because that the TV system has the ch~ac~ristic response time of a silicon target for electron multiplication which amounts to about 17 ms, which is nearly equal to the interval between frames of the TV system. Thus, the real advancement of hiss enhancement before aurora1 brightening is about 30 ms. The 30 ms delay of aurora1 luminosity behind hiss enhancement is likely to come from the contribution of 5577 A aurora1 green line. Though the quantum efficiency of the imaging surface of the TV camera is maximum at a wave length of 4200 A, it is still as much as 213 of the maximum at 5577 A. Hence, the average contribution of the green line emission in the total light is estimated to be about a few per cent to 10 per cent. Since the luminosity changes with a time comparable to the green line ch~rae~risti~ time (~1 s) a cont~bution of 10 per cent would yield a time delay in the total recorded intensity of about 30 min. Consequently, we can conclude that most of the spike-like hiss enhancements show an extremely good simultaneity with the increases in local aurora1 electron precipitations within a time accuracy of few tens of milliseconds on the average. CONCLUDINU REMARKS The local aurora1 activities during a breakup phase were found to be associated with specific spike-like increase in hiss intensity, and no systematic time difference was found between enhancements of local aurora1 electron precipitations and those of hiss intensities provided that the contribution of the forbidden line is about 10% or less of the total light recorded by the TV system. This result, first of all, may suggest that due to the p~cipitation of aurora1 particles, a high electron density column is produced which serves as a wave duct which permits hiss waves to easily penetrate the ionosphere. However, the fact that the amoral hiss is associated with specific characteristics of aurora, such as rapid brightening and rapid motions, and that the aurora1 hiss is not associated with quiet aurora (which have no remarkable motions and changes in luminosity), even when very luminous, suggests that the ducting of hiss waves due to the precipitation may not be the exclusive interpretation of the hiss enhancements simultaneous with the local aurora1 activities. Generation and amplification mechanisms of hiss waves may possibly be found in the mechanisms for the speciflo, rapid acceleration of aurora1 electrons in rather low altitudes (~5000 km) as suggested by GURNETT and FEANH (1972) on the basis of the cutoff frequency ~st~butio~ of hiss obse~ed at satellite altitudes. The facts mentioned above also do not allow an interpre~tion
768
TAKASI &UT1
of aurora1 hiss, at least, in terms of ‘stationary’ emissions due to stationary electron precipitations, since rapid increases in electron flux as well as rapid motions of the precipitation regions are necessary conditions for hiss emissions. If this is the case, the rapid increase and decay in local aurora1 luminosity within a small volume as well as the rapid motions would suggest that the amoral electrons act somewhat collectively as a small electron cloud, which may give rise to a high efficiency for producing aurora1 hiss emissions, in terms of coherent Cerenkov radiation.
BURTON
E.T.md BOARDMAN E.M. GURNETCC D. A. and FRANE L. A. HARANU L. and LARSEN R. HELLWELL R. A.
1933 1972 1965 1965
HIRASAWA T. and NAQATA T. JORQENSENT. S. KOKUXJN S., M&ITA K. and HIRASAWA T. MOROZUMIH. M. ROSENBERGT. J.
1972 1966 1972
Proc. IRE 21,1476. J. geophys. Re-s. 77, 172. J. atmos. tew. Phys. 27, 481. Whistlers and Related Ionosphetic Phenomena. Stanford University Press. JARE Sci. Rept. A, No. 10, 1. J. geophys. Res. 71, 1367. Rept. Ionos. Space Res., Japan, 28, 138.
1965 1968
Rept. Ionos. Spa.ce Res., Japan Planet. Space Sci. 16, 1419.
19,286.
Hiss emitting
aurora1 activity
TAKASI OGUTI Geophysics Research Laboratory, University of Tokyo, Tokyo 113, Japan (Received12 August
1974; in revisedform
17 October 1974)
Abstract-Real-time auroral records on a video tape obtained by use of a highly sensitive TV camera, and simultaneous records of VLF waves on the sound track of the same video tape were analyzed and specificsmall scale aurora1activities were found to be associatedwith specific burst-like hiss enhancements with durations of 0.1-l s. The hiss emitting aurora1 activities were identified by cross correlation analysis between the temporal variations in luminosity of aurora1 structures and the temporal variations of hiss intensities. The hiss emitting aurora were long-rayed sheet fragments or folds of a few km to several tens of km in horizontal scale which rapidly changed in luminosity, and displayed rapid motions such as splittings and rotations. The enhancements of local aurora1electron precipitations responsible for the local aurora1 activity and of hiss emissions are concluded to be simultaneous within a time accuracy of 200 ms at worst and within a few tens of ms at best.
A CLOSE relation
between aurora1 activity and VLF hiss emission was first found by BURTON and BOARDMAN (1933). Since then, many efforts have been made to clarify the relation not only between aurora and hiss observed on the ground (MOROZUMI, 1965; HELUWELL, 1965; HARANG and LARSEN, 1965; JORGENSEN, 1966; ROSENBERG, 1968; HIRASAWA and NAGATA, 1972; KOKTJBUN et al., 1972) but also between aurora1 particles and hiss at satellite altitudes (e.g. GURNETT and FRY, 1972). The relation so far reported, however, is somewhat statistical in nature due mainly to the limit in temporal and spatial resolutions of the observations of auroras or aurora1 particles. Generally speaking, the characteristic scale of aurora1 activity, as we shall call it, can be as small as a few km in horizontal scale during the breakup phase, while the characteristic time of the temporal variation can be as brief as 0.1-10 s. Many active arc fragments brighten, rotate and decay from place to place in the fragments and from time to time. Corresponding to the rapid variations of small-scale fragments, the aurora1 hiss shows burst-like enhancements in intensity consisting of many spikes with duration from 0.1-l s. The relation between aurora1 activity and hiss enhancement with duration of a few minutes so far reported (ROSENBERG, 1968; HIRASAWA and NAGATA, 1972) is thus understood to be a relation between a group of these hiss spikes and a group of aurora1 activities, consisting of many ‘elementary’ activities, occurring over an extended area of at least several tenshundreds of km. A recently developed technique for observing aurora, a highly sensitive TV camera system has allowed us to record aurora with a high resolution both in time (60 frames/s) and in space ( ~1 km). VLF emissions can simultaneously be recorded on the sound track of the same video-tape. A direct comparison between hiss spikes and related ‘elementary’ aurora1 activities has thus become possible. 6
761
Observations of this type have been carried out at Syowa Station (Geographm lat. 69’00’~, Xong, 39O35’E; Geomagnetic Iat;. -6&7”, long. 72-5“) since April 1971, and many examples of extremely good correlation between specific auroral a~ti~ty and hiss emission have been obtained. This paper desoribes the identi6cation of hiss emitting aurora1 activities, the characteristics of hiss emitting aurora, and the simultaneity of the brightening of aurora1 arc fragments (or folds) and hiss spikes.
The examples used in this paper are those obtained from 00~~005~ WT on 46 June 1971. The general magnetict activity was modera~ t~o~gho~t the night as shown in Fig. 1. A small but sharp negative bay started at 0047, with the maximum depression of horizontal component (- 135 y) at 0051. The ~o~es~o~di~
aword breaknp started at O&Min a region. about 300 km north ~~~~~~~~~d~ of Syowa station. The aurora then expanded pofeward, passing the magnetic zenith of Syowcb about 0048, TV images indicate that the front of the aurora1 expansion consisted of arc slatting or smaff-scale up-turning. The split sheets rotated elockwisely viewed &om below, and the original src gradually decayed into arc fragments. As the expansion and the fragmentation proceeded, many fragments split within the moving front and eventually were left behind. Sometimes the fragments abruptly brightened and rotated. The region of strong hydrogen (Ham emission (over 10 R/A) appeared behind the po~eward expanding front and it also expanded in a fashion similar to that of a sheet aurora just followiug an electron aurora1 front. AU sky camera photo~~phs of aurora during the breakup are shown in Fig. 2. The hiss comity began to rise at abont the same instant as the onset of aurora1 focal
4 3
Pig. 4. Sequential photographs roprodu~od ovcry & s from the video-tape. The viewing angle is 60° and the center of the frame is nearly coincident with the magnetic zenith. The rapidly varying portions designated il sntl 73are the specific hiss emitting am*oras.
26 June 00:50?27-3lslQ?l *.
26
June
00:49~2?-325
Hiss emitting amoral a&ivity
763
Fig. 3. General hiss activity during the IocaJ.aurora1breakup shown in Fig. 2. Arrows desigmtsd at II and III at the bottom of the 32 kHz channel indicate the TV obae~ation periods given in Figs. 4 and 0.
breakup. The intensity increased with a maximum power spectral density of IO-*6watt/msHz in the 8-16 kHz frequency range at 0048 and decayed at 0052. The average intensity of hiss was maximum within the time interval when the poleward expanding front passed over the magnetic zenith. The general hiss enhancement for the period is shown in Fig. 3. OBSERVATIONAL RESULTS In order to identify a specific aurora1 activity which corresponds with a specific spike-like enhancement of hiss emission, the time variations of the aurora1 luminosities of small areas of the observed auroras and narrow band, hiss densities were compared. Figure 4 shows sequential pho~~aphs of auroras with a viewing angle of GO” with its center nearly pointing to the magnetic zenith. The pictures are samples at a rate of 21 frames/s selected from the original video-tape records of 60 frames/s, during the period from 0050 : 2’7-3 1. It is clearly seen in the figure that a sheet fragment designated as A, abruptly brightened at 0050: 28.2. The luminosity attained a maximum level of about 10 kR for the 4278 A line at 28.4 s, then faded out at 28.9 s. The portion A at its maximum Iuminosity was the brightest part of the aurora in the field of view at that moment. The portion B appeared at nearIy the same times as A and its luminosity rapidly increased over the luminosities of surrounding anroras at 29.1 s with the maximum at 29.5 s; then, it decayed to the surrounding Iuminosity level at 29.8 s. The time variations of the lu~o~~es at A and B, are reproduced at the top of Fig. 5 along with those of the narrow band hiss intensities in the frequency ranges
TKIUSI OUUTI
764
6.8-9.8,Ktiz *&~iJb$&~z_ . . _
a,
._..
_
.~_.
_
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26June0050:Uf
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._
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34
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1971
Fig. 5. l’ime variations of luminosity of A and B (at the top) in Fig. 4, and those of hiss intensity in frequency bands of 24-3~4 kHz, 6*8-S-8 kHz and M-13.6 kHz (at the bottom). 2.4-3~4 kHz, 6+--9*S kRz and 93-13.7 kHz in arbitary scales. The brightening of small aurora1 fragments A and B are clearly found to be simultaneou~y associated with the hiss enhancements. Similar examples of hiss emitting aurora1 activities are seen in Fig. 6 designated as C, D and E. These examples, in contrast to A and B, however, were charatlterized not only by rapid brightening but also by expansion or movement of the bright portions along a thin arc. Although the characteristic motion is not clear in the pictures because their size is too small, the motion of the bright portions is found to consist of splitting of the arc and clockwise unfolding or rotation as the luminosity increased. The time variations of luminosities of these moving portions are illustrated in Fig. 7 in a manner similar to those in Fig. 5. These examples are typical hiss emitting aurora1 activities. The ~haracter~tic feature of these activities is the rapid brigh~n~g associated with fast motions. The local increasing rate of luminosities exceeds 10 kR/s, and the expanding (or rotational) speed is over 30 km/s. The characteristic scale of the hiss emitting aurora is in a range from a few km up to several tens of km in horizontal length and from 1 to several tens of km in horizontal width. With regard to the field-aligned extent of the luminous region, examples A and B are long rayed sheet fragments and C, D and E are also quite long. Hence, a highly extended vertical structure seems likely to be one of the required conditions for hiss emitting aurora. The simultaneity of the aurora1brightening and hiss enhancement is summarized in Fig. 8 in terms of time-shifted cross-correlations. Solid curves with triangles, crosses and circles indicate the time-s~f~d correlation between aurora1 luminosities and the hiss intensities in the frequency bands 2-4-3-4 kHz, 6+--9-S kHz and
765
Hiss emitting auroral aotivity
D
.9.6-13.6
_Khz E
_ _ __ .-_._. 1.7-2.4
-. _______--
Khz
I.__..
26 iune 00 49:UT 1971 Fig. 7. Time variations of luminosity of C, D and E (at the top) in Fig. 6 and corresponding hiss enhancements (at the bottom).
9%13.6 kHz, respectively. Dashed curves with crosses in Figs. S(a,b) are those from smoothed hiss intensities. What is meant by smoothing here is the elimination of the steep spikes with period shorter than 250 ms, several of which are seen in the hiss intensities of Fig. 5, especially on the curve of the 64-94 kHz band. The adoption of smoothed hiss intensity is based on an assumption that the overlapping shorter spikes of hiss intensity are due to the cont~butio~s from portions of aurora. other than A. and 23, which are most likely to be outside the field of view. The appreciable improvement of the correlation coefficient by the smoothing suggests that the assumption is correct. One remarkable point in these figures is the rather high correlation coefficient, in almost all oases exceeding 0% This indicates that the association of the local aurora1 activities with hiss enhancements is real. Moreover, it may be somewhat surprising to learn that these examples with high correlations are not special eases but rather are very common along the discrete aurora1 arc near the high latitude boundary of the aurora1 activity in the dusk to midnight sector. As a matter of fact, more than 70 examples of this type of aurora1 activity (small sheet fragment with luminosity variation > 10 kRfs or expansion speed > 30 km/s with luminosity > 5 kR) were readily discernible in 55 h of VTR records and of these greeter than 80 per cent were associated with simultaneous spike-like enhancements of hiss emission. High correlation, however, is rarely obtained unless the field of view of the TV covers the zenith and then only when there is general aurorsl activity near the zenith. Also, the correlation must be made within a few seconds, because otherwise the spatial resolutions of both the aurora1 structure and the related hiss enhancement region become signScantly reduced leading to a reduction in the correlation due to the contamination of hiss emissions from other activities. In other words, the high correlation is observable only when the aurora1 activity near the zenith
I
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+-
I
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I
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Hiss advance
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ioo
I
Hiss delay
2oc ‘Ins
’ -100
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Hiss advaiice -k
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HISS advance -Cc
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-300
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HISS delay
100
I
Hiss delay
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t -200
I
-100
Hiss advance Jr
00
f
0
I
HISS delay
100
0,
Fig. 8. Time-shifted cross correlation between the aurora1luminosities and hiss enhancements for auroral regions A, 23, 0, D and E. Triangles, crosses and circles indicate the correlation is taken between aurora1 luminosities and narrow band hiss intensity in the frequency bands N-3+4 kHz, 6.8-S-8 kI& and S-813.0 kH2, respectively.
0
5-
3-
HISS advance -4-
mr
E&f3 emitting k-sxord activity
767
is at a much higher level than surrounding regions and when there are only a few characteristic hiss emitting activities on the field of view of the TV system. It is clear that the examples of this paper fall, for the most part, under the above conditions, That the peak to peak relation between them has not been clearly seen previously and accordingly that the correlation heretofore reported has not been very high may be due to the observational limitations on the spatial and time resolutions of the aurora1 structure and its time variations. Generally many such hiss emitting activities can exist simultaneously in widespread aurora1 forms. A second point is the slight (above 50 ms) advancement in time of hiss enhancement over aurora1 brigh~~g on the average, although scattered from -80 to + 160 ms in these examples. The 50 ms advancement of hiss, however, is not all real. This is because that the TV system has the ch~ac~ristic response time of a silicon target for electron multiplication which amounts to about 17 ms, which is nearly equal to the interval between frames of the TV system. Thus, the real advancement of hiss enhancement before aurora1 brightening is about 30 ms. The 30 ms delay of aurora1 luminosity behind hiss enhancement is likely to come from the contribution of 5577 A aurora1 green line. Though the quantum efficiency of the imaging surface of the TV camera is maximum at a wave length of 4200 A, it is still as much as 213 of the maximum at 5577 A. Hence, the average contribution of the green line emission in the total light is estimated to be about a few per cent to 10 per cent. Since the luminosity changes with a time comparable to the green line ch~rae~risti~ time (~1 s) a cont~bution of 10 per cent would yield a time delay in the total recorded intensity of about 30 min. Consequently, we can conclude that most of the spike-like hiss enhancements show an extremely good simultaneity with the increases in local aurora1 electron precipitations within a time accuracy of few tens of milliseconds on the average. CONCLUDINU REMARKS The local aurora1 activities during a breakup phase were found to be associated with specific spike-like increase in hiss intensity, and no systematic time difference was found between enhancements of local aurora1 electron precipitations and those of hiss intensities provided that the contribution of the forbidden line is about 10% or less of the total light recorded by the TV system. This result, first of all, may suggest that due to the p~cipitation of aurora1 particles, a high electron density column is produced which serves as a wave duct which permits hiss waves to easily penetrate the ionosphere. However, the fact that the amoral hiss is associated with specific characteristics of aurora, such as rapid brightening and rapid motions, and that the aurora1 hiss is not associated with quiet aurora (which have no remarkable motions and changes in luminosity), even when very luminous, suggests that the ducting of hiss waves due to the precipitation may not be the exclusive interpretation of the hiss enhancements simultaneous with the local aurora1 activities. Generation and amplification mechanisms of hiss waves may possibly be found in the mechanisms for the speciflo, rapid acceleration of aurora1 electrons in rather low altitudes (~5000 km) as suggested by GURNETT and FEANH (1972) on the basis of the cutoff frequency ~st~butio~ of hiss obse~ed at satellite altitudes. The facts mentioned above also do not allow an interpre~tion
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of aurora1 hiss, at least, in terms of ‘stationary’ emissions due to stationary electron precipitations, since rapid increases in electron flux as well as rapid motions of the precipitation regions are necessary conditions for hiss emissions. If this is the case, the rapid increase and decay in local aurora1 luminosity within a small volume as well as the rapid motions would suggest that the amoral electrons act somewhat collectively as a small electron cloud, which may give rise to a high efficiency for producing aurora1 hiss emissions, in terms of coherent Cerenkov radiation.
BURTON
E.T.md BOARDMAN E.M. GURNETCC D. A. and FRANE L. A. HARANU L. and LARSEN R. HELLWELL R. A.
1933 1972 1965 1965
HIRASAWA T. and NAQATA T. JORQENSENT. S. KOKUXJN S., M&ITA K. and HIRASAWA T. MOROZUMIH. M. ROSENBERGT. J.
1972 1966 1972
Proc. IRE 21,1476. J. geophys. Re-s. 77, 172. J. atmos. tew. Phys. 27, 481. Whistlers and Related Ionosphetic Phenomena. Stanford University Press. JARE Sci. Rept. A, No. 10, 1. J. geophys. Res. 71, 1367. Rept. Ionos. Space Res., Japan, 28, 138.
1965 1968
Rept. Ionos. Spa.ce Res., Japan Planet. Space Sci. 16, 1419.
19,286.