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Pulsating aurora and geomagnetic pulsations in the daytime high-latitude region
PULSATING AURORA AND GEOMAGNETIC PULSATIONS TN THE DAYTIME HIGH-LATITUDE REGION 0. 1. YAGODKINA, Y. G. VOKOBJEV and S. V. LEONTIEV Polar Geophysical Institute. Apatity. 184200, U.S.S.R.
Abstract-The structure and dynamics of the aurora1 precipitation regions associated with the pulsating aurora, as well as the relationship between aurora1 and geomagnetic pulsations have been investigated on the basis of the data obtained at Heiss Island during the winter season of 1985~~1986. It is shown that the aurora1 pulsations in the period range 5- 50 s arc generally observed in the prenoon sector and are accompanied by Pi I magnetic pulsations. Both geomagnetic and aurora1 pulsations are generated in the region of the aurora1 ratio Ihl,jU/li~77 < I equatorward of the intensive 630.0 nm luminosity band. During observations of the pulsating aurora there appeared an aurora1 intensity trough between the mid-day red aurora1 band and the region of the diffuse 557.7 nm luminosity formed by the hard electron precipitation. Two types of the aurora1 pulsations can be distinguished in optical observations. The first type is recorded by photometers in the form of a quasi-periodic irregular increasing of the aurora1 luminosity generally in 557.7 and 427.8 nm emissions. These pulsations are associated with short-living diffuse aurora1 bands which arc formed in an aurora1 intensity trough. The second type of the aurora1 pulsations is formed by a quasi-periodic expansion of the region of the hard electron precipitation. The nor~l,ern boundary of this region undergoes high-frequency modifications that arc recorded as amplitude-modulated pulsations.
I. INTRODUCTION Magnetic and aurora1 pulsations observed in the highlatitude daytime region have been under discussion in a number of papers (e.g. Vorobjev ct al., 1984; Chernouss et ~1.. 1985 : Arnoldy et cd., 1986 ; Engebretson et al., 1986; Olson, 1986; Bolshakova rt al.. 1987). Bolshakova et al. ( 1987) have discovered the relationship bctwecn Pi IC pulsations and the field aligned currents in the daytime sector of the aurora] oval. The authors concluded that Pi I C pulsations are a typical oscillating regime in the polar cusp. Arnoldy ct rd. (1986) and Olson ct ~1. (1986) considered the polar cusp to bc a source of irregular pulsations. Engebrctson ef ul. (1986), however, analyscd simultaneous ground and satellite data and did not discover any connection between the aurora] pulsation activity and the location of the polar cusp. It has been found that irregular geomagnetic pulsations of a period of 0.540 s and the precipitation of the energctic (E > IO keV) electrons are observed simultaneously. Raspopov et al. (1978) have found that geomagnetic pulsations may be accompanied by auroral pulsations of the same morphological type. From the data obtained at Spitzbergen, Vorobjev et ul. (1984) demonstrated that pulsating auroras arc located equatorward of the mid-day discrete aurora] forms which arise near the equatorial boundary of the dayside red aurora1 band (Vorobjev et rd.. 1983).
Many investigators have found that the mid-day red auroras seem to be identified with the dayside polar cusp. The present paper gives an analysis of optical and magnetic data obtained at Heiss Island (gcomagnctic latitude 74.5 N) during the winter season of lY851986 and is aimed at investigating the correlation between aurora1 and magnetic pulsations and studying the morphology and disposition of the pulsating aurora in the structure of the daytime precipitation.
2. THE DATA USED
The data were obtained from simultaneous optical and magnetic observations. Geomagnetic pulsations were rccordcd using a two-channel magnetometer supplied with filters of high and low frequency and sensitivities of 0.1 and I .3 nT mm- ‘. respectively. Pulsations with 5-300 s periods have been analysed. Aurora] pulsations were recorded by narrow-field (2/I = 6 ) photometers with interference filters in the main aurora1 emissions. The photometers were oricnted to the magnetic zenith (ZF) and 30’ towards the South (SF) along the geomagnetic meridian. The sensitivity of the photometers in i&557.7 nm is about 60 R. Morphological characteristics of the aurora have been investigated by all-sky and TV cameras. The
I 50
FIG. 1. PROBABILITY
OF APPEAKANCE OF AURORAL AND ~~EOMAGNETIC (Pit) I'ULSATIONSIN THE DAYSIDE SECTOR DERIVED FROMTHE DATAOHTAINED AT Htrss ISLAND DURING THE WINTEKSEASONOF 1985-1986.
spectral structure of the aurora1 luminosity has been investigated by a meridional scanning photometer in 427.8, 557.7, and 630.0 nm emissions and by a spectrophotometer with the sensitivity in the green line of the spectrum of about 30 R and oriented to the magnetic zenith.
3.
THE REGION OF LOCALIZATION PULSATING
OF DAYTIME
AURORA
During the season of observations (1985-1986) 64 events of geomagnetic pulsations and 13 events of pulsating auroras were detected. The geomagnetic data were divided into several groups: pulsations absent; geomagnetic pulsations which appear as an irregular pattern with the dominating period of I(& 50 s (Pil); gcomagnctic pulsations which appear as regular, almost sinusoidal variations (PC): and pulsations with a mixed pattern. An analysis of the data showed that irregular Pil pulsations are the main oscillating regime of the daytime. As seen from Fig. 1 the distribution of probability of Pi1 pulsations at Hciss Island essentially increases before noon and reaches its maximum at 10:00~12:00 M.L.T. There is a second maximum on the histogram ; it is connected with the substorm activity and is observed around midnight. Arnoldy ct cd (1986), Olson (1986) and Engebretson rt d. (1986) published similar results on the distribution probability of high-latitude irregular geomagnetic pulsations. The probability of observations of aurora1 pulsations (PA) at Heiss Island is significantly smaller than that of geomagnetic pulsations. As seen from Fig. 1 the aurora1 pulsations have maximum prob-
ability at 08:OOGl2:00 M.L.T. This coincides with the results obtained at Barenzburg (Vorobjev et al., 1984) and Heiss Island (Chernouss et al., 1985). The aurora1 pulsations are accompanied by the geomagnetic Pi1 pulsations in 72% of cases. In I3 and 1.5% of cases either Pc3 or no pulsating activity, respectively. is rcgistercd. Figure 2 shows the intervals of pulsations and featurcs of the daytnnc aurora1 luminosity. The shading at the top shows the band of 2630.0 nm emission luminosity which can be identified with the soft clectron precipitation pattern. The boundary of the red luminosity band was constructed from the data obtained by the meridional scanning photometer. It was supposed that the altitude of emission n630.0 nm is 250 km (Grentzberg and McEwen, 1978). The station’s zenith is marked by the broken line. Figure 2A gives HII idea of the am-oral situation on I8 December 1985. The black stripe indicates the time interval when irregular geomagnetic pulsations of IO30 s period were dctectcd. No aurora1 pulsations were recorded during that time. The red luminosity band was slightly poleward of the station’s zenith during the observations. Variations of the i.557.7 nm and 630.0 nm emission intensities in the zenith of the station registered by the spectrometer are shown at the bottom of Fig. 2A. It wx found that during gcomagnetic pulsations 1(5577) > 1(6300). After 08:OO U.T. a rapid cquatorward propagation of the red luminosity band started. The appearance of red luminosity in the zenith is detected as a sharp increase of the J630.0 nm intensity in the zenith. Geomagnetic pulsations disappeared at 08:lO U.T. The blackcncd stripe in Fig. 2B shows the interval
Aurora
12
and geomagnetic
pulsations in daytime high-latitude
14 ML1
151
region
I2 ML1
FIG. 2. ALW>KAL SITUATIONoh IX DEWMH~K 19X5 (A) ANN 17 DMTMHEK 1985 (B). The black stripe indicates the time interval when the pulsations were detected: (A) only irregular magnetic pulsations of IO ~30 s period; (B) both auroral and geomagnetic pulsations.
both the irregular geomagnetic and amoral pulsations, registered on 17 December 1985. Both processes were broken at the same time after 06:OO U.T. As before. the red luminosity band was located poleward of the station’s zenith when the pulsations were detected. The intensity of 557.7 nm emission at the station’s zenith was rather intensive. The ratio 1(6300)/1(5577) z 0.5 which indicates rather hard electron precipitations. Just after 06:OO U.T. the red luminosity band shifted towards the zenith where a decrease of 557.7 nm intensity and a sharp increase of the 1(6300)//(5577) ratio were observed. The appearance of the red luminosity band at the station’s zenith was accompanied by the disappearance of geomagnetic and aurora1 pulsations. Thus, the present results show that daytime pulsations can be detected in the region of hard electron precipitations equatorward of the red luminosity band. A statistical analysis of geomagnetic and auroral pulsations was performed to prove this assumption. Twelve intervals with distinctly manifested sharp stoppings of pulsations were selected. It should be noted that the events of geomagnetic and aurora1 pulsations did not coincide fully since (a) the moment of the geomagnetic pulsation stopping could not always be determined exactly ; and (b) 15% of aurora1 pulsations were not accompanied by any oscillating regime in the geomagnetic field. The results of the statistical analysis are presented in Fig. 3 where the ratio 1(6300)/1(5577) is shown together with the interof
gco-
vals of _t I h from the pulsation stopping. According to Rees and Luckey (I 974) this ratio is determined by hardness of the electron precipitation and provides good monitoring of the aurora1 situation. The curves were constructed by the method of epoches. The moment of the pulsation stopping was adopted as zero, and the points denote minute values of the ratio 1(6300)/1(5577) averaged from the I2 intervals taken. The solid line presents approximation of the experimental data according to the method of the trapezium. As seen from Fig. 3 the aurora1 and geomagnetic pulsations are obscrvcd during the time when /(6300)/1(5577) < I which indicates that the flux of the precipitating electrons is rather hard (E = I keV, Rees and Luckcy. 1974). When the red luminosity band appeared in the station’s zenith the pulsations ceased. Both the increase of the ratio 1(6300)/1(5577) and the appearance of the red luminosity are connected with the softening of the electron precipitation flux. According to this ratio the energy of the electron precipitation was 0.3-0.5 keV after the pulsation stopping. The aurora1 pulsations disappeared simultaneously with the softening of the electron precipitations at 1(6300)/1(5577) > 1. On the other hand, the geomagnetic pulsations were being recorded for a few more minutes after that. This phenomenon can be accounted for by assuming that the aurora1 and geomagnetic pulsations
0.
.-
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. . . ‘.
i
I
fk;.
et
1
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A
nPil
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0
3. THE K~~TIO/(63(N) ‘/(577)
30
60
\VITHIZ f I II INTIXVAI. AfJKOI
were gencratcd in the region of cipitations where 1(6300)/1(5577) rather localized. In this case the sation stopping is determined by the registration devices.
-60
(B)
hard electron prc< I, and they arc moment of the pulthe field of view of
4. SPECTHAI. CHARACTERISTICS OF THE DAYTIHE P~~LSATIONS
According to the optical data the pulsating auroras were divided into two direrent morphological types. Figure 4 illustrates photometric records and the spcctral distribution of the two pulsation types. The upper panel of Fig. 4A presents variation?; oS557.7 nm intcnsity obtained by zenith and slanting photometers. These data were recorded as quasi-periodic irregular increases of the luminosity intensities. Note that thcsc records are incoherent though the optical axis of the zenith and the slanting photometers are distanced at 70 km along the magnetic meridian at an altitude of 120 km. This tcstifics to the small-scale nature of the pulsating auroras. Panel 3 of Fig. 4A shows spectral distribution ol the aurora1 pulsations caleuiatcd by Fast Furic transformation. The spectral distributi[~n manifests a large set of harmonics characterized by a slight tendency to a decreasing of the harmonic power under increasing frcqucncy. On the panel the broken line shows the spectra! distribution of the geomagnetic pulsations in the ~~~ornponent of the high-frcquen~y (HF) mapnetometcr channel for the same period. Similarity of the geomagnetic and aurora1 pulsations spectra is clearly manifested. but the relationship bctwccn the amplitudes of certain harmonics in the spectral distrihlition is not retained. The bottom p:ffnel of Fig. JA dcmonstratcs the spcc-
-30 INIM
0
‘WI:sToi’f’IN(;OI:
30 GI:OM~~;~F~TIC
60min (A) ~1‘11
f’I,LSATIOYS.
tral distribution of the geomagnetic variations in the Y-component of the low frequency (LF) magnetometer channel. There is an increase of the power ;rt low frequency though these amplitudes do not cxcecd the ampiitudcs of high-frequency harmonics in the HF channel. The upper panels (1 and 7) of Fig. 4B show pulsating aurora1 intensity records dctccted as quasi-periodic variations of the aurora1 luminosity with the pllls~ltin~ period of I O-40 s which arc modulated by long-period variations. As can be seen from Fig. 4B the shortperiod variations arc recorded only by the zenith photomcter, whcreus the slanting photometer dctectcd long-period variations. The spectra! ~iistriblftioli oi aurora1 pulsations consists of a number of harmonics with a significant increase of power at low frcqucncics. Spectral analysis based on the data from 19 events shows that the powerful low-frequency component has a 2-5 min period. The spectral distr~buti~~lis of the geomagnetic puisations in the LF and HF channels are similar to the spectrum ol‘[hc aurora1 pulsations seen in Fig. 4B. The analysis of the statistical data confirms this conclusion for two types of the pulsating aurora. Note that somctimes the geomagnetic pulsation spectrum displays harmonics which arc absent from the amoral pulsation spectrum, whereas analogous recordings of the geomagnetic field contain variations which are not detected in the pulsating aurora. This may bc due to the difference in the field of view of the devices. 5.
MERIUIONAI. STRL’CTCRE 01: THE AURORAI, l,UMINOSIT\’
IN THE DAYTIME SECTOR
Figure 5 presents meridional profiles of the aurora1 luminosity in the emissions 557.7 and 630.0 nm during
Aurora
and geomagnetic
pulsations
in daytime
high-latitude
region
153
FIG. 4. AUKOKAL SITLATION ON 9 JANI.ARY 19X6 (A) AN) 17 DFCEMHER 1986 (R). (2)--records of 557.7 nm intensity pulsations obtained by the zenith and slanting photometers, respectively ; (3)--spectral distributions of aurora1 (ZF) and geomagnetic (HF) pulsations. The solid line and the broken line mark the distribution of aurora1 and geomagnetic pulsations, respectively; (4)spectral distributions of geomagnetic pulsations in the LF channel. (I),
recordings of the pulsating aurora on 17 December 1985 obtained from the scanning photometer data. The scanograms demonstrate the typical structure of the luminosity distribution along the meridian. The distribution of the 557.7 nm intensity has two maxima which are divided by the region of the lower intensity (Fig. 5, upper panel). The poleward maximum is closely correlated with a sharp increase of the intensity in 630.0 nm emission and thus is connected with the region of the red amoral band in the daytime sector. Figure 2 illustrates the location of this region in the coordinates’ geomagnetic latitude-local geomagnetic time. The equatorial maximum of the green line intensity is accompanied by an insignificant increase of 630.0 nm emission which is very typical
for the hard electron precipitation zone. According to Rees and Luckey (1974) an average energy of the precipitating electrons is 0.5-0.6 keV in the red luminosity band, whereas it is 4-6 and l-2 keV in the equatorial zone of the hard electron precipitation and in the luminosity trough, respectively. The meridional structure of the daytime aurora1 luminosity during the recording of the pulsating aurora may be obtained in the following way: the zone of the relatively hard electron precipitation with luminosity prevailing in 557.7 nm emission is shifted equatorward for several degrees in latitude from the red luminosity band. Both regions are separated by an intensity trough. The intensity 557.7 nm in the aurora1 trough is about I kR, that is,
154
I.
YAGODKINA
et
al.
I. nR
2 1
S,477” U/7 FIG. 5.
MERII~IOP;AL
‘7.6’ W’N
0
I’ROFILES
OF THE
K~~RI)INC;S 1,630.O
nm
ot
AURORAL
n
L~MIKOSITY
THF I’IJLSATINC;
AURORA
4n
Bl7’N
swan
IN EMISSIONS
0 557.7
ON I7 DKI:MLII:R
emission luminosity is shown by shading. The universal titnc is an&s
significantly glow.
s 6no w
higher
arc marked along the horizontal
than the Ievcl of the night-time
Al KORA
Daytime pulsating auroras are usually obscrvcd in small IO-20 min series for several hours. This phenomenon. as a rule. starts with the first type of pulsation, then irregular variations of luminosity (somctirncs quasi-sinusoidal forms) are transferred into pulsations with the modulated amplitude or mixed pulsations. The event on 17 Dccembcr 1985 had a similar pattern. Figures 5a and 5b.c present scanograms obtained during the first and the second aurora1 types, rcspectivcly. The dynamics of the luminosity during one intensity burst observed by the zenith photometer is shown in Fig. 5. The region which is controlled by the slanting photometer is denoted by the vertical broken line. As seen from Fig. 5A the first type of pulsation is generated by the impulse precipitation in the am-oral trough. At 04:2X U.T. an additional maximum of 557.7 nm luminosity was registered around the station’s zenith. The width of the luminosity region for an altitude of 120 km was about 100 km. At 04:28:40 U.T. the luminosity in the zenith regained its initial level and bright intensifications appeared in the field of view of the slanting photometer. The allsky TV camera shows that the bright intensification at 04:28:40 U.T. recorded by the slanting photometer (30’S) is an independent intensification and it was not
AND
w
b’L’=N
630.0
tl”?
DURING
1985.
used cverywhcrr.
The
zenith
axis.
connected with the luminosity observed at 04:2X:30 U.T. in the station’s zenith. The analysis of simultaneous photometer and TV data gives the basis for associating the pulsating aurora with the appearance of the short-living East West oriented diffuse bands and patches. The scqucntial aurora1 pictures from 03:55:48 to 03:56:16 U.T. and from 03:57:00 to 03:57:30 U.T. on 17 December I985 arc presented in Fig. 6. Each picture is an image taken every 2 s. As seen from these pictures the bands appeared at different places within the aurora1 trough. Somctimcs Iwo to three bands appcarcd simut~~ncously showjinp ;I weak tcndcncy to cquatorward shifting. The width ofditfilsc bands is 50~ 150 km and the lalitude width of lhc aurora1 trough is aboul 6 Figure 5B.C shop scanograms obtained during aurora1 pulsations with a modulated amplitude. The dcvelopmcnt of a single impuisc preceding the aurora1 pulsation series is shown in Fig. 5B. According to scanograms the pulsations in the aurora1 luminosity result from the impulsive poleward expansion of the boundary of the hard electron precipitation. The width of the aurora1 trough during this period is about 300 km, which is significantly narrower compared with that of the first type of aurora1 pulsations. When the aurora1 pulsations of the second type arc detected (Fig. 5B) the luminosity trough is periodically filled with luminosity. Figure 7 presents a detailed development of the aurora1 luminosity during the pulsating auroras given in Fig. 4B. The upper panel dcmonstratcs aurora1 formations obtained from the TV-monitor along the
Aurora and geomagnetic pulsations in daytime high-latitude region
03.55.48
03.56.16 FIG. 6. THE SEQUENTIAL AURORALPICTURES FROM03:55:48 U.T. TO 03:56:16 U.T. AND FKOM03:57:00 TO 03:57:30 U.T. ON 17 DECEMBER1985. Geomagnetic coordinates are marked in the upper left corner of the figure.
U.T
Aurora
and geomagnetic
pulsations
meridian (Chernouss et al., 1982). The shaded regions denote the aurora1 luminosity. The aurora1 development in 557.7 nm emission from IO s data obtained by the meridian scanning photometer is presented at the bottom of Fig. 7. As seen from the figure the long-period variations recorded by the slanting photometer (panel 2. Fig. 4B) arc caused by a rapid poleward expansion of the equatorward boundary of the hard electron precipitation. At the same time the small-scale impulse variations of the poleward boundary of the expansion region are rcgistcred by the zenith photometer in the form of highfrequency pulsations. The aurora1 intensification caused by the energy electron precipitation tends to fill up the aurora1 trough and. in the given example. it tills the trough up for 2 min after 04:57 U.T. During such periods a periodical degradation of the luminosity is caused by a periodical formation of holes in the diffuse aurora and they rapidly expand in different directions. Sometimes long-period variations may occur without the high-frcqucncy component. Twominute variations observed after 05:OO U.T. may serve 3s an example.
in daytime
157
Leigh-l~~titud~ region
geomagnetic
Classifications of the daytime pulsating aurora have been proposed by a number of authors (Chernouss et al., 1985; Vorobjev et ul., 1984). Chernouss et ~11. (1985) have proposed two morphological categories : (1) Pi(A)-type irregular pulsations (IO s period) occurring in connection with aurora1 small-scale patches; (2) Pc(A)-type irregular pulsations (IO-40 s period) related to diffuse large-scale auroras. The PC(A)regime signifi~dntly resembles the first type of aurora1 pulsations described in this paper; however, Pc(A)type pulsations studied by Chcrnouss LJI ul. (1984) have mainly been observed at the aurora1 zone stations. Vorobjcv rt d. (1984) observed daytime pulsations at Spitzbergen and described them as luminosity variations with 20110 s quasi-periods and often modulated by long-period (7‘ 2 5 min) variations. Vorobjev ct (E/. (I 984) suggested a global pattern of pulsation appearance for different magnetic activity levels. The present paper proinotes this invcsti~ation. The types of aurora1 pulsations proposed in the present paper resemble the two types of pulsations in
N
I
S
FIG. 7. DEVELOPMENT MEKII~AN (THE UPPER
OF THE AUKORAL LUMINOSITY FROM THE TV MONITOR ALONG THE GEOMAGNETIC FROM 10 s OATA OF THE MERIDIAN SCANNING PHOTOMETER (THE LOWER PANEL)DURING THE PULSATING AURORA ON 17 DECEMBEK 1985.
PANEL) AND
The data of the scanning photometer present: along the vertical axis, the zenith angles (left); and the distance from the zenith at an altitude of 120 km (right). Thick shading marks the regions with the luminosity intensity in 557.7 urn emission higher than 2 kR. whereas thin shading indicates to the regions of 1.5. 2.0 kR intensity, unshaded areas stand for the regions with the luminosity intensity lower than 1.5 kR.
the aurora1 zone (Yamamoto, 19%). The shape of the photometer recordings and the impulse duration of the first type arc similar to that given by Yamamoto (1988). Note that Yamamoto has associated this type of pulsation with rapidly moving east- or westward aurora1 patches which are bent by a diffuse band from the poleward side. This differs significantly from the given morphological picture of the daytime pulsations. The shape of the photometer recordings for the second type is very similar, but the periods of the modulated pulsations (the negative impulses as presented by Yamamoto, 1988) differ remarkably. Moreover, Yamamoto (I 988) has suggested that this type of pulsation in the morning sector is transrormed into torch structures by aurora1 patches, whereas in the daytime sector this happens due to the polcward expansion of the hard precipitation boundary and/or due to quasi-periodic erosion of diffuse luminosity. No typical features of torch structures have been observed in the daytime sector, but we still do not reject the possibility that the daytime pulsating structures might be iust the same, but only a transformed type of the aurora1 zone pulsations. Thus, invcstigations of the longitudinal structure of the pulsating aurora seem to be of considerable importance. The prcscnt paper shows that a typical latitudinal distribution of the aurbraf luminosity is distinguished at registration of pulsating auroras. The zone of red auroras. which is connected with precipitations of 0.3 kcV electrons, is located on the polcward side. Some degrees equatorward from the zone of red auroras, there is a band of intensive luminosity in the 557.7 nm emission which is caused by the precipitation ofrather energetic electrons. The existence of the luminosity trough between these two zones is cxtremcly significant. The energy of the precipitating electrons in the trough is evaluated as being IL2 keV. Earlier the existence of the luminosity trough was marked on TV data by Chornouss et d. ( 1986). The appcarancc of pulsating auroras is connected with either more hard precipitations inside the trough or with filling up of the intensity trough with more hard precipitations. Such structures of the aurora1 luminosity might be definitely connected with the construction of the magnetosphere and, to our minds, agrees well with the scheme suggested by Feldstein and Gafperin (1985). According to Feldstein and Galperin the structure of the diffuse luminosity equatorward of the oval in the morning sector is rather complicated. The luminosity located just equatorward ofthe oval co-exists with the fuminosity band stretched nearly along the geomagnetic parallel from the niidnight to the dayside sector. Such a situation arises due to the precipitation
of highly energetic eastward drifting electrons from midnight to noon in the region of the outer radiation belt during and after magnetospheric substortns. Within this scheme the region of the red luminosity may bc connected with the magnetospheric entry layer and/or the polar cusp. In this case the luminosity in the region ofthc intensity trough will becauscd by soft cfcctrons drilting under the effect of non-stationary convection during disturbances from the plasma sheet towards the inner ma~~letosphere. Withit this scheme the aurora1 pulsations, which arc observed in the region of the intensity trough, might be generated on the closed field lines and should be connected with non-stationary processes on the external boundary of the radiation belt.
The paper suggests the following results. (I) The auro,ral pulsations observed in the dayside sector on the latitude of Heiss Island are mainly accompanied by Pi I geomagnetic pulsations. Both aurora1 and geomagnetic pulsations are most fikcly to be observed during the prcnoon hours of local gcomagnctic titnc. (2) According to the photometric data the pufsating auroras and geomagnetic pulsations are observed in the region of the precipitation of rather hard electrons wtth ;kn energy of several keV. (3) The sources of aurora1 and gcomagnctic pulsations are located cq~latorward of the intensive red aurora1 band in the daytime sector. (4) During the registration of pulsating auroras the meridional structure of the aurora1 luminosity rcpresents a red luminosity band (soft precipitations with a mean energy of 0.54.6 keV); equatorward of this band there is situated a zone of hard (4-6 keV) clectron precipitation with the luminosity mainly in the 557.7 nm emission. These zones arc separated by the trough of luminosity where the mean energy of precipitating electrons is I-2 keV. (5) The daytime pLlfsating auroras are registered in the region of the luminosity trough and may be divided into two morphofogicaf types: a quasi-periodic irregular increasing of luminosity and intensity osciflations modulated by more long-period variations. (6) Pulsations of the first type are connected with formation of short-living diffuse bands (SO-IS0 km width) in the region of the intensity trough. (7) Pulsations of the second type are observed at impulsive polcward expansions of the hard prccipitation region; these expansions manifest themselves in a quasi-periodic form with a duration of 25 min. During this period the pofeward precipitation
Aurora
and gcomaynetic pulsation s in daytime high-latitude
boundary undergoes variations of a higher frequency ; variations are registered as aurora1 pulsations with a modulated amplitude. (8) The spectrum of the pulsating auroras is similar to that of the geomagnetic pulsations and there is a set of harmonics demonstrating the tendency to diminish the harmonics’ power when the frcqucncy increases. A powerful low-frequency harmonic with a period of 2-5 min is observed in the spectrum of the pulsations of the second type. these
REFERENCES Arnoldy. R. L., Cahill, L. I.. Eather. R. H. and Engcbretson, M. I. (1986) 0.1 Hz ULF magnetic pulsations measured at South Pole, Antarctica. J. ,qc~ph~,s. Rex. A91. 5700. Bolshakova. 0. V., Borovkova, 0. K., Troitskaya, V. A. and Khorosheva, 0. V. (1987) PilC-type pulsations in the dayside sector of the aurora1 trough. Gronqn. Aerononzicr 27. 109. Chcrnouss. S. A., Taglro\. V. R., Chernouss, M. A.. Kangas. J.. Lcinoncn. J. and Ki\ inen. M. (1985) Aurora1 and magnetic pulsations in the morning sector of the aurora1 /one. GcWph~~.\ictr21 . 19. C‘hernousa, S. A.. Taglrov, V. R.. Kornilova. T. A.. Pigin. E. V. and AIIIOSOV, L. G. (lYX2) Hardwirc and softwirc 01 automatization of geophysical rcscarch. A/x/t;/?,. p. 6. Chcrnouss. S. A.. Vorobjev. V. G.. Tagirov, V. R.. Rothwcll, P.. Grcavcs. C. and Lanchestcr. B. S. (19X6) TV and photo-
region
I.59
metric obscr\atlons of transitIon region between discrete and pulsation aurorae m the dayside. Polar geomagnetic phenomena. Intcrnatlonal Symposium. Sourdal. U.S.S.R.
.-lh.Y/ro~~f.\. p. 53. Engebrctson. M. J., Mcng. C.-l., Arnoldy. R. L. and Cahill, L. J. (1986) Pc3 pulsations observed near the South polar. J. ,qeoph~. Rm. 91 . X009. Feldstein. Ya. I. and Galperin, Yu. 1. (1985) The aurora1 luminosity structure in the high-latitude upper atmosphere: its dynamics and relationship to the large-scale structure of the Earth’s magnetosphere. Rw. Gcop/~y.v. 23, 117. Grcntzbcrg, F. and McEwen. D. J. (197X) A rocket-ground study of electron precipitation in the cleft region. Sptrcr Rev. 18. 309. Olson. J. V. (19X6) VLF signatures of the polar cusp. .I. yc~ph~. Rrs. 91. 10055. Rospopov, 0. M., Chcrnouss. S. A.. Roldugin, V. K. and Pokhotelo\. 0. A. ( 1978) Prrr/ick, f’~/lvtrr/,~,qF/t,.\-c,.vit1 f/r<, ,2ltryrrc,/f~s/l,h~,~c, rr& /o~ro.c~hc~. p. 240. Nnuka. Leningrad. Rees, M. H. and Luckcy. D. (1974) Aurora1 electron energy derived from ratio of spectroscopic emissions. I. Model computations. J. qc~ph~.s. Rex. 79. 5 IX I Vorobjev, V. G., Tagirov. V. R. and Chernouss, S. A. (1984) Relative position of the zones of soft precipitation and aurora1 pulsations in the daytime polar region. Geon~rqn. .4cwmmiu 24. 284. Vorobjev. V. G. and Turyansky, V. A. (1983) Peculiarities of the bchaviour of emission [01] i,6300 A and i5577 A in the daytime aurorae. Gt~mcqn. Aeronomicr 23, 957 (in Russian). Yamamoto. T. (198X) On the temporal fluctuations of pulsating aurora1 luminosity. J. qycopI~~,s.Rc,.v.93. 897.
Abstract-The structure and dynamics of the aurora1 precipitation regions associated with the pulsating aurora, as well as the relationship between aurora1 and geomagnetic pulsations have been investigated on the basis of the data obtained at Heiss Island during the winter season of 1985~~1986. It is shown that the aurora1 pulsations in the period range 5- 50 s arc generally observed in the prenoon sector and are accompanied by Pi I magnetic pulsations. Both geomagnetic and aurora1 pulsations are generated in the region of the aurora1 ratio Ihl,jU/li~77 < I equatorward of the intensive 630.0 nm luminosity band. During observations of the pulsating aurora there appeared an aurora1 intensity trough between the mid-day red aurora1 band and the region of the diffuse 557.7 nm luminosity formed by the hard electron precipitation. Two types of the aurora1 pulsations can be distinguished in optical observations. The first type is recorded by photometers in the form of a quasi-periodic irregular increasing of the aurora1 luminosity generally in 557.7 and 427.8 nm emissions. These pulsations are associated with short-living diffuse aurora1 bands which arc formed in an aurora1 intensity trough. The second type of the aurora1 pulsations is formed by a quasi-periodic expansion of the region of the hard electron precipitation. The nor~l,ern boundary of this region undergoes high-frequency modifications that arc recorded as amplitude-modulated pulsations.
I. INTRODUCTION Magnetic and aurora1 pulsations observed in the highlatitude daytime region have been under discussion in a number of papers (e.g. Vorobjev ct al., 1984; Chernouss et ~1.. 1985 : Arnoldy et cd., 1986 ; Engebretson et al., 1986; Olson, 1986; Bolshakova rt al.. 1987). Bolshakova et al. ( 1987) have discovered the relationship bctwecn Pi IC pulsations and the field aligned currents in the daytime sector of the aurora] oval. The authors concluded that Pi I C pulsations are a typical oscillating regime in the polar cusp. Arnoldy ct rd. (1986) and Olson ct ~1. (1986) considered the polar cusp to bc a source of irregular pulsations. Engebrctson ef ul. (1986), however, analyscd simultaneous ground and satellite data and did not discover any connection between the aurora] pulsation activity and the location of the polar cusp. It has been found that irregular geomagnetic pulsations of a period of 0.540 s and the precipitation of the energctic (E > IO keV) electrons are observed simultaneously. Raspopov et al. (1978) have found that geomagnetic pulsations may be accompanied by auroral pulsations of the same morphological type. From the data obtained at Spitzbergen, Vorobjev et ul. (1984) demonstrated that pulsating auroras arc located equatorward of the mid-day discrete aurora] forms which arise near the equatorial boundary of the dayside red aurora1 band (Vorobjev et rd.. 1983).
Many investigators have found that the mid-day red auroras seem to be identified with the dayside polar cusp. The present paper gives an analysis of optical and magnetic data obtained at Heiss Island (gcomagnctic latitude 74.5 N) during the winter season of lY851986 and is aimed at investigating the correlation between aurora1 and magnetic pulsations and studying the morphology and disposition of the pulsating aurora in the structure of the daytime precipitation.
2. THE DATA USED
The data were obtained from simultaneous optical and magnetic observations. Geomagnetic pulsations were rccordcd using a two-channel magnetometer supplied with filters of high and low frequency and sensitivities of 0.1 and I .3 nT mm- ‘. respectively. Pulsations with 5-300 s periods have been analysed. Aurora] pulsations were recorded by narrow-field (2/I = 6 ) photometers with interference filters in the main aurora1 emissions. The photometers were oricnted to the magnetic zenith (ZF) and 30’ towards the South (SF) along the geomagnetic meridian. The sensitivity of the photometers in i&557.7 nm is about 60 R. Morphological characteristics of the aurora have been investigated by all-sky and TV cameras. The
I 50
FIG. 1. PROBABILITY
OF APPEAKANCE OF AURORAL AND ~~EOMAGNETIC (Pit) I'ULSATIONSIN THE DAYSIDE SECTOR DERIVED FROMTHE DATAOHTAINED AT Htrss ISLAND DURING THE WINTEKSEASONOF 1985-1986.
spectral structure of the aurora1 luminosity has been investigated by a meridional scanning photometer in 427.8, 557.7, and 630.0 nm emissions and by a spectrophotometer with the sensitivity in the green line of the spectrum of about 30 R and oriented to the magnetic zenith.
3.
THE REGION OF LOCALIZATION PULSATING
OF DAYTIME
AURORA
During the season of observations (1985-1986) 64 events of geomagnetic pulsations and 13 events of pulsating auroras were detected. The geomagnetic data were divided into several groups: pulsations absent; geomagnetic pulsations which appear as an irregular pattern with the dominating period of I(& 50 s (Pil); gcomagnctic pulsations which appear as regular, almost sinusoidal variations (PC): and pulsations with a mixed pattern. An analysis of the data showed that irregular Pil pulsations are the main oscillating regime of the daytime. As seen from Fig. 1 the distribution of probability of Pi1 pulsations at Hciss Island essentially increases before noon and reaches its maximum at 10:00~12:00 M.L.T. There is a second maximum on the histogram ; it is connected with the substorm activity and is observed around midnight. Arnoldy ct cd (1986), Olson (1986) and Engebretson rt d. (1986) published similar results on the distribution probability of high-latitude irregular geomagnetic pulsations. The probability of observations of aurora1 pulsations (PA) at Heiss Island is significantly smaller than that of geomagnetic pulsations. As seen from Fig. 1 the aurora1 pulsations have maximum prob-
ability at 08:OOGl2:00 M.L.T. This coincides with the results obtained at Barenzburg (Vorobjev et al., 1984) and Heiss Island (Chernouss et al., 1985). The aurora1 pulsations are accompanied by the geomagnetic Pi1 pulsations in 72% of cases. In I3 and 1.5% of cases either Pc3 or no pulsating activity, respectively. is rcgistercd. Figure 2 shows the intervals of pulsations and featurcs of the daytnnc aurora1 luminosity. The shading at the top shows the band of 2630.0 nm emission luminosity which can be identified with the soft clectron precipitation pattern. The boundary of the red luminosity band was constructed from the data obtained by the meridional scanning photometer. It was supposed that the altitude of emission n630.0 nm is 250 km (Grentzberg and McEwen, 1978). The station’s zenith is marked by the broken line. Figure 2A gives HII idea of the am-oral situation on I8 December 1985. The black stripe indicates the time interval when irregular geomagnetic pulsations of IO30 s period were dctectcd. No aurora1 pulsations were recorded during that time. The red luminosity band was slightly poleward of the station’s zenith during the observations. Variations of the i.557.7 nm and 630.0 nm emission intensities in the zenith of the station registered by the spectrometer are shown at the bottom of Fig. 2A. It wx found that during gcomagnetic pulsations 1(5577) > 1(6300). After 08:OO U.T. a rapid cquatorward propagation of the red luminosity band started. The appearance of red luminosity in the zenith is detected as a sharp increase of the J630.0 nm intensity in the zenith. Geomagnetic pulsations disappeared at 08:lO U.T. The blackcncd stripe in Fig. 2B shows the interval
Aurora
12
and geomagnetic
pulsations in daytime high-latitude
14 ML1
151
region
I2 ML1
FIG. 2. ALW>KAL SITUATIONoh IX DEWMH~K 19X5 (A) ANN 17 DMTMHEK 1985 (B). The black stripe indicates the time interval when the pulsations were detected: (A) only irregular magnetic pulsations of IO ~30 s period; (B) both auroral and geomagnetic pulsations.
both the irregular geomagnetic and amoral pulsations, registered on 17 December 1985. Both processes were broken at the same time after 06:OO U.T. As before. the red luminosity band was located poleward of the station’s zenith when the pulsations were detected. The intensity of 557.7 nm emission at the station’s zenith was rather intensive. The ratio 1(6300)/1(5577) z 0.5 which indicates rather hard electron precipitations. Just after 06:OO U.T. the red luminosity band shifted towards the zenith where a decrease of 557.7 nm intensity and a sharp increase of the 1(6300)//(5577) ratio were observed. The appearance of the red luminosity band at the station’s zenith was accompanied by the disappearance of geomagnetic and aurora1 pulsations. Thus, the present results show that daytime pulsations can be detected in the region of hard electron precipitations equatorward of the red luminosity band. A statistical analysis of geomagnetic and auroral pulsations was performed to prove this assumption. Twelve intervals with distinctly manifested sharp stoppings of pulsations were selected. It should be noted that the events of geomagnetic and aurora1 pulsations did not coincide fully since (a) the moment of the geomagnetic pulsation stopping could not always be determined exactly ; and (b) 15% of aurora1 pulsations were not accompanied by any oscillating regime in the geomagnetic field. The results of the statistical analysis are presented in Fig. 3 where the ratio 1(6300)/1(5577) is shown together with the interof
gco-
vals of _t I h from the pulsation stopping. According to Rees and Luckey (I 974) this ratio is determined by hardness of the electron precipitation and provides good monitoring of the aurora1 situation. The curves were constructed by the method of epoches. The moment of the pulsation stopping was adopted as zero, and the points denote minute values of the ratio 1(6300)/1(5577) averaged from the I2 intervals taken. The solid line presents approximation of the experimental data according to the method of the trapezium. As seen from Fig. 3 the aurora1 and geomagnetic pulsations are obscrvcd during the time when /(6300)/1(5577) < I which indicates that the flux of the precipitating electrons is rather hard (E = I keV, Rees and Luckcy. 1974). When the red luminosity band appeared in the station’s zenith the pulsations ceased. Both the increase of the ratio 1(6300)/1(5577) and the appearance of the red luminosity are connected with the softening of the electron precipitation flux. According to this ratio the energy of the electron precipitation was 0.3-0.5 keV after the pulsation stopping. The aurora1 pulsations disappeared simultaneously with the softening of the electron precipitations at 1(6300)/1(5577) > 1. On the other hand, the geomagnetic pulsations were being recorded for a few more minutes after that. This phenomenon can be accounted for by assuming that the aurora1 and geomagnetic pulsations
0.
.-
N/.
. . . ‘.
i
I
fk;.
et
1
,:
-30
YAC;OI)KINA
A
nPil
-60
I.
0
3. THE K~~TIO/(63(N) ‘/(577)
30
60
\VITHIZ f I II INTIXVAI. AfJKOI
were gencratcd in the region of cipitations where 1(6300)/1(5577) rather localized. In this case the sation stopping is determined by the registration devices.
-60
(B)
hard electron prc< I, and they arc moment of the pulthe field of view of
4. SPECTHAI. CHARACTERISTICS OF THE DAYTIHE P~~LSATIONS
According to the optical data the pulsating auroras were divided into two direrent morphological types. Figure 4 illustrates photometric records and the spcctral distribution of the two pulsation types. The upper panel of Fig. 4A presents variation?; oS557.7 nm intcnsity obtained by zenith and slanting photometers. These data were recorded as quasi-periodic irregular increases of the luminosity intensities. Note that thcsc records are incoherent though the optical axis of the zenith and the slanting photometers are distanced at 70 km along the magnetic meridian at an altitude of 120 km. This tcstifics to the small-scale nature of the pulsating auroras. Panel 3 of Fig. 4A shows spectral distribution ol the aurora1 pulsations caleuiatcd by Fast Furic transformation. The spectral distributi[~n manifests a large set of harmonics characterized by a slight tendency to a decreasing of the harmonic power under increasing frcqucncy. On the panel the broken line shows the spectra! distribution of the geomagnetic pulsations in the ~~~ornponent of the high-frcquen~y (HF) mapnetometcr channel for the same period. Similarity of the geomagnetic and aurora1 pulsations spectra is clearly manifested. but the relationship bctwccn the amplitudes of certain harmonics in the spectral distrihlition is not retained. The bottom p:ffnel of Fig. JA dcmonstratcs the spcc-
-30 INIM
0
‘WI:sToi’f’IN(;OI:
30 GI:OM~~;~F~TIC
60min (A) ~1‘11
f’I,LSATIOYS.
tral distribution of the geomagnetic variations in the Y-component of the low frequency (LF) magnetometer channel. There is an increase of the power ;rt low frequency though these amplitudes do not cxcecd the ampiitudcs of high-frequency harmonics in the HF channel. The upper panels (1 and 7) of Fig. 4B show pulsating aurora1 intensity records dctccted as quasi-periodic variations of the aurora1 luminosity with the pllls~ltin~ period of I O-40 s which arc modulated by long-period variations. As can be seen from Fig. 4B the shortperiod variations arc recorded only by the zenith photomcter, whcreus the slanting photometer dctectcd long-period variations. The spectra! ~iistriblftioli oi aurora1 pulsations consists of a number of harmonics with a significant increase of power at low frcqucncics. Spectral analysis based on the data from 19 events shows that the powerful low-frequency component has a 2-5 min period. The spectral distr~buti~~lis of the geomagnetic puisations in the LF and HF channels are similar to the spectrum ol‘[hc aurora1 pulsations seen in Fig. 4B. The analysis of the statistical data confirms this conclusion for two types of the pulsating aurora. Note that somctimes the geomagnetic pulsation spectrum displays harmonics which arc absent from the amoral pulsation spectrum, whereas analogous recordings of the geomagnetic field contain variations which are not detected in the pulsating aurora. This may bc due to the difference in the field of view of the devices. 5.
MERIUIONAI. STRL’CTCRE 01: THE AURORAI, l,UMINOSIT\’
IN THE DAYTIME SECTOR
Figure 5 presents meridional profiles of the aurora1 luminosity in the emissions 557.7 and 630.0 nm during
Aurora
and geomagnetic
pulsations
in daytime
high-latitude
region
153
FIG. 4. AUKOKAL SITLATION ON 9 JANI.ARY 19X6 (A) AN) 17 DFCEMHER 1986 (R). (2)--records of 557.7 nm intensity pulsations obtained by the zenith and slanting photometers, respectively ; (3)--spectral distributions of aurora1 (ZF) and geomagnetic (HF) pulsations. The solid line and the broken line mark the distribution of aurora1 and geomagnetic pulsations, respectively; (4)spectral distributions of geomagnetic pulsations in the LF channel. (I),
recordings of the pulsating aurora on 17 December 1985 obtained from the scanning photometer data. The scanograms demonstrate the typical structure of the luminosity distribution along the meridian. The distribution of the 557.7 nm intensity has two maxima which are divided by the region of the lower intensity (Fig. 5, upper panel). The poleward maximum is closely correlated with a sharp increase of the intensity in 630.0 nm emission and thus is connected with the region of the red amoral band in the daytime sector. Figure 2 illustrates the location of this region in the coordinates’ geomagnetic latitude-local geomagnetic time. The equatorial maximum of the green line intensity is accompanied by an insignificant increase of 630.0 nm emission which is very typical
for the hard electron precipitation zone. According to Rees and Luckey (1974) an average energy of the precipitating electrons is 0.5-0.6 keV in the red luminosity band, whereas it is 4-6 and l-2 keV in the equatorial zone of the hard electron precipitation and in the luminosity trough, respectively. The meridional structure of the daytime aurora1 luminosity during the recording of the pulsating aurora may be obtained in the following way: the zone of the relatively hard electron precipitation with luminosity prevailing in 557.7 nm emission is shifted equatorward for several degrees in latitude from the red luminosity band. Both regions are separated by an intensity trough. The intensity 557.7 nm in the aurora1 trough is about I kR, that is,
154
I.
YAGODKINA
et
al.
I. nR
2 1
S,477” U/7 FIG. 5.
MERII~IOP;AL
‘7.6’ W’N
0
I’ROFILES
OF THE
K~~RI)INC;S 1,630.O
nm
ot
AURORAL
n
L~MIKOSITY
THF I’IJLSATINC;
AURORA
4n
Bl7’N
swan
IN EMISSIONS
0 557.7
ON I7 DKI:MLII:R
emission luminosity is shown by shading. The universal titnc is an&s
significantly glow.
s 6no w
higher
arc marked along the horizontal
than the Ievcl of the night-time
Al KORA
Daytime pulsating auroras are usually obscrvcd in small IO-20 min series for several hours. This phenomenon. as a rule. starts with the first type of pulsation, then irregular variations of luminosity (somctirncs quasi-sinusoidal forms) are transferred into pulsations with the modulated amplitude or mixed pulsations. The event on 17 Dccembcr 1985 had a similar pattern. Figures 5a and 5b.c present scanograms obtained during the first and the second aurora1 types, rcspectivcly. The dynamics of the luminosity during one intensity burst observed by the zenith photometer is shown in Fig. 5. The region which is controlled by the slanting photometer is denoted by the vertical broken line. As seen from Fig. 5A the first type of pulsation is generated by the impulse precipitation in the am-oral trough. At 04:2X U.T. an additional maximum of 557.7 nm luminosity was registered around the station’s zenith. The width of the luminosity region for an altitude of 120 km was about 100 km. At 04:28:40 U.T. the luminosity in the zenith regained its initial level and bright intensifications appeared in the field of view of the slanting photometer. The allsky TV camera shows that the bright intensification at 04:28:40 U.T. recorded by the slanting photometer (30’S) is an independent intensification and it was not
AND
w
b’L’=N
630.0
tl”?
DURING
1985.
used cverywhcrr.
The
zenith
axis.
connected with the luminosity observed at 04:2X:30 U.T. in the station’s zenith. The analysis of simultaneous photometer and TV data gives the basis for associating the pulsating aurora with the appearance of the short-living East West oriented diffuse bands and patches. The scqucntial aurora1 pictures from 03:55:48 to 03:56:16 U.T. and from 03:57:00 to 03:57:30 U.T. on 17 December I985 arc presented in Fig. 6. Each picture is an image taken every 2 s. As seen from these pictures the bands appeared at different places within the aurora1 trough. Somctimcs Iwo to three bands appcarcd simut~~ncously showjinp ;I weak tcndcncy to cquatorward shifting. The width ofditfilsc bands is 50~ 150 km and the lalitude width of lhc aurora1 trough is aboul 6 Figure 5B.C shop scanograms obtained during aurora1 pulsations with a modulated amplitude. The dcvelopmcnt of a single impuisc preceding the aurora1 pulsation series is shown in Fig. 5B. According to scanograms the pulsations in the aurora1 luminosity result from the impulsive poleward expansion of the boundary of the hard electron precipitation. The width of the aurora1 trough during this period is about 300 km, which is significantly narrower compared with that of the first type of aurora1 pulsations. When the aurora1 pulsations of the second type arc detected (Fig. 5B) the luminosity trough is periodically filled with luminosity. Figure 7 presents a detailed development of the aurora1 luminosity during the pulsating auroras given in Fig. 4B. The upper panel dcmonstratcs aurora1 formations obtained from the TV-monitor along the
Aurora and geomagnetic pulsations in daytime high-latitude region
03.55.48
03.56.16 FIG. 6. THE SEQUENTIAL AURORALPICTURES FROM03:55:48 U.T. TO 03:56:16 U.T. AND FKOM03:57:00 TO 03:57:30 U.T. ON 17 DECEMBER1985. Geomagnetic coordinates are marked in the upper left corner of the figure.
U.T
Aurora
and geomagnetic
pulsations
meridian (Chernouss et al., 1982). The shaded regions denote the aurora1 luminosity. The aurora1 development in 557.7 nm emission from IO s data obtained by the meridian scanning photometer is presented at the bottom of Fig. 7. As seen from the figure the long-period variations recorded by the slanting photometer (panel 2. Fig. 4B) arc caused by a rapid poleward expansion of the equatorward boundary of the hard electron precipitation. At the same time the small-scale impulse variations of the poleward boundary of the expansion region are rcgistcred by the zenith photometer in the form of highfrequency pulsations. The aurora1 intensification caused by the energy electron precipitation tends to fill up the aurora1 trough and. in the given example. it tills the trough up for 2 min after 04:57 U.T. During such periods a periodical degradation of the luminosity is caused by a periodical formation of holes in the diffuse aurora and they rapidly expand in different directions. Sometimes long-period variations may occur without the high-frcqucncy component. Twominute variations observed after 05:OO U.T. may serve 3s an example.
in daytime
157
Leigh-l~~titud~ region
geomagnetic
Classifications of the daytime pulsating aurora have been proposed by a number of authors (Chernouss et al., 1985; Vorobjev et ul., 1984). Chernouss et ~11. (1985) have proposed two morphological categories : (1) Pi(A)-type irregular pulsations (IO s period) occurring in connection with aurora1 small-scale patches; (2) Pc(A)-type irregular pulsations (IO-40 s period) related to diffuse large-scale auroras. The PC(A)regime signifi~dntly resembles the first type of aurora1 pulsations described in this paper; however, Pc(A)type pulsations studied by Chcrnouss LJI ul. (1984) have mainly been observed at the aurora1 zone stations. Vorobjcv rt d. (1984) observed daytime pulsations at Spitzbergen and described them as luminosity variations with 20110 s quasi-periods and often modulated by long-period (7‘ 2 5 min) variations. Vorobjev ct (E/. (I 984) suggested a global pattern of pulsation appearance for different magnetic activity levels. The present paper proinotes this invcsti~ation. The types of aurora1 pulsations proposed in the present paper resemble the two types of pulsations in
N
I
S
FIG. 7. DEVELOPMENT MEKII~AN (THE UPPER
OF THE AUKORAL LUMINOSITY FROM THE TV MONITOR ALONG THE GEOMAGNETIC FROM 10 s OATA OF THE MERIDIAN SCANNING PHOTOMETER (THE LOWER PANEL)DURING THE PULSATING AURORA ON 17 DECEMBEK 1985.
PANEL) AND
The data of the scanning photometer present: along the vertical axis, the zenith angles (left); and the distance from the zenith at an altitude of 120 km (right). Thick shading marks the regions with the luminosity intensity in 557.7 urn emission higher than 2 kR. whereas thin shading indicates to the regions of 1.5. 2.0 kR intensity, unshaded areas stand for the regions with the luminosity intensity lower than 1.5 kR.
the aurora1 zone (Yamamoto, 19%). The shape of the photometer recordings and the impulse duration of the first type arc similar to that given by Yamamoto (1988). Note that Yamamoto has associated this type of pulsation with rapidly moving east- or westward aurora1 patches which are bent by a diffuse band from the poleward side. This differs significantly from the given morphological picture of the daytime pulsations. The shape of the photometer recordings for the second type is very similar, but the periods of the modulated pulsations (the negative impulses as presented by Yamamoto, 1988) differ remarkably. Moreover, Yamamoto (I 988) has suggested that this type of pulsation in the morning sector is transrormed into torch structures by aurora1 patches, whereas in the daytime sector this happens due to the polcward expansion of the hard precipitation boundary and/or due to quasi-periodic erosion of diffuse luminosity. No typical features of torch structures have been observed in the daytime sector, but we still do not reject the possibility that the daytime pulsating structures might be iust the same, but only a transformed type of the aurora1 zone pulsations. Thus, invcstigations of the longitudinal structure of the pulsating aurora seem to be of considerable importance. The prcscnt paper shows that a typical latitudinal distribution of the aurbraf luminosity is distinguished at registration of pulsating auroras. The zone of red auroras. which is connected with precipitations of 0.3 kcV electrons, is located on the polcward side. Some degrees equatorward from the zone of red auroras, there is a band of intensive luminosity in the 557.7 nm emission which is caused by the precipitation ofrather energetic electrons. The existence of the luminosity trough between these two zones is cxtremcly significant. The energy of the precipitating electrons in the trough is evaluated as being IL2 keV. Earlier the existence of the luminosity trough was marked on TV data by Chornouss et d. ( 1986). The appcarancc of pulsating auroras is connected with either more hard precipitations inside the trough or with filling up of the intensity trough with more hard precipitations. Such structures of the aurora1 luminosity might be definitely connected with the construction of the magnetosphere and, to our minds, agrees well with the scheme suggested by Feldstein and Gafperin (1985). According to Feldstein and Galperin the structure of the diffuse luminosity equatorward of the oval in the morning sector is rather complicated. The luminosity located just equatorward ofthe oval co-exists with the fuminosity band stretched nearly along the geomagnetic parallel from the niidnight to the dayside sector. Such a situation arises due to the precipitation
of highly energetic eastward drifting electrons from midnight to noon in the region of the outer radiation belt during and after magnetospheric substortns. Within this scheme the region of the red luminosity may bc connected with the magnetospheric entry layer and/or the polar cusp. In this case the luminosity in the region ofthc intensity trough will becauscd by soft cfcctrons drilting under the effect of non-stationary convection during disturbances from the plasma sheet towards the inner ma~~letosphere. Withit this scheme the aurora1 pulsations, which arc observed in the region of the intensity trough, might be generated on the closed field lines and should be connected with non-stationary processes on the external boundary of the radiation belt.
The paper suggests the following results. (I) The auro,ral pulsations observed in the dayside sector on the latitude of Heiss Island are mainly accompanied by Pi I geomagnetic pulsations. Both aurora1 and geomagnetic pulsations are most fikcly to be observed during the prcnoon hours of local gcomagnctic titnc. (2) According to the photometric data the pufsating auroras and geomagnetic pulsations are observed in the region of the precipitation of rather hard electrons wtth ;kn energy of several keV. (3) The sources of aurora1 and gcomagnctic pulsations are located cq~latorward of the intensive red aurora1 band in the daytime sector. (4) During the registration of pulsating auroras the meridional structure of the aurora1 luminosity rcpresents a red luminosity band (soft precipitations with a mean energy of 0.54.6 keV); equatorward of this band there is situated a zone of hard (4-6 keV) clectron precipitation with the luminosity mainly in the 557.7 nm emission. These zones arc separated by the trough of luminosity where the mean energy of precipitating electrons is I-2 keV. (5) The daytime pLlfsating auroras are registered in the region of the luminosity trough and may be divided into two morphofogicaf types: a quasi-periodic irregular increasing of luminosity and intensity osciflations modulated by more long-period variations. (6) Pulsations of the first type are connected with formation of short-living diffuse bands (SO-IS0 km width) in the region of the intensity trough. (7) Pulsations of the second type are observed at impulsive polcward expansions of the hard prccipitation region; these expansions manifest themselves in a quasi-periodic form with a duration of 25 min. During this period the pofeward precipitation
Aurora
and gcomaynetic pulsation s in daytime high-latitude
boundary undergoes variations of a higher frequency ; variations are registered as aurora1 pulsations with a modulated amplitude. (8) The spectrum of the pulsating auroras is similar to that of the geomagnetic pulsations and there is a set of harmonics demonstrating the tendency to diminish the harmonics’ power when the frcqucncy increases. A powerful low-frequency harmonic with a period of 2-5 min is observed in the spectrum of the pulsations of the second type. these
REFERENCES Arnoldy. R. L., Cahill, L. I.. Eather. R. H. and Engcbretson, M. I. (1986) 0.1 Hz ULF magnetic pulsations measured at South Pole, Antarctica. J. ,qc~ph~,s. Rex. A91. 5700. Bolshakova. 0. V., Borovkova, 0. K., Troitskaya, V. A. and Khorosheva, 0. V. (1987) PilC-type pulsations in the dayside sector of the aurora1 trough. Gronqn. Aerononzicr 27. 109. Chcrnouss. S. A., Taglro\. V. R., Chernouss, M. A.. Kangas. J.. Lcinoncn. J. and Ki\ inen. M. (1985) Aurora1 and magnetic pulsations in the morning sector of the aurora1 /one. GcWph~~.\ictr21 . 19. C‘hernousa, S. A.. Taglrov, V. R.. Kornilova. T. A.. Pigin. E. V. and AIIIOSOV, L. G. (lYX2) Hardwirc and softwirc 01 automatization of geophysical rcscarch. A/x/t;/?,. p. 6. Chcrnouss. S. A.. Vorobjev. V. G.. Tagirov, V. R.. Rothwcll, P.. Grcavcs. C. and Lanchestcr. B. S. (19X6) TV and photo-
region
I.59
metric obscr\atlons of transitIon region between discrete and pulsation aurorae m the dayside. Polar geomagnetic phenomena. Intcrnatlonal Symposium. Sourdal. U.S.S.R.
.-lh.Y/ro~~f.\. p. 53. Engebrctson. M. J., Mcng. C.-l., Arnoldy. R. L. and Cahill, L. J. (1986) Pc3 pulsations observed near the South polar. J. ,qeoph~. Rm. 91 . X009. Feldstein. Ya. I. and Galperin, Yu. 1. (1985) The aurora1 luminosity structure in the high-latitude upper atmosphere: its dynamics and relationship to the large-scale structure of the Earth’s magnetosphere. Rw. Gcop/~y.v. 23, 117. Grcntzbcrg, F. and McEwen. D. J. (197X) A rocket-ground study of electron precipitation in the cleft region. Sptrcr Rev. 18. 309. Olson. J. V. (19X6) VLF signatures of the polar cusp. .I. yc~ph~. Rrs. 91. 10055. Rospopov, 0. M., Chcrnouss. S. A.. Roldugin, V. K. and Pokhotelo\. 0. A. ( 1978) Prrr/ick, f’~/lvtrr/,~,qF/t,.\-c,.vit1 f/r<, ,2ltryrrc,/f~s/l,h~,~c, rr& /o~ro.c~hc~. p. 240. Nnuka. Leningrad. Rees, M. H. and Luckcy. D. (1974) Aurora1 electron energy derived from ratio of spectroscopic emissions. I. Model computations. J. qc~ph~.s. Rex. 79. 5 IX I Vorobjev, V. G., Tagirov. V. R. and Chernouss, S. A. (1984) Relative position of the zones of soft precipitation and aurora1 pulsations in the daytime polar region. Geon~rqn. .4cwmmiu 24. 284. Vorobjev. V. G. and Turyansky, V. A. (1983) Peculiarities of the bchaviour of emission [01] i,6300 A and i5577 A in the daytime aurorae. Gt~mcqn. Aeronomicr 23, 957 (in Russian). Yamamoto. T. (198X) On the temporal fluctuations of pulsating aurora1 luminosity. J. qycopI~~,s.Rc,.v.93. 897.