- Email: [email protected]
Pulsations of precipitating energetic electrons: ACTIVE satellite data
Journal of ATMOSPHERIC AND PERGAMON
Journal of Atmospheric and Solar-Terrestrial Physics 60 (1998) 643-653
Pulsations
of precipitating energetic electrons ACTIVE satellite data
K. Kudelaa,b*, I. M. Martinb, F. K. Shuiskaya”, F. Jiricekd
:
M. M. Mogilevsky”
aInstitute of Experimental Physics, SAS, Kofice, Slovakia b VNICAMP, Campinas, SP, Brazil ‘Space Research Institute, Russian Academy of Sciences, Moscow, Russia ‘Institute of Atmospheric Physics, Bocni II, Prague, Czech Republic
Received 22 July 1996, in revised forms 5 May 1997 and 11 December 1997 accepted 19 December 1997
Abstract We present the patterns of high frequency energetic electron (- 20 to - 300 keV) pulsations in middle latitudes, in subauroral and aurora1 regions, based on low altitude nearly polar orbiting satellite (Intercosmos-24, project ACTIVE) measurements. Two detectors with different orientations allowed the fluxes within and outside the loss cone to be compared. The trapped electrons do not exhibit the strong pulsations observed within the bounce loss cone. The pitch angle diffusion coefficient changes significantly on time scales comparable to or even shorter than the bounce period of electrons. The same is valid for changes of precipitating electron energy spectra. There are indications that both pitch angle and energy diffusion are in operation. VLF emissions atf< 3.5 kHz with a complicated structure are observed simultaneously with the electron pulsations, but there is no clear correlation between waves and particles. If the characteristics of the electron pulsations and VLF emissions are of a temporal character, they are qualitatively consistent with the models of nonlinear wave particle interactions, or with the concept of a flowing cyclotron maser. However, the pulsations may originate from fine spatial structure. Variability of the precipitation flux implies short time and/or small scale changes of ionization at altitudes down to 70 km. 0 1998 Elsevier Science Ltd. All rights reserved
1. Introduction The pulsating patches in the polar morningside aurora during the recovery phase of geomagnetic storms are caused by electrons precipitating from the outer radiation belt (see e.g. review of Davidson, 1990). The pitch angle diffusion of electrons, bringing them into the loss cone, due to cyclotron resonance with waves at the equator (Rycroft, 1991, and references therein) is accompanied by pulsations of ELF and VLF (f = 10 Hz-10 kHz) radio waves. These phenomena have been studied for over three decades. A considerable list of works related to the experimental investigations of these phenomena and their
* Corresponding author. Tel. : 0042 1 95 6224554 ; fax : 0042 1 95 6336292 ; e-mail : [email protected]
theoretical modelling can be found in reviews (e.g. Yamamoto, 1988 ; Demekhov and Trakhtengerts, 1994). In recent years there has been renewed interest in the above mentioned problems. Model calculations which detail the mechanism of resonant wave-particle interactions in the pulsating patches in the polar aurora have been carried out (Davidson and Chiu, 1991 ; Trakhtengerts, 1992, 1995). Electron precipitation pulsations have been studied for long time (e.g. Coroniti and Kennel, 1970). Further progress in experimental studies of patterns of precipitating electrons has also been reached recently. Precipitation of electrons connected to the pulsating aurora have been reported (e.g. Sandahl, 1984). The relativistic electron precipitation of short duration (less than 1 s) have been reported by Imhof et al. (1992). CRRES measurement have shown that bursts of precipitating electrons and wave intensities are highly correlated over a wide frequency range of waves and energy of electrons (Imhof et
S1364-6826/98 $19.00 0 1998 Published by Elsevier Science Ltd. All rights reserved PII: S1364&6826(98)00003-0
644
K. Kudela et a/./Journal
qf Atmospheric
and Solar-Terrestiral
al., 1994). The measurements of precipitating > 1 MeV electrons provided the estimate of their total energy input to the atmosphere (Imhof and Gaines, 1993). For the study of particle precipitation dynamics at low altitude orbits, high temporal resolution of the measurements is important. The temporal resolution of the ACTIVE satellite measurement is 0.1 s. Here we present the characteristics of the energetic electron pulsations using the measurements on this satellite which allowed comparison of the fluxes of precipitating and trapped particles using high temporal resolution over a range of energies. The character and possible causes of the strong and fast pulsations of electrons within the local loss cone, as well as their consequences for the upper atmosphere, are discussed.
2. Data Intercosmos(ACTIVE) was launched on 22 September 1989 into an orbit with perigee 500 km, apogee 2500 km and inclination i = 82.5”. The satellite has a 3axis orientation with respect to the Earth (x-axis along the velocity, and z-axis along the Earth’s radius vector). Energetic particles measurements were carried out with the SPEl instrument (Kudela et al., 1992). The particle flux was measured by means of silicon solid state detectors The differential energy spectrum was determined by amplitude analysis of the pulses from the arriving particles. The angular distributions are estimated from simultaneous measurements made by three pairs of detectors having different orientations, namely 39”, 69”, 99” with respect to the z-axis. The angle between the x-axis and axes of the detectors with 157.5”. In each pair a magnetic filter is put in front of one detector to deflect electrons with energies below 650 keV (the proton detector with the thickness of 100 pm), in front of the second detector of the pair there is an aluminium foil to absorb ions with energy up to 700 keV (the electron detector has the thickness 300 pm). Each detector has the field of view
Physics 60 (1998) 643-653
angle 20”. The diameter of each detector is 8 mm. The geometrical factor of each detector is 0.03 cm’ ster. Energy thresholds of particle registration vary depending on individual characteristics of the detectors and foils. Energy ranges for protons are from 25 to 800 keV and for the electrons from 20 to 400 keV. Three operational modes named A, B and C, differing in number of energy ranges and time resolution. have been used during the flight. The energy ranges were divided quasilogarithmically and their number was 7 in the A-mode, 15 in the B-mode and 3 1 in the C-mode. The best time resolution was in the A mode used in this work. The maximum count per channel was 255. To enlarge the dynamic range the pulse counters are stopped when any one of the channels reached 255 and the corresponding time is recorded (this is done separately for electron and proton channels). Active cooling of the detectors was performed by means of Peltier elements to suppress the thermal noise. Other details of the experiment can be found in (Kudela et al., 1992). The energy ranges of the detectors 1 and 3 are listed in Table 1. For comparison we report here also examples of wave measurements. VLF electromagnetic fields were observed by the NVK-ONCH receiver. Only the data of the waveform electric field in the frequency range 10 Hz-20 kHz are used here. The measurement of the electric field components from 8 Hz to 20 kHz was achieved by means of pairs of spherical vitreous carbons sensors, located at the ends of insulated booms. The distance between the sensors was 2 m. Each sensor was equipped with a unit gain preamplifier. Signals coming from the preamplifiers were processed in the electronic unit located inside the spacecraft. The difference between the signals from the pair of preamplifiers was filtered and then amplified. The output signal fed in parallel a set of narrowband filters and was amplified by a 2 step amplifier which could be positioned either in a free run mode, with automatic gain control, or in a fixed gain mode by telecommand. The processed signal was transmitted by direct modulation of the telemetry transmitter, recorded at the ground tel-
Table 1 Energy ranges of electron
detectors
Energy channel number
Range of detector 1 (keV) (trapped electrons*)
Range of detector 3 (precipitating electrons*)
1
17.7-27.9 27.944.2 44.2-69.9 69.9-l 11 Ill-175 175-277 r271
20.6-32.0 32.0-49.6 49.6-16.9 76.9-l 19 119-185 185-287 >287
2 3 4 5 6 7 * In the analyzed
portion
of the orbit presented
here.
K. Kudela et al.lJournal
qf Atmospheric and Solar-Terrestiral
emetry station and processed in the laboratory. The sensitivity of NVK-ONCH receiver electric channel was 0.2 to 2 pV/(m * Hz) in the frequency range 100 Hz-15 kHz and the dynamic range was 40 dB. The highest time resolution of particle and wave measurements was obtained during intervals when the satellite was in the region of radiovisibility of Panska Ves station, Czech Republic (50”32’N, 14’34’E). Depending on the actual orbit (and especially the altitude above the receiving station) the duration of the measurements in this mode was variable, but was typically 15520 min. This allowed energetic particle data with 7 energy channels and temporal resolution of 0.1 to be obtained for the northern hemisphere for the intervals of latitude typically from 30” to 75”. The data coverage is from October 1989 until July 1992. There were, however, many gaps. Nevertheless, measurements from a total 240 passes with 0.1 s resolution are available, and the coverage of different latitudes as well as local times is sufficient for more extensive studies of pulsating precipitation than that presented here.
3. Characteristics of the pulsation event The data on the measurements of electrons during available magnetic storms in 1990 were analyzed to search for pulsating precipitations of electrons from the outer radiation belt on the morningside of the magnetosphere. Usually during the recovery phase of magnetic storms there were several cases of highly structured flow of precipitating electrons, with the modulation factor of 3-5, simultaneously with the pulsating VLF emissions. The intensity of trapped particles changed only rather slightly during these periods. For the case of orbit 2179 on 22 March 1990, starting from 0742 until 0749 the orientation of the detector 1 was 103”-91< with respect to the magnetic field B, while the angle between the axis of detector 3 and B was 162”15 1“, both varying slowly. Thus for the whole period the fluxes of electrons precipitating (detector 3) and trapped (detector 1) have been measured simultaneously. For the period until 0748, detector 3 was observing particles within the local loss cone (assuming the altitude 200 km as the boundary of the atmosphere). The altitude of the satellite (1930 km-1600 km) was suitable for measuring the stably trapped particles. The minimum altitude of mirror points for the whole period is above 1200 km. Figure 1 compares electron fluxes at the two detectors for three selected energy ranges. One second averages are plotted. Starting from an L-shell of about 4 and up to aurora1 latitudes the precipitation of the electrons is accompanied with considerable intensity increases of duration about 5520 s. The difference in the profiles of precipitating and trapped electrons is very apparent. While the trapped particles show a relatively smoothed profile,
645
Physics 60 (1998) 643-653
I(keV.s)-' 100
0.0001
0.00001 0742 L=3.2 Altitude 1920
0744 4.0
0746 5.1
0748 7.5
1660
1740
166Oh
UT
Fig. 1. Count rate profiles of electrons in units (SK’ keV_‘) at selected energy channels on 22 March 1990 observed by SPEl. For conversion to intensities (in cmm2 s I sr -’ keV. ‘) the geometry factor 0.01 cm* sr can be used. One second averages are used and the count rates are divided by the corresponding energy interval width. D means the detector, CH means its energy channel. Intensities for channels 3 and 5 are reduced with respect to those for channel 1 by a factor 3.10m2 and 9.10m4, respectively. The orientation of Dl with respect to B is changing from 103 to 91’ (left to right), while D3 had 162” to 151’, varying smoothly. The latitude and longitude interval covered is 51.8’ to 73’N. and 358” to 10’E. The local time is 07.30 to 08.30. Time divisions are at 5 and 20 s intervals.
the spiky structure of flux within the loss cone is seen at all three energy ranges. At lower latitudes (L < 5.5) the duration of spikes is short. Starting from L = 5.6 the character of the increases changes, single intensity increases change to multistructured ones. In some events the loss cone appears to be full (the precipitating intensity reaching the trapped intensity). Isotropy is reached, especially at lower energies. The occurrence of the spikes of precipitating electrons indicating strong pitch angle diffusion is different for different energies. The lowest L value at which this effect appears has the tendency to increase with increasing energy. At high energies, 1755 287 keV (not shown), an isotropic angular distribution is not reached, even at the highest latitudes. However, the spiky structure of electron precipitation remains. The observations were performed during the recovery phase of a magnetic storm with a sudden commencement. However, the pulsations of precipitating electrons have been found not exclusively in the time intervals connected to substorm activity. The character of the energy spectra of precipitating and trapped electrons (for simplicity based on the count rate ratios of the different energy channels) is shown in
K. Kudela et al./Journal of Atmospheric and Solar-Terrestiral Physics 60 (1998) 643453
646
1 0742
0744
0746
0748
m
Fig. 2. The ratios of 1 s count rate averages of different energy channels for detectors Dl and D3 for the same time period as plotted in Fig. 1.
Fig. 2 for channels 1, 3 and 5 for the same time interval as in Fig. 1. The energy spectra of the trapped (Dl) electrons are relatively stable in character and the change with latitude is smooth. In contrast, the energy spectra of the precipitating electrons is significantly more variable both at low and higher energies. Fluctuations of the ratios by a factor >3 within a few seconds are present. Although the general course of the latitudinal variation of the ratios are similar for trapped and precipitating particles, a much larger variability and fast changes of the precipitating spectra are evident. Precipitating electron spectra at low energies at L < 5.6 (before 0746) are softer than those of the trapped particles.
4. Fine structure of the precipitation Figure 3 (a, b, c) shows the intensity of the precipitating electrons in all 7 energy channels in linear scale for the three consecutive minutes, 07 : 4547 : 48, with the highest temporal resolution available, namely 0.1 s. The sequence of very short duration increases, namely on time scales less than 1 s, many of them less than 0.2-0.3 s, until approximately 07 : 46 is seen for the precipitating electrons. A linear scale with a shift of 200 impulses between the subsequent energy records is used to show the variability of the spikes. In many cases the increases are up to one order above the ‘quiet’ intensities. Very fast processes, or highly structured spatial structures of precipitating electrons, are apparent. In some of the spikes only low energy electrons are detected, in others high energies are also seen. There are also cases of fast electron pulsations only at high energies without lower energy increases. With the increasing latitude the pulses are of
longer duration, as was also apparent from Fig. 1. However, the short duration spikes are still present, superimposed on those fluxes. Thus, the roughly estimated periodicities of 5-20 s mentioned according to Fig 1 are not only ones in the precipitation structure. In addition high frequency fluctuations with characteristic time periods of < 1 s are usually observed, too. These fast pulsations are, however, much more variable in the energy spectra. The patterns of the short time (or small scale) variations in simultaneous measurements of the trapped and precipitating electron fluxes at two different energy intervals are shown in Fig. 4 (a, b, c) for the same intervals as plotted in Fig. 3. In one case the short interval pulsations of precipitating electrons at higher energies were found to precede those at lower energies. This shift is demonstrated in Fig. 5. The cross-correlations of the count rate at different energies for a 10 s interval, namely 07 : 45.2& 07 : 45.30 UT, show clear asymmetry with respect to a time lag = 0. If the lowest energy range is compared with those at higher energies, the shift to at least 0.1 s is more pronounced. The high energy spikes precede the spikes at lower energies. It has to be mentioned, however, that this type of phase shift is not clearly apparent in all spiky structures of precipitation. The correlation between the lowest and highest energy profiles is in many cases lost. However the comparison of two neighbouring energy ranges confirms this tendency in general, for L < 6. The forward energy dispersion of the precipitating spikes is also seen in the evolution of the energy spectra. Figure 6 presents the electromagnetic emissions measured simultaneously with the electron flux pulsations. This figure gives a general view of the spectral evolution during 105 s from 07 : 45.00 to 07 : 46.45. The spiky structure of the emissions is observed in different frequency ranges. The correlation between the emission structures and electron precipitation is, however, not clear. A different pattern, namely emissions in wider frequency ranges, with longer duration and more continuous, are observed close to the end time of the record, i.e. after 07:46.35. The first pulse with longer duration (about 10 s) is observed at the same time as an electron pulsation on SPEl.
5. Discussion For these measurements of highly structured fluxes of precipitating electrons there were no observations of the aurora1 forms. However, the characteristics of the measurements such as: the appearance on the morningside of the magnetosphere during the recovery phase of a magnetic storm in the range of L-shells corresponding to the outer belt and aurora1 latitudes; the source of intensity bursts near the equator; anisotropy
K. Kudela et al.lJournal of Atmospheric and Solar-Terrestiral Physics 60 (1998) 6436.53
647
CR(S.‘) 2.10
Fig. 3. 100 ms resolution data (count rates) on electron precipitation for the interval 07 : 4547 : 46 (a), 07 : 4&07 : 47 (b), and 07 : 4707 : 48 (c). The count rates (CR) obtained during the 0.1 s periods in different channels of Dl are plotted linearly with shifts by 200
between subsequent energy ranges.
of electron fluxes during the burst minimum and their isotropy for resonance energies during burst maximum ; the upper value of the resonance energy reaching 100 keV and sometimes 200 keV in the maximum of the intensity burst ; the duration of the intensity flash about several seconds, fine structure of the increase consisting of elementary bursts with an ‘on time’ of about 0.2 s and with a modulation frequency of about 1.5 Hz ; simultaneous measurements of the electromagnetic emission chorus in frequency range l-3.5 kHz-are all circumstantial evidence that our observations refer to the phenomenon of ‘pulsating patches’ of the polar aurora. The features of the precipitating electrons and electromagnetic emissions in pulsating patches analogous to
our case were reported, for example, in earlier papers (Bryant et al., 1971 ; Lepine et al., 1980; Sandahl et al., 1980; Tsuruda et al., 1981; Sandahl, 1984; Evans et al., 1987; Yamamoto, 1988 ; Hansen et al., 1988 Imhof et al., 1994). The key question in the possible explanation of the observations of the fast strong pulsations of energetic electrons observed on a single low altitude satellite, even with high temporal resolution, is the ambiguity between temporal and spatial character of these fluctuations. Pulses with periods 5-20 s correspond to measurements over distances of a few tens to more than one hundred kilometers. Such distances are comparable with the typical size for the patches of aurora1 pulsations. Thus it is
648
K. Kudela et al/Journal qf’Atmospheric and Solar-Terrestiral Physics 60 (1998) 6436.53
Fig. 4. The count rates for energy channels 1 and 3 of the trapped (Dl) and precipitating (D3) electrons are compared for the three intervals covered in Fig. 3. Data of Dl are shifted upwards by 200 with respect to D3. Precipitating electrons are shown by dots (CHI)
and by crosses (CH3), while lines are labelled and connect the profiles of the trapped electrons. Although the 1saverages of precipitating electrons usually do not exceed the fluxes of trapped ones, in several short spikes they clearly do.
impossible to exclude the spatial character of subauroral and aurora1 inhomogenities contributing to the observed fluctuations. The morningside aurorae at latitudes below invariant latitudes 70” display complex structures which are believed to be formed by the trapped electrons injected in aurora1 substorms (e.g. review by Davidson, 1990), which is not the case for the evening or the nighttime sectors. However, the occurrence of strong precipitating electron pulsations is not exclusively in the morningside. The probability of pulsations in the midnight time sector is also high according to ACTIVE satellite data (Kudela, 1997a). Pulsations of relativistic elec-
trons have also been observed in the nighttime sector by other experiments. According to Imhof et al. (1991), significant fluxes of precipitating relativistic electrons > 1 MeV within the loss cone are much more frequent near midnight than noon. They generally occur in narrow spikes < 100 km in width, typically at L values between 4 and 6 near the radiation belt boundary. Thus there is not an exclusive connection between the pulsating character of precipitating electrons and the aurora1 and subauroral morningside spatial inhomogenities during the substorm events. The narrow widths of the precipitating regions of elec-
K. Kudela et al./Joumal
of Atmosphericand
CC+A 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -3
-2
-1
1
0
2
3
time lag (in 0.1s)
Fig. 5. The cross-correlations (CC) between the intensities (in #/keV s)) of the energy channel 1 (lowest energies, first time series, X,) vs the intensities in higher energy channels (the second time series, Y,,,, t and k are in units of 0.1 s). The crosscorrelations are shifted vertically by A. Those with channel 2 (full circles, A = 0.6), channel 3 (crosses, A = 0.4) channel 4 (asterisks, A = 0.2) and channel 5 (open circles, A = 0) of detector 3 detect precipitating electrons during the 10 s interval after 07: 45.20 UT. 100 points with 0.1 s resolution are taken. The coordinates are: L = 4.74.8, altitude 1750 km, longitude l”E, magnetic local time is 9.1. The tendency for higher energies to be observed before the lower energies (negative time lag between the respective pairs of two time series) is more pronounced with increasing energy.
trons (smaller than 40 km) has been observed indirectly through bremsstrahlung X rays (Datlowe et al., 1993). In that paper significant variations on kilometer scale sizes were also reported. The data from SPEl help to confirm this feature from direct electron measurements. From Fig. 3 it is clear that, within 1 s, corresponding to the scale of few kilometers, the energy spectra of precipitating electrons is strongly changing. According to Fig. 6 the high energy (> 100 keV) precipitating electrons at 1800 km are advancing the low energy (- 20 keV) pattern by 0.1 s. It may be possible that an additional acceleration of electrons can contribute to that pattern. According to Swift and Gorney (1989) the acceleration of electrons to energies in excess of 100 keV sometimes occurs on the field lines associated with discrete aurora1 arcs, and the process can occur in the altitude range from the topside ionosphere up to 5.500 km. If the acceleration takes place in narrow altitude regions and above the observation point, forward velocity dispersion at ionospheric altitudes is expected. In a few examples SPEl is observing the short
spikes
flux of electrons bilities
of precipitating outside
of the acceleration
electrons
the loss cone.
in narrow
One
exceeding
the
of the possi-
regions would give
Solar-Terrestiral
Ph?asics 60 (1998)
643453
649
this forward velocity dispersion assuming temporal variations in the process energizing the electrons in the auroral arc. The uncertainty due to the individual characteristics of the detectors, namely not exactly the same geometric factor and not identical energy ranges, puts some limits on this comparison. However, in several short spikes the precipitating electron flux is a factor of 1.552 larger than that of the trapped particles. In one second averages, as seen in Fig. 1, the maximum precipitating electron flux reaches-but does not exceed--the trapped flux, and the isotropization expected during times of strong pitch angle diffusion can be identified. The pattern of short time precipitating flux exceeding the trapped flux at the same energy has to be checked using a larger amount of SPEl data (see Fig. 4). Such a pattern is difficult to explain in the frame of pure pitch angle diffusion due to wave particle interactions without the redistribution of energy in periods shorter then the bounce period. According to Swift and Gorney (1989) the waves efficiently scatter the electron beam in both energy and pitch angle. We suggest that combined pitch angle and energy diffusion is indicated in the short spike patterns presented by SPEl. However, Fig. 5 shows only one clear example of the short time velocity dispersion of precipitating electrons. The detailed analysis of occurrence of the values and of the sense of dispersion requires a statistical analysis which is now in progress. Together with the improvement of temporal resolution, this is important for resolving the fine temporal versus spatial structure of electron precipitation. The most common mechanism for pitch angle diffusion of electrons originally trapped and subsequently precipitating is cyclotron resonance between the trapped electrons and whistler mode waves. This process can lead not only to the reduction of electron pitch angles but simultaneously to the amplification of the wave emissions. The resonance is usually assumed to take place near the equatorial plane. The direct ionisation due to precipitating electrons in the upper atmosphere can lead to the modifications of amplitudes and phases of the VLF waves in the ionosphere and a feedback situation can occur (Rycroft, 1991). For the time period 07:45.00 to 07:46.40 UT (Fig. 5) the VLF emissions have higher amplitudes (if compared with other passes through the same L interval) ; however, the pattern of VLF emissions observed on the same satellite in the same time period when precipitating pulsations occur is far from having a clear correspondence to only a temporal (or only a spatial) structure of electron precipitation. The typical chorus time corresponds to time scales of electron flux pulsation, but there is no peak-to-peak conformity between chorus and electron pulsations. Such a correspondence should be expected only if the measurement of waves and electrons is done simultaneously at a fixed geomagnetic position for at least a few bounce periods of the electrons. The precipitating pattern is of very small
650
K. Kudela et al./Journal
of Atmospheric
and Solar-Terrestiral
Physics 60 (1998) 643-653
Amplitude IK-24 orb 2179
074.630 06 02 4.7 1763
Fig. 6. The VLF emissions atf<
074666 OS 10 6.1 1768
5 kHz for the available *Hz).
22.3.1990
074630 06 16 6.6 1743
set of data overlapping
07 47 00 6627 6.0 t71a
with the electron
UT OMT L AL1 (km]
fluxes observed
at the same satellite.
The signal is given in units of pV/(m
scale and/or of very fine temporal structure ; assuming that the interaction is taking place at the equatorial plane, the difference between the time of wave propagation and of scattered electrons from the interaction region to the observation point is masked by the motion of the satellite within the time of one bounce period by several kilometres. There exist theoretical approaches which are not in contradiction with the electron pulsations presented here; however, the SPEl measurements, even if compared with VLF emissions in this particular case on the satellite, are not providing the definite evidence for their exclusive validity. Several theoretical papers come to the conclusion that the pulsating patches are the result of the whistler cyclotron instability development in field-aligned ducts with enhanced plasma density and in the presence of a source of energetic particles in the region of instability. Davidson (1979) pointed out the importance of strong diffusion and tried to connect the pulsation period with the filling and emptying of the loss cone. The pitch angle diffusion model for morningside aurorae described in Davidson (1986a, b) predicts cyclic or repetitive pulsations for a wide range of conditions, with periods of 330 s. The pulsation periods observed here, if they are of
a temporal character, are consistent with that model. The aspects of relations between temporal and spatial structures in morningside aurorae can be explained with the aid of a nonlinear model describing the interactions between energetic electrons and whistler waves (Davidson and Chiu, 1991). In that paper the existence of spatial structures having sufficiently fine scales can provide a repetitive perturbation that maintains the temporal variations. In these theoretical approaches the importance of the loss cone distribution and its dynamics for the explanation of the pulsations is stressed. The measurements discussed here show a very fast and/or highly structured pattern of electrons within the loss cone if compared to the simultaneously measured trapped electron dynamics. This is in qualitative consistency with the above-mentioned theoretical suggestions. The paper by Demekhov (1991) shows that, to explain the autopulsating mode of quasilinear relaxations, the nonlinear modulation of the particle distribution outside the loss cone is important. A list of the theories of the pulsating aurorae is presented in Demekhov and Trakhtengerts (1994). The specific mechanism of pulsating aurora for the recovery phase of a substorm is presented by Trakhtengerts (1992). The selfconsistent theory of the cyclotron
K. Kudela et al.lJournal of Atmospheric and Solar-Terrestiral Physics 60 (1998) 643-653
maser operation is presented describing the step-like regime of whistler wave generation and energetic particle precipitation, explaining the pulsating aurora1 patches in the morningside magnetosphere. In the model of the flow cyclotron maser a duct with enhanced plasma density serves as a resonance cavity and the inclusion of fresh resonant particles into the process of whistler cyclotron instability development is considered. The role of new resonant particles is attributed to energetic electrons with an anisotropic distribution (for example, empty or nearly empty loss cone), which enter the duct through its side during the process of magnetic gradient drift. The quasilinear modification of the pitch angle distribution function determines the pulsating character of the whistler cyclotron instability. The flux increase is finished when a major portion of the energetic electrons in the duct has interacted with waves. This portion of electrons pours into the dense layers of the atmosphere via the loss cone. The wave attenuates, leaving the resonator, and is absorbed against the isotropic background. Recently a new generation model of the magnetosphere cyclotron maser has been suggested, based on phase coherence effects in wave-particle systems with the step-like deformations of electron velocity distribution system (Trakhtengerts, 1995). This mechanism may be involved in the precipitating electron structures observed here. However, for its definite selection as a single candidate of the interpretation there are not sufficient experimental data in the present study. Namely, testing its validity requires one to measure in detail the phase relations of the wave and particle spikes at given L, which is impossible on a single low altitude satellite due to its motion which confuses spatial and temporal structures. Thus our measurements can be assumed only as being qualitatively not contradicting the existing theoretical approaches, but being unable to single out any one of them. For the progress in discriminating between existing theoretical models, either very high temporal resolution measurements of the particles (at different pitch angles) and the waves on two mutually not distant satellites with similar devices are needed, or, in some situations, comparisons with ground based measurements are necessary. It has to be pointed out that the conditions of the cyclotron resonance and the critical energy of particles involved in the wave particle interaction is also dependent on the plasma density in the regions of interaction. Thus data on the plasma density structure, at least indirect information from ground-based measurements and/or the plasma density inferred from the same satellite data have to be taken into a more proper testing of the resonant conditions. The precipitation of energetic electrons has an influence on the ionization state of the upper atmosphere. Vampola and Gorney (1983) have shown that at 70-90 km the mid latitude ionization due to these precipitating electrons in the energy range 36-3 17 eV is comparable to
651
that due to the solar H Lyman alpha emissions. The compilation of the production rate of ion pairs based on the papers of Baker et al. (1987) Sheldon et al. (1988) and Vampola and Gorney (1983) also show the importance of precipitating electron fluxes for this altitude interval. The f. dependence of the input from precipitating electrons to the upper atmosphere was found by Imhof and Gaines (1993). The fluxes of electron precipitation as shown from the ACTIVE satellite data are showing strong pulsations with intensity changes up to one order of magnitude during times shorter than 1 s over regions with a characteristic scale of a few km. During the electron bursts the energy spectra are strongly changing and thus the ionization at different altitudes is affected variably so that highly variable patterns of ionization are expected. The results presented here are from the northern hemisphere due to the telemetry restrictions on ACTIVE satellite. According to the study of Vampola and Gorney (1983) southern hemisphere precipitation dominates that in the North for 1.1 < L < 6. Thus, it would be interesting to measure in the future electron fluxes within and outside the loss cone in the southern hemisphere with high temporal resolution to examine electron pulsations there. Such a possibility will probably arise in the forthcoming COMPASS satellite experiment with the MEPl particle device (Kudela et al., 1997b).
6. Summary Strong, short time and/or small scale size pulsations of energetic electrons in different energy ranges in 2&300 keV within the loss cone on the low altitude ACTIVE satellite have been observed during the recovery phase of a substorm in the morningside aurora1 and subauroral regions. Simultaneous measurements outside the local loss cone at approximately the same energy do not exhibit such structure. The energy spectra of trapped electrons evolve smoothly in the range of L = 3.5-7, while the precipitating electron spectra are changing very fast, ins ome cases within a few tenths of a second. According to one second averages, fast changes of pitch angle diffusion coefficients, with the ratio of outside/inside loss cone flux of one order of magnitude, are observed and during the spikes the strong pitch angle diffusion signature, isotropisation, is reached. The duration of the spikes at L > 5.5 is longer than at lower L. At energies > 185 keV the strong pitch angle diffusion mode is not reached, although the spikes are persistent also at these high energies. The pulsations have typical periods of 5 to 20 s; however, using the 0.1 s resolution, much shorter precipitation pulsations (time duration of 0.3 to 0.5 s) are seen to be superimposed on the low frequency pattern. The cross-correlation of pulsations indicates in a few cases that the high energy precipitating electrons are in advance of the lower energies by 0.1 s, and that a few
652
K. Kudela et al./Journal
of Atmosphericand Solar-Terrestiral
short (0.2-0.3 s) individual spikes of precipitating electrons have a larger intensity than the trapped electrons. This probably means the combination of pitch angle and energy diffusion. Although some qualitative consistency with theoretical predictions, like the nonlinear oscillation model of electrons and waves and the concept of the cyclotron maser, can be deduced from the precipitation pattern observed, this single low altitude satellite measurement even in the high temporal resolution does not allow the separation of the spatial from temporal structure and we thus cannot establish the phase relations of the localized phenomena clearly. The short strong pulses of precipitating energetic electrons with fast variability of their energy spectra have implications for highly structured ionization at different altitudes above 70 km.
Acknowledgments The paper has been supported by the FAPESP Agency, SP, Brazil, 94/6212-7 and two of the coauthors (K.K. and I.M.M.) wish to acknowledge this support. This work was supported also by Slovak VEGA No 1353 grant and the Russian Foundation for Fundamental Research under grant 94-02-04299. The authors would like to thank J. Rojko for the design and construction of the SPEl device, L. Rudnevskaya, V. Maslov, M. Pivovarov, J. Stetiarova, T. Romantsova, 0. Akentieva for their assistance in preparing data for scientific analysis, and Yu. I. Galperin for useful discussions.
References Baker, D.N., Blake, J.B., Gorney,
D.J., Higbie, P.R., Klebesadel, R.W., King, J.H., 1987. Highly relativistic magnetospheric electrons : a role in coupling to the middle atmosphere? Geophys. Res. Lett. 14, 1027-1030. Bryant, D.A., Courtier, G.M., Bennet, G., 1971. Equatorial modulation of electrons in a pulsating aurora. J. Atmos. Terr. Phys. 33, 849-858. Coroniti, F.V., C.F. Kennel, 1970. Electron precipitation pulsations J. Geophys. Res. 75, 127991288. Datlowe, D.W., Imhof, W.L., Voss, D.H., 1993. Kilometer scale structures in aurora1 X rays arcs detected by combining satellite X-ray images with in situ energetic electron fluxes. J. Geophys. Res. 989259-9266. Davidson, G.T., 1979. Self-modulated VLF wave-electron interactions in the magnetosphere : a cause of amoral pulsations. J. Geophys. Res. 84,6517-6523. Davidson, G.T., 1986a. Pitch angle diffusion in morningside aurorae. 1. The role of the loss cone in the formation of impulsive bursts of precipitation. J. Geophys. Res. 91, 44134427. Davidson, G.T., 1986b. Pitch angle diffusion in morningside aurorae. 2. The formation of repetitive aurora1 pulsations. J. Geophys. Res. 91,442994436.
Physics 60 (1998) 643653
Davidson, G.T., 1990. Pitch angle diffusion and the origin of temporal and spatial structures in morningside aurorae. Space Sci. Rev. 53,45582. Davidson, G.T., Chiu, Y.T., 1991. An unusual nonlinear system in the magnetosphere : a possible driver for amoral pulsations. J. Geophys. Res. 96, 19353319362. Demekhov, A.G., 1991. About the role of loss cone in the forming of pulsating regime of cyclotron instability of whistlers. Geomagn. Aeron. 31, 1099-l 108. Demekhov, A.G., Trakhtengerts, V. Yu., 1994. A mechanism of formation of pulsating aurorae. J. Geophys. Res. 99, 58315841. Evans, D.S., Davidson, G.T., Voss, H.D., Imhof, W.L., Mobilia, J., Chiu, Y.T., 1987. Interpretation of electron spectra in morningside pulsating aurorae. J. Geophys. Res. 92, 122955 12306. Hansen, H.J., Mravlag, E., Scourfield, M.W.J., 1988. Coupled 3- and 1.3-Hz components in aurora1 pulsations. J. Geophys. Res. 93, 10029-10034. Imhof, W.L., Voss, H.D., Mobilia, J., Datlowe, D.W., Gaines, E.E., 1991. The precipitation of relativistic electrons near the trapping boundary. J. Geophys. Res. 96, 5619-5629. Imhof, W.L., Voss, H.D., Mobilia, J., Datlowe, D.W., Gaines, E.E., McGlennon, J.P., Inan, U.S., 1992. Relativistic electron microbursts. J. Geophys. Res. 97, 13829913837. Imhof, W.L. and Gaines, E.E., 1993. Inputs to the atmosphere from relative electrons. J. Geophys Res. 98, 13575-13580. Imhof, W.L., Robinson, R.M., Collin, H.L., Wygant, J.R., Anderson, R.R., 1994. Simultaneous measurements of waves and precipitating electrons near the equator in the outer radiation belt. J. Geophys. Res. 99, 241552427. Kudela, K., Matisin, J., Shuiskaya, F.K., Akentieva, O.S., Romantsova, T.V., Venkatesan, D., 1992. Inner zone electron peaks observed by the ‘Active’ satellite. J. Geophys. Res. 97, 8681-8685. Kudela, K., Slivka, M., Martin, I.M., Jiricek, F., Triska, P., Shuiskaya, F.K., 1997a. Strong fluctuations of energetic electrons at low altitudes. Adv. Space Res. 20, 499-503. Kudela, K., Balaz, J., Strharsky, I., Stetiarova, J., Martin, I.M., Gladyshev, V., Shuiskaya, F.K., Stepanov, A., Klepikov, V., 1997b. MEPl experiment : scientific tasks and basic characteristics. Preprint of IEP SAS, KoSice, April 1997, UEF-0297. Lepine, D.R., Bryant, D.A, Hall, D.S., 1980. A 2.2 Hz modulation of aurora1 electrons imposed at the geomagnetic equator. Nature 286, 5772,469471. Rycroft, M.J., 1991. Interactions between whistler-mode waves and energetic electrons in the coupled system formed by the magnetosphere, ionosphere and atmosphere. J. Atmos. Terr. Phys. 53, 849-858. Sandahl, I., Eliasson, L., Lundin, R., 1980. Rocket observations of precipitating electrons over a pulsating aurora. Geophys. Res. Lett. 7, 3099312. Sandahl, I., 1984. Pitch angle scattering and particle precipitation over a pulsating aurora-an experimental study. Kiruna Geophys. Inst., Kiruna, Sweden, Rep. 185. Sheldon, W.R., Benbrook, J.R., Bering, E.A., 1988. Rocket investigations of electron-precipitation and VLF waves in the Antarctic upper-atmosphere. Rev. Geophys. 26, 519-533. Swift, D.W., Gorney, D.J., 1989. Production of very energetic electrons in discrete aurora, J. Geophys. Res. 94, 26962702.
K. Kudela et al./Journal of Atmospheric and Solar-Terrestiral Physics 60 (1998) 643-653 Trakhtengerts, V. Yu., 1992. Mechanism of formation of pulsating auroras. Proc. of the Int. Conf. on Substorms (ICS-I), Kiruna, Sweden, 23-27 March 1992, ESA SP 335, May 1992, p. 463. Trakhtengerts, V. Yu., 1995. Magnetosphere cyclotron maser: backward wave oscillator generation regime. J. Geophys. Res. 100, 17205517210. Tsuruda, K., Machida, S., Oguti, T., Kokubun, S., Hayashi, K.,
653
Kitamura, T., Saka, O., Watanabe, T., 1981. Correlations between the very low frequency chorus and pulsating aurora observed by low-light-level televison at L approx. 4.4. Can. J. Phys. 59, 1042-1048. Vampola, A.L., Gorney, D.J., 1983. Electron energy deposition in the middle atmosphere, J. Geophys. Res. 88, 6267-6274. Yamamoto, T, 1988. On the temporal fluctuations of pulsating aurora1 luminosity. J. Geophys. Res. 93, 897791 I.
Journal of Atmospheric and Solar-Terrestrial Physics 60 (1998) 643-653
Pulsations
of precipitating energetic electrons ACTIVE satellite data
K. Kudelaa,b*, I. M. Martinb, F. K. Shuiskaya”, F. Jiricekd
:
M. M. Mogilevsky”
aInstitute of Experimental Physics, SAS, Kofice, Slovakia b VNICAMP, Campinas, SP, Brazil ‘Space Research Institute, Russian Academy of Sciences, Moscow, Russia ‘Institute of Atmospheric Physics, Bocni II, Prague, Czech Republic
Received 22 July 1996, in revised forms 5 May 1997 and 11 December 1997 accepted 19 December 1997
Abstract We present the patterns of high frequency energetic electron (- 20 to - 300 keV) pulsations in middle latitudes, in subauroral and aurora1 regions, based on low altitude nearly polar orbiting satellite (Intercosmos-24, project ACTIVE) measurements. Two detectors with different orientations allowed the fluxes within and outside the loss cone to be compared. The trapped electrons do not exhibit the strong pulsations observed within the bounce loss cone. The pitch angle diffusion coefficient changes significantly on time scales comparable to or even shorter than the bounce period of electrons. The same is valid for changes of precipitating electron energy spectra. There are indications that both pitch angle and energy diffusion are in operation. VLF emissions atf< 3.5 kHz with a complicated structure are observed simultaneously with the electron pulsations, but there is no clear correlation between waves and particles. If the characteristics of the electron pulsations and VLF emissions are of a temporal character, they are qualitatively consistent with the models of nonlinear wave particle interactions, or with the concept of a flowing cyclotron maser. However, the pulsations may originate from fine spatial structure. Variability of the precipitation flux implies short time and/or small scale changes of ionization at altitudes down to 70 km. 0 1998 Elsevier Science Ltd. All rights reserved
1. Introduction The pulsating patches in the polar morningside aurora during the recovery phase of geomagnetic storms are caused by electrons precipitating from the outer radiation belt (see e.g. review of Davidson, 1990). The pitch angle diffusion of electrons, bringing them into the loss cone, due to cyclotron resonance with waves at the equator (Rycroft, 1991, and references therein) is accompanied by pulsations of ELF and VLF (f = 10 Hz-10 kHz) radio waves. These phenomena have been studied for over three decades. A considerable list of works related to the experimental investigations of these phenomena and their
* Corresponding author. Tel. : 0042 1 95 6224554 ; fax : 0042 1 95 6336292 ; e-mail : [email protected]
theoretical modelling can be found in reviews (e.g. Yamamoto, 1988 ; Demekhov and Trakhtengerts, 1994). In recent years there has been renewed interest in the above mentioned problems. Model calculations which detail the mechanism of resonant wave-particle interactions in the pulsating patches in the polar aurora have been carried out (Davidson and Chiu, 1991 ; Trakhtengerts, 1992, 1995). Electron precipitation pulsations have been studied for long time (e.g. Coroniti and Kennel, 1970). Further progress in experimental studies of patterns of precipitating electrons has also been reached recently. Precipitation of electrons connected to the pulsating aurora have been reported (e.g. Sandahl, 1984). The relativistic electron precipitation of short duration (less than 1 s) have been reported by Imhof et al. (1992). CRRES measurement have shown that bursts of precipitating electrons and wave intensities are highly correlated over a wide frequency range of waves and energy of electrons (Imhof et
S1364-6826/98 $19.00 0 1998 Published by Elsevier Science Ltd. All rights reserved PII: S1364&6826(98)00003-0
644
K. Kudela et a/./Journal
qf Atmospheric
and Solar-Terrestiral
al., 1994). The measurements of precipitating > 1 MeV electrons provided the estimate of their total energy input to the atmosphere (Imhof and Gaines, 1993). For the study of particle precipitation dynamics at low altitude orbits, high temporal resolution of the measurements is important. The temporal resolution of the ACTIVE satellite measurement is 0.1 s. Here we present the characteristics of the energetic electron pulsations using the measurements on this satellite which allowed comparison of the fluxes of precipitating and trapped particles using high temporal resolution over a range of energies. The character and possible causes of the strong and fast pulsations of electrons within the local loss cone, as well as their consequences for the upper atmosphere, are discussed.
2. Data Intercosmos(ACTIVE) was launched on 22 September 1989 into an orbit with perigee 500 km, apogee 2500 km and inclination i = 82.5”. The satellite has a 3axis orientation with respect to the Earth (x-axis along the velocity, and z-axis along the Earth’s radius vector). Energetic particles measurements were carried out with the SPEl instrument (Kudela et al., 1992). The particle flux was measured by means of silicon solid state detectors The differential energy spectrum was determined by amplitude analysis of the pulses from the arriving particles. The angular distributions are estimated from simultaneous measurements made by three pairs of detectors having different orientations, namely 39”, 69”, 99” with respect to the z-axis. The angle between the x-axis and axes of the detectors with 157.5”. In each pair a magnetic filter is put in front of one detector to deflect electrons with energies below 650 keV (the proton detector with the thickness of 100 pm), in front of the second detector of the pair there is an aluminium foil to absorb ions with energy up to 700 keV (the electron detector has the thickness 300 pm). Each detector has the field of view
Physics 60 (1998) 643-653
angle 20”. The diameter of each detector is 8 mm. The geometrical factor of each detector is 0.03 cm’ ster. Energy thresholds of particle registration vary depending on individual characteristics of the detectors and foils. Energy ranges for protons are from 25 to 800 keV and for the electrons from 20 to 400 keV. Three operational modes named A, B and C, differing in number of energy ranges and time resolution. have been used during the flight. The energy ranges were divided quasilogarithmically and their number was 7 in the A-mode, 15 in the B-mode and 3 1 in the C-mode. The best time resolution was in the A mode used in this work. The maximum count per channel was 255. To enlarge the dynamic range the pulse counters are stopped when any one of the channels reached 255 and the corresponding time is recorded (this is done separately for electron and proton channels). Active cooling of the detectors was performed by means of Peltier elements to suppress the thermal noise. Other details of the experiment can be found in (Kudela et al., 1992). The energy ranges of the detectors 1 and 3 are listed in Table 1. For comparison we report here also examples of wave measurements. VLF electromagnetic fields were observed by the NVK-ONCH receiver. Only the data of the waveform electric field in the frequency range 10 Hz-20 kHz are used here. The measurement of the electric field components from 8 Hz to 20 kHz was achieved by means of pairs of spherical vitreous carbons sensors, located at the ends of insulated booms. The distance between the sensors was 2 m. Each sensor was equipped with a unit gain preamplifier. Signals coming from the preamplifiers were processed in the electronic unit located inside the spacecraft. The difference between the signals from the pair of preamplifiers was filtered and then amplified. The output signal fed in parallel a set of narrowband filters and was amplified by a 2 step amplifier which could be positioned either in a free run mode, with automatic gain control, or in a fixed gain mode by telecommand. The processed signal was transmitted by direct modulation of the telemetry transmitter, recorded at the ground tel-
Table 1 Energy ranges of electron
detectors
Energy channel number
Range of detector 1 (keV) (trapped electrons*)
Range of detector 3 (precipitating electrons*)
1
17.7-27.9 27.944.2 44.2-69.9 69.9-l 11 Ill-175 175-277 r271
20.6-32.0 32.0-49.6 49.6-16.9 76.9-l 19 119-185 185-287 >287
2 3 4 5 6 7 * In the analyzed
portion
of the orbit presented
here.
K. Kudela et al.lJournal
qf Atmospheric and Solar-Terrestiral
emetry station and processed in the laboratory. The sensitivity of NVK-ONCH receiver electric channel was 0.2 to 2 pV/(m * Hz) in the frequency range 100 Hz-15 kHz and the dynamic range was 40 dB. The highest time resolution of particle and wave measurements was obtained during intervals when the satellite was in the region of radiovisibility of Panska Ves station, Czech Republic (50”32’N, 14’34’E). Depending on the actual orbit (and especially the altitude above the receiving station) the duration of the measurements in this mode was variable, but was typically 15520 min. This allowed energetic particle data with 7 energy channels and temporal resolution of 0.1 to be obtained for the northern hemisphere for the intervals of latitude typically from 30” to 75”. The data coverage is from October 1989 until July 1992. There were, however, many gaps. Nevertheless, measurements from a total 240 passes with 0.1 s resolution are available, and the coverage of different latitudes as well as local times is sufficient for more extensive studies of pulsating precipitation than that presented here.
3. Characteristics of the pulsation event The data on the measurements of electrons during available magnetic storms in 1990 were analyzed to search for pulsating precipitations of electrons from the outer radiation belt on the morningside of the magnetosphere. Usually during the recovery phase of magnetic storms there were several cases of highly structured flow of precipitating electrons, with the modulation factor of 3-5, simultaneously with the pulsating VLF emissions. The intensity of trapped particles changed only rather slightly during these periods. For the case of orbit 2179 on 22 March 1990, starting from 0742 until 0749 the orientation of the detector 1 was 103”-91< with respect to the magnetic field B, while the angle between the axis of detector 3 and B was 162”15 1“, both varying slowly. Thus for the whole period the fluxes of electrons precipitating (detector 3) and trapped (detector 1) have been measured simultaneously. For the period until 0748, detector 3 was observing particles within the local loss cone (assuming the altitude 200 km as the boundary of the atmosphere). The altitude of the satellite (1930 km-1600 km) was suitable for measuring the stably trapped particles. The minimum altitude of mirror points for the whole period is above 1200 km. Figure 1 compares electron fluxes at the two detectors for three selected energy ranges. One second averages are plotted. Starting from an L-shell of about 4 and up to aurora1 latitudes the precipitation of the electrons is accompanied with considerable intensity increases of duration about 5520 s. The difference in the profiles of precipitating and trapped electrons is very apparent. While the trapped particles show a relatively smoothed profile,
645
Physics 60 (1998) 643-653
I(keV.s)-' 100
0.0001
0.00001 0742 L=3.2 Altitude 1920
0744 4.0
0746 5.1
0748 7.5
1660
1740
166Oh
UT
Fig. 1. Count rate profiles of electrons in units (SK’ keV_‘) at selected energy channels on 22 March 1990 observed by SPEl. For conversion to intensities (in cmm2 s I sr -’ keV. ‘) the geometry factor 0.01 cm* sr can be used. One second averages are used and the count rates are divided by the corresponding energy interval width. D means the detector, CH means its energy channel. Intensities for channels 3 and 5 are reduced with respect to those for channel 1 by a factor 3.10m2 and 9.10m4, respectively. The orientation of Dl with respect to B is changing from 103 to 91’ (left to right), while D3 had 162” to 151’, varying smoothly. The latitude and longitude interval covered is 51.8’ to 73’N. and 358” to 10’E. The local time is 07.30 to 08.30. Time divisions are at 5 and 20 s intervals.
the spiky structure of flux within the loss cone is seen at all three energy ranges. At lower latitudes (L < 5.5) the duration of spikes is short. Starting from L = 5.6 the character of the increases changes, single intensity increases change to multistructured ones. In some events the loss cone appears to be full (the precipitating intensity reaching the trapped intensity). Isotropy is reached, especially at lower energies. The occurrence of the spikes of precipitating electrons indicating strong pitch angle diffusion is different for different energies. The lowest L value at which this effect appears has the tendency to increase with increasing energy. At high energies, 1755 287 keV (not shown), an isotropic angular distribution is not reached, even at the highest latitudes. However, the spiky structure of electron precipitation remains. The observations were performed during the recovery phase of a magnetic storm with a sudden commencement. However, the pulsations of precipitating electrons have been found not exclusively in the time intervals connected to substorm activity. The character of the energy spectra of precipitating and trapped electrons (for simplicity based on the count rate ratios of the different energy channels) is shown in
K. Kudela et al./Journal of Atmospheric and Solar-Terrestiral Physics 60 (1998) 643453
646
1 0742
0744
0746
0748
m
Fig. 2. The ratios of 1 s count rate averages of different energy channels for detectors Dl and D3 for the same time period as plotted in Fig. 1.
Fig. 2 for channels 1, 3 and 5 for the same time interval as in Fig. 1. The energy spectra of the trapped (Dl) electrons are relatively stable in character and the change with latitude is smooth. In contrast, the energy spectra of the precipitating electrons is significantly more variable both at low and higher energies. Fluctuations of the ratios by a factor >3 within a few seconds are present. Although the general course of the latitudinal variation of the ratios are similar for trapped and precipitating particles, a much larger variability and fast changes of the precipitating spectra are evident. Precipitating electron spectra at low energies at L < 5.6 (before 0746) are softer than those of the trapped particles.
4. Fine structure of the precipitation Figure 3 (a, b, c) shows the intensity of the precipitating electrons in all 7 energy channels in linear scale for the three consecutive minutes, 07 : 4547 : 48, with the highest temporal resolution available, namely 0.1 s. The sequence of very short duration increases, namely on time scales less than 1 s, many of them less than 0.2-0.3 s, until approximately 07 : 46 is seen for the precipitating electrons. A linear scale with a shift of 200 impulses between the subsequent energy records is used to show the variability of the spikes. In many cases the increases are up to one order above the ‘quiet’ intensities. Very fast processes, or highly structured spatial structures of precipitating electrons, are apparent. In some of the spikes only low energy electrons are detected, in others high energies are also seen. There are also cases of fast electron pulsations only at high energies without lower energy increases. With the increasing latitude the pulses are of
longer duration, as was also apparent from Fig. 1. However, the short duration spikes are still present, superimposed on those fluxes. Thus, the roughly estimated periodicities of 5-20 s mentioned according to Fig 1 are not only ones in the precipitation structure. In addition high frequency fluctuations with characteristic time periods of < 1 s are usually observed, too. These fast pulsations are, however, much more variable in the energy spectra. The patterns of the short time (or small scale) variations in simultaneous measurements of the trapped and precipitating electron fluxes at two different energy intervals are shown in Fig. 4 (a, b, c) for the same intervals as plotted in Fig. 3. In one case the short interval pulsations of precipitating electrons at higher energies were found to precede those at lower energies. This shift is demonstrated in Fig. 5. The cross-correlations of the count rate at different energies for a 10 s interval, namely 07 : 45.2& 07 : 45.30 UT, show clear asymmetry with respect to a time lag = 0. If the lowest energy range is compared with those at higher energies, the shift to at least 0.1 s is more pronounced. The high energy spikes precede the spikes at lower energies. It has to be mentioned, however, that this type of phase shift is not clearly apparent in all spiky structures of precipitation. The correlation between the lowest and highest energy profiles is in many cases lost. However the comparison of two neighbouring energy ranges confirms this tendency in general, for L < 6. The forward energy dispersion of the precipitating spikes is also seen in the evolution of the energy spectra. Figure 6 presents the electromagnetic emissions measured simultaneously with the electron flux pulsations. This figure gives a general view of the spectral evolution during 105 s from 07 : 45.00 to 07 : 46.45. The spiky structure of the emissions is observed in different frequency ranges. The correlation between the emission structures and electron precipitation is, however, not clear. A different pattern, namely emissions in wider frequency ranges, with longer duration and more continuous, are observed close to the end time of the record, i.e. after 07:46.35. The first pulse with longer duration (about 10 s) is observed at the same time as an electron pulsation on SPEl.
5. Discussion For these measurements of highly structured fluxes of precipitating electrons there were no observations of the aurora1 forms. However, the characteristics of the measurements such as: the appearance on the morningside of the magnetosphere during the recovery phase of a magnetic storm in the range of L-shells corresponding to the outer belt and aurora1 latitudes; the source of intensity bursts near the equator; anisotropy
K. Kudela et al.lJournal of Atmospheric and Solar-Terrestiral Physics 60 (1998) 6436.53
647
CR(S.‘) 2.10
Fig. 3. 100 ms resolution data (count rates) on electron precipitation for the interval 07 : 4547 : 46 (a), 07 : 4&07 : 47 (b), and 07 : 4707 : 48 (c). The count rates (CR) obtained during the 0.1 s periods in different channels of Dl are plotted linearly with shifts by 200
between subsequent energy ranges.
of electron fluxes during the burst minimum and their isotropy for resonance energies during burst maximum ; the upper value of the resonance energy reaching 100 keV and sometimes 200 keV in the maximum of the intensity burst ; the duration of the intensity flash about several seconds, fine structure of the increase consisting of elementary bursts with an ‘on time’ of about 0.2 s and with a modulation frequency of about 1.5 Hz ; simultaneous measurements of the electromagnetic emission chorus in frequency range l-3.5 kHz-are all circumstantial evidence that our observations refer to the phenomenon of ‘pulsating patches’ of the polar aurora. The features of the precipitating electrons and electromagnetic emissions in pulsating patches analogous to
our case were reported, for example, in earlier papers (Bryant et al., 1971 ; Lepine et al., 1980; Sandahl et al., 1980; Tsuruda et al., 1981; Sandahl, 1984; Evans et al., 1987; Yamamoto, 1988 ; Hansen et al., 1988 Imhof et al., 1994). The key question in the possible explanation of the observations of the fast strong pulsations of energetic electrons observed on a single low altitude satellite, even with high temporal resolution, is the ambiguity between temporal and spatial character of these fluctuations. Pulses with periods 5-20 s correspond to measurements over distances of a few tens to more than one hundred kilometers. Such distances are comparable with the typical size for the patches of aurora1 pulsations. Thus it is
648
K. Kudela et al/Journal qf’Atmospheric and Solar-Terrestiral Physics 60 (1998) 6436.53
Fig. 4. The count rates for energy channels 1 and 3 of the trapped (Dl) and precipitating (D3) electrons are compared for the three intervals covered in Fig. 3. Data of Dl are shifted upwards by 200 with respect to D3. Precipitating electrons are shown by dots (CHI)
and by crosses (CH3), while lines are labelled and connect the profiles of the trapped electrons. Although the 1saverages of precipitating electrons usually do not exceed the fluxes of trapped ones, in several short spikes they clearly do.
impossible to exclude the spatial character of subauroral and aurora1 inhomogenities contributing to the observed fluctuations. The morningside aurorae at latitudes below invariant latitudes 70” display complex structures which are believed to be formed by the trapped electrons injected in aurora1 substorms (e.g. review by Davidson, 1990), which is not the case for the evening or the nighttime sectors. However, the occurrence of strong precipitating electron pulsations is not exclusively in the morningside. The probability of pulsations in the midnight time sector is also high according to ACTIVE satellite data (Kudela, 1997a). Pulsations of relativistic elec-
trons have also been observed in the nighttime sector by other experiments. According to Imhof et al. (1991), significant fluxes of precipitating relativistic electrons > 1 MeV within the loss cone are much more frequent near midnight than noon. They generally occur in narrow spikes < 100 km in width, typically at L values between 4 and 6 near the radiation belt boundary. Thus there is not an exclusive connection between the pulsating character of precipitating electrons and the aurora1 and subauroral morningside spatial inhomogenities during the substorm events. The narrow widths of the precipitating regions of elec-
K. Kudela et al./Joumal
of Atmosphericand
CC+A 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -3
-2
-1
1
0
2
3
time lag (in 0.1s)
Fig. 5. The cross-correlations (CC) between the intensities (in #/keV s)) of the energy channel 1 (lowest energies, first time series, X,) vs the intensities in higher energy channels (the second time series, Y,,,, t and k are in units of 0.1 s). The crosscorrelations are shifted vertically by A. Those with channel 2 (full circles, A = 0.6), channel 3 (crosses, A = 0.4) channel 4 (asterisks, A = 0.2) and channel 5 (open circles, A = 0) of detector 3 detect precipitating electrons during the 10 s interval after 07: 45.20 UT. 100 points with 0.1 s resolution are taken. The coordinates are: L = 4.74.8, altitude 1750 km, longitude l”E, magnetic local time is 9.1. The tendency for higher energies to be observed before the lower energies (negative time lag between the respective pairs of two time series) is more pronounced with increasing energy.
trons (smaller than 40 km) has been observed indirectly through bremsstrahlung X rays (Datlowe et al., 1993). In that paper significant variations on kilometer scale sizes were also reported. The data from SPEl help to confirm this feature from direct electron measurements. From Fig. 3 it is clear that, within 1 s, corresponding to the scale of few kilometers, the energy spectra of precipitating electrons is strongly changing. According to Fig. 6 the high energy (> 100 keV) precipitating electrons at 1800 km are advancing the low energy (- 20 keV) pattern by 0.1 s. It may be possible that an additional acceleration of electrons can contribute to that pattern. According to Swift and Gorney (1989) the acceleration of electrons to energies in excess of 100 keV sometimes occurs on the field lines associated with discrete aurora1 arcs, and the process can occur in the altitude range from the topside ionosphere up to 5.500 km. If the acceleration takes place in narrow altitude regions and above the observation point, forward velocity dispersion at ionospheric altitudes is expected. In a few examples SPEl is observing the short
spikes
flux of electrons bilities
of precipitating outside
of the acceleration
electrons
the loss cone.
in narrow
One
exceeding
the
of the possi-
regions would give
Solar-Terrestiral
Ph?asics 60 (1998)
643453
649
this forward velocity dispersion assuming temporal variations in the process energizing the electrons in the auroral arc. The uncertainty due to the individual characteristics of the detectors, namely not exactly the same geometric factor and not identical energy ranges, puts some limits on this comparison. However, in several short spikes the precipitating electron flux is a factor of 1.552 larger than that of the trapped particles. In one second averages, as seen in Fig. 1, the maximum precipitating electron flux reaches-but does not exceed--the trapped flux, and the isotropization expected during times of strong pitch angle diffusion can be identified. The pattern of short time precipitating flux exceeding the trapped flux at the same energy has to be checked using a larger amount of SPEl data (see Fig. 4). Such a pattern is difficult to explain in the frame of pure pitch angle diffusion due to wave particle interactions without the redistribution of energy in periods shorter then the bounce period. According to Swift and Gorney (1989) the waves efficiently scatter the electron beam in both energy and pitch angle. We suggest that combined pitch angle and energy diffusion is indicated in the short spike patterns presented by SPEl. However, Fig. 5 shows only one clear example of the short time velocity dispersion of precipitating electrons. The detailed analysis of occurrence of the values and of the sense of dispersion requires a statistical analysis which is now in progress. Together with the improvement of temporal resolution, this is important for resolving the fine temporal versus spatial structure of electron precipitation. The most common mechanism for pitch angle diffusion of electrons originally trapped and subsequently precipitating is cyclotron resonance between the trapped electrons and whistler mode waves. This process can lead not only to the reduction of electron pitch angles but simultaneously to the amplification of the wave emissions. The resonance is usually assumed to take place near the equatorial plane. The direct ionisation due to precipitating electrons in the upper atmosphere can lead to the modifications of amplitudes and phases of the VLF waves in the ionosphere and a feedback situation can occur (Rycroft, 1991). For the time period 07:45.00 to 07:46.40 UT (Fig. 5) the VLF emissions have higher amplitudes (if compared with other passes through the same L interval) ; however, the pattern of VLF emissions observed on the same satellite in the same time period when precipitating pulsations occur is far from having a clear correspondence to only a temporal (or only a spatial) structure of electron precipitation. The typical chorus time corresponds to time scales of electron flux pulsation, but there is no peak-to-peak conformity between chorus and electron pulsations. Such a correspondence should be expected only if the measurement of waves and electrons is done simultaneously at a fixed geomagnetic position for at least a few bounce periods of the electrons. The precipitating pattern is of very small
650
K. Kudela et al./Journal
of Atmospheric
and Solar-Terrestiral
Physics 60 (1998) 643-653
Amplitude IK-24 orb 2179
074.630 06 02 4.7 1763
Fig. 6. The VLF emissions atf<
074666 OS 10 6.1 1768
5 kHz for the available *Hz).
22.3.1990
074630 06 16 6.6 1743
set of data overlapping
07 47 00 6627 6.0 t71a
with the electron
UT OMT L AL1 (km]
fluxes observed
at the same satellite.
The signal is given in units of pV/(m
scale and/or of very fine temporal structure ; assuming that the interaction is taking place at the equatorial plane, the difference between the time of wave propagation and of scattered electrons from the interaction region to the observation point is masked by the motion of the satellite within the time of one bounce period by several kilometres. There exist theoretical approaches which are not in contradiction with the electron pulsations presented here; however, the SPEl measurements, even if compared with VLF emissions in this particular case on the satellite, are not providing the definite evidence for their exclusive validity. Several theoretical papers come to the conclusion that the pulsating patches are the result of the whistler cyclotron instability development in field-aligned ducts with enhanced plasma density and in the presence of a source of energetic particles in the region of instability. Davidson (1979) pointed out the importance of strong diffusion and tried to connect the pulsation period with the filling and emptying of the loss cone. The pitch angle diffusion model for morningside aurorae described in Davidson (1986a, b) predicts cyclic or repetitive pulsations for a wide range of conditions, with periods of 330 s. The pulsation periods observed here, if they are of
a temporal character, are consistent with that model. The aspects of relations between temporal and spatial structures in morningside aurorae can be explained with the aid of a nonlinear model describing the interactions between energetic electrons and whistler waves (Davidson and Chiu, 1991). In that paper the existence of spatial structures having sufficiently fine scales can provide a repetitive perturbation that maintains the temporal variations. In these theoretical approaches the importance of the loss cone distribution and its dynamics for the explanation of the pulsations is stressed. The measurements discussed here show a very fast and/or highly structured pattern of electrons within the loss cone if compared to the simultaneously measured trapped electron dynamics. This is in qualitative consistency with the above-mentioned theoretical suggestions. The paper by Demekhov (1991) shows that, to explain the autopulsating mode of quasilinear relaxations, the nonlinear modulation of the particle distribution outside the loss cone is important. A list of the theories of the pulsating aurorae is presented in Demekhov and Trakhtengerts (1994). The specific mechanism of pulsating aurora for the recovery phase of a substorm is presented by Trakhtengerts (1992). The selfconsistent theory of the cyclotron
K. Kudela et al.lJournal of Atmospheric and Solar-Terrestiral Physics 60 (1998) 643-653
maser operation is presented describing the step-like regime of whistler wave generation and energetic particle precipitation, explaining the pulsating aurora1 patches in the morningside magnetosphere. In the model of the flow cyclotron maser a duct with enhanced plasma density serves as a resonance cavity and the inclusion of fresh resonant particles into the process of whistler cyclotron instability development is considered. The role of new resonant particles is attributed to energetic electrons with an anisotropic distribution (for example, empty or nearly empty loss cone), which enter the duct through its side during the process of magnetic gradient drift. The quasilinear modification of the pitch angle distribution function determines the pulsating character of the whistler cyclotron instability. The flux increase is finished when a major portion of the energetic electrons in the duct has interacted with waves. This portion of electrons pours into the dense layers of the atmosphere via the loss cone. The wave attenuates, leaving the resonator, and is absorbed against the isotropic background. Recently a new generation model of the magnetosphere cyclotron maser has been suggested, based on phase coherence effects in wave-particle systems with the step-like deformations of electron velocity distribution system (Trakhtengerts, 1995). This mechanism may be involved in the precipitating electron structures observed here. However, for its definite selection as a single candidate of the interpretation there are not sufficient experimental data in the present study. Namely, testing its validity requires one to measure in detail the phase relations of the wave and particle spikes at given L, which is impossible on a single low altitude satellite due to its motion which confuses spatial and temporal structures. Thus our measurements can be assumed only as being qualitatively not contradicting the existing theoretical approaches, but being unable to single out any one of them. For the progress in discriminating between existing theoretical models, either very high temporal resolution measurements of the particles (at different pitch angles) and the waves on two mutually not distant satellites with similar devices are needed, or, in some situations, comparisons with ground based measurements are necessary. It has to be pointed out that the conditions of the cyclotron resonance and the critical energy of particles involved in the wave particle interaction is also dependent on the plasma density in the regions of interaction. Thus data on the plasma density structure, at least indirect information from ground-based measurements and/or the plasma density inferred from the same satellite data have to be taken into a more proper testing of the resonant conditions. The precipitation of energetic electrons has an influence on the ionization state of the upper atmosphere. Vampola and Gorney (1983) have shown that at 70-90 km the mid latitude ionization due to these precipitating electrons in the energy range 36-3 17 eV is comparable to
651
that due to the solar H Lyman alpha emissions. The compilation of the production rate of ion pairs based on the papers of Baker et al. (1987) Sheldon et al. (1988) and Vampola and Gorney (1983) also show the importance of precipitating electron fluxes for this altitude interval. The f. dependence of the input from precipitating electrons to the upper atmosphere was found by Imhof and Gaines (1993). The fluxes of electron precipitation as shown from the ACTIVE satellite data are showing strong pulsations with intensity changes up to one order of magnitude during times shorter than 1 s over regions with a characteristic scale of a few km. During the electron bursts the energy spectra are strongly changing and thus the ionization at different altitudes is affected variably so that highly variable patterns of ionization are expected. The results presented here are from the northern hemisphere due to the telemetry restrictions on ACTIVE satellite. According to the study of Vampola and Gorney (1983) southern hemisphere precipitation dominates that in the North for 1.1 < L < 6. Thus, it would be interesting to measure in the future electron fluxes within and outside the loss cone in the southern hemisphere with high temporal resolution to examine electron pulsations there. Such a possibility will probably arise in the forthcoming COMPASS satellite experiment with the MEPl particle device (Kudela et al., 1997b).
6. Summary Strong, short time and/or small scale size pulsations of energetic electrons in different energy ranges in 2&300 keV within the loss cone on the low altitude ACTIVE satellite have been observed during the recovery phase of a substorm in the morningside aurora1 and subauroral regions. Simultaneous measurements outside the local loss cone at approximately the same energy do not exhibit such structure. The energy spectra of trapped electrons evolve smoothly in the range of L = 3.5-7, while the precipitating electron spectra are changing very fast, ins ome cases within a few tenths of a second. According to one second averages, fast changes of pitch angle diffusion coefficients, with the ratio of outside/inside loss cone flux of one order of magnitude, are observed and during the spikes the strong pitch angle diffusion signature, isotropisation, is reached. The duration of the spikes at L > 5.5 is longer than at lower L. At energies > 185 keV the strong pitch angle diffusion mode is not reached, although the spikes are persistent also at these high energies. The pulsations have typical periods of 5 to 20 s; however, using the 0.1 s resolution, much shorter precipitation pulsations (time duration of 0.3 to 0.5 s) are seen to be superimposed on the low frequency pattern. The cross-correlation of pulsations indicates in a few cases that the high energy precipitating electrons are in advance of the lower energies by 0.1 s, and that a few
652
K. Kudela et al./Journal
of Atmosphericand Solar-Terrestiral
short (0.2-0.3 s) individual spikes of precipitating electrons have a larger intensity than the trapped electrons. This probably means the combination of pitch angle and energy diffusion. Although some qualitative consistency with theoretical predictions, like the nonlinear oscillation model of electrons and waves and the concept of the cyclotron maser, can be deduced from the precipitation pattern observed, this single low altitude satellite measurement even in the high temporal resolution does not allow the separation of the spatial from temporal structure and we thus cannot establish the phase relations of the localized phenomena clearly. The short strong pulses of precipitating energetic electrons with fast variability of their energy spectra have implications for highly structured ionization at different altitudes above 70 km.
Acknowledgments The paper has been supported by the FAPESP Agency, SP, Brazil, 94/6212-7 and two of the coauthors (K.K. and I.M.M.) wish to acknowledge this support. This work was supported also by Slovak VEGA No 1353 grant and the Russian Foundation for Fundamental Research under grant 94-02-04299. The authors would like to thank J. Rojko for the design and construction of the SPEl device, L. Rudnevskaya, V. Maslov, M. Pivovarov, J. Stetiarova, T. Romantsova, 0. Akentieva for their assistance in preparing data for scientific analysis, and Yu. I. Galperin for useful discussions.
References Baker, D.N., Blake, J.B., Gorney,
D.J., Higbie, P.R., Klebesadel, R.W., King, J.H., 1987. Highly relativistic magnetospheric electrons : a role in coupling to the middle atmosphere? Geophys. Res. Lett. 14, 1027-1030. Bryant, D.A., Courtier, G.M., Bennet, G., 1971. Equatorial modulation of electrons in a pulsating aurora. J. Atmos. Terr. Phys. 33, 849-858. Coroniti, F.V., C.F. Kennel, 1970. Electron precipitation pulsations J. Geophys. Res. 75, 127991288. Datlowe, D.W., Imhof, W.L., Voss, D.H., 1993. Kilometer scale structures in aurora1 X rays arcs detected by combining satellite X-ray images with in situ energetic electron fluxes. J. Geophys. Res. 989259-9266. Davidson, G.T., 1979. Self-modulated VLF wave-electron interactions in the magnetosphere : a cause of amoral pulsations. J. Geophys. Res. 84,6517-6523. Davidson, G.T., 1986a. Pitch angle diffusion in morningside aurorae. 1. The role of the loss cone in the formation of impulsive bursts of precipitation. J. Geophys. Res. 91, 44134427. Davidson, G.T., 1986b. Pitch angle diffusion in morningside aurorae. 2. The formation of repetitive aurora1 pulsations. J. Geophys. Res. 91,442994436.
Physics 60 (1998) 643653
Davidson, G.T., 1990. Pitch angle diffusion and the origin of temporal and spatial structures in morningside aurorae. Space Sci. Rev. 53,45582. Davidson, G.T., Chiu, Y.T., 1991. An unusual nonlinear system in the magnetosphere : a possible driver for amoral pulsations. J. Geophys. Res. 96, 19353319362. Demekhov, A.G., 1991. About the role of loss cone in the forming of pulsating regime of cyclotron instability of whistlers. Geomagn. Aeron. 31, 1099-l 108. Demekhov, A.G., Trakhtengerts, V. Yu., 1994. A mechanism of formation of pulsating aurorae. J. Geophys. Res. 99, 58315841. Evans, D.S., Davidson, G.T., Voss, H.D., Imhof, W.L., Mobilia, J., Chiu, Y.T., 1987. Interpretation of electron spectra in morningside pulsating aurorae. J. Geophys. Res. 92, 122955 12306. Hansen, H.J., Mravlag, E., Scourfield, M.W.J., 1988. Coupled 3- and 1.3-Hz components in aurora1 pulsations. J. Geophys. Res. 93, 10029-10034. Imhof, W.L., Voss, H.D., Mobilia, J., Datlowe, D.W., Gaines, E.E., 1991. The precipitation of relativistic electrons near the trapping boundary. J. Geophys. Res. 96, 5619-5629. Imhof, W.L., Voss, H.D., Mobilia, J., Datlowe, D.W., Gaines, E.E., McGlennon, J.P., Inan, U.S., 1992. Relativistic electron microbursts. J. Geophys. Res. 97, 13829913837. Imhof, W.L. and Gaines, E.E., 1993. Inputs to the atmosphere from relative electrons. J. Geophys Res. 98, 13575-13580. Imhof, W.L., Robinson, R.M., Collin, H.L., Wygant, J.R., Anderson, R.R., 1994. Simultaneous measurements of waves and precipitating electrons near the equator in the outer radiation belt. J. Geophys. Res. 99, 241552427. Kudela, K., Matisin, J., Shuiskaya, F.K., Akentieva, O.S., Romantsova, T.V., Venkatesan, D., 1992. Inner zone electron peaks observed by the ‘Active’ satellite. J. Geophys. Res. 97, 8681-8685. Kudela, K., Slivka, M., Martin, I.M., Jiricek, F., Triska, P., Shuiskaya, F.K., 1997a. Strong fluctuations of energetic electrons at low altitudes. Adv. Space Res. 20, 499-503. Kudela, K., Balaz, J., Strharsky, I., Stetiarova, J., Martin, I.M., Gladyshev, V., Shuiskaya, F.K., Stepanov, A., Klepikov, V., 1997b. MEPl experiment : scientific tasks and basic characteristics. Preprint of IEP SAS, KoSice, April 1997, UEF-0297. Lepine, D.R., Bryant, D.A, Hall, D.S., 1980. A 2.2 Hz modulation of aurora1 electrons imposed at the geomagnetic equator. Nature 286, 5772,469471. Rycroft, M.J., 1991. Interactions between whistler-mode waves and energetic electrons in the coupled system formed by the magnetosphere, ionosphere and atmosphere. J. Atmos. Terr. Phys. 53, 849-858. Sandahl, I., Eliasson, L., Lundin, R., 1980. Rocket observations of precipitating electrons over a pulsating aurora. Geophys. Res. Lett. 7, 3099312. Sandahl, I., 1984. Pitch angle scattering and particle precipitation over a pulsating aurora-an experimental study. Kiruna Geophys. Inst., Kiruna, Sweden, Rep. 185. Sheldon, W.R., Benbrook, J.R., Bering, E.A., 1988. Rocket investigations of electron-precipitation and VLF waves in the Antarctic upper-atmosphere. Rev. Geophys. 26, 519-533. Swift, D.W., Gorney, D.J., 1989. Production of very energetic electrons in discrete aurora, J. Geophys. Res. 94, 26962702.
K. Kudela et al./Journal of Atmospheric and Solar-Terrestiral Physics 60 (1998) 643-653 Trakhtengerts, V. Yu., 1992. Mechanism of formation of pulsating auroras. Proc. of the Int. Conf. on Substorms (ICS-I), Kiruna, Sweden, 23-27 March 1992, ESA SP 335, May 1992, p. 463. Trakhtengerts, V. Yu., 1995. Magnetosphere cyclotron maser: backward wave oscillator generation regime. J. Geophys. Res. 100, 17205517210. Tsuruda, K., Machida, S., Oguti, T., Kokubun, S., Hayashi, K.,
653
Kitamura, T., Saka, O., Watanabe, T., 1981. Correlations between the very low frequency chorus and pulsating aurora observed by low-light-level televison at L approx. 4.4. Can. J. Phys. 59, 1042-1048. Vampola, A.L., Gorney, D.J., 1983. Electron energy deposition in the middle atmosphere, J. Geophys. Res. 88, 6267-6274. Yamamoto, T, 1988. On the temporal fluctuations of pulsating aurora1 luminosity. J. Geophys. Res. 93, 897791 I.