We report the first satellite observations of relativistic (>1 MeV) electron precipitation in microbursts with measured durations of less than 1 s. Microbursts of lower-energy electrons (10-100 keV) have been found to occur preferentially in the early daylight hours and to be closely associated with VLF chorus emissions. In contrast, the relativistic electron microbursts occurred more frequently near 2230 LT than 1030 LT, and no association was found with ELF/VLF chorus, consistent with the fact that resonant interactions with --• 1-MeV electrons require significantly lower frequencies. The available data on these relativistic microbursts thus appear to indicate that many of the bursts may be due to wave-particle interaction not with whistler mode chorus but possibly with other waveforms. The locations of many of the relativistic microbursts are concentrated at the outer edge of the trapped radiation belt, where the gyroradii of the electrons are comparable to the curvature of the magnetic field lines and stable trapping may therefore not occur. The preferred location of the microbursts, which may be primarily spatial in character, implies the possible importance of irregularities in the magnetic field lines near the trapping boundary as the responsible mechanism. 13,829 13,830 IMHOF ET AL.' RELATIVISTIC ELECTRON MICROBURSTS

The solar X-ray experiment on the satellite OSO-7 has provided extensive observations of hard and soft X-ray bursts. We give a general description of the hard X-ray data here, in parallel with the description of the soft X-ray data already published (Datlowe et al., 1974). The data for this study consist of 123 hard X-ray bursts which occurred between 10 October 1971 and 6 June 1972. We examine the behavior of a typical event in terms of its spectral and flux variations. For the whole data sample, we find that ~-of the soft X-ray bursts have detectable hard X-ray emission. We present the distributions of frequency of occurrence of peak flux, spectral index, collisional energy loss, burst duration and the duration at half maximum of the flux profile. No correlation was found between the flux and the spectral slope of an individual data sample, nor was there a correlation between the peak flux and the full width at half maximum of a burst.We have utilized Ha flare identifications to study longitude variations. No statistically significant variation was found in the relative frequency of hard or soft X-ray bursts with longitude. However, the spectral slope does exhibit a center-to-limb variation, with limb spectra tending to be softer.We have compared the growth of energy in the hot flare plasma (soft X-ray source) with the collisional energy deposition (hard X-ray source) for the entire sample. This analysis shows that collision loss within the hot plasma is not the principal source of its heating.

An investigation has been made of electron and associated ion precipitation spikes near the plasmapause that are narrow in L shell and in which relativistic electrons are favored. The electron energy spectra during the spikes sometimes had equivalent e fold energies in excess of 500 keV. In approximately 31% of these spike events observed from the low‐altitude polar orbiting satellites P72‐1, P78‐1, and S81‐1, nearly simultaneous precipitation was measured in energetic ions above ∼30 keV at about the same L value. Several of the precipitation spikes occurred primarily in the drift loss cone, but in some cases, significant precipitation (∼10−2 ergs/cm² s) was also observed in the bounce loss cone. The electron spikes occurred preferentially in the evening sector, and all of the associated narrow ion spikes were in that local time interval. Narrow relativistic electron spikes were observed on less than 1% of the crossings of the plasmapause. From the set of S81‐1 events a search was made for those also observed on the NOAA 6 spacecraft. On rare occasions, nearly simultaneous (<2000 s) narrow spikes with hard electron spectra were found at approximately the same L value from both spacecraft and at longitudes differing by 8°–47°. These findings suggest a patchy profile, sometimes with an arc structure which may extend over longitude intervals as great as 25° and time intervals as long as 2000 s. From consideration of the AE index for 17 events, 12 were found to occur close to the times of substorms. The spike precipitation is interpreted in terms of cyclotron resonance wave‐particle interactions involving radiation belt particles, the narrow widths being associated with fine structure in the cold plasma density profiles near the plasmapause and the energy selectivity associated with an upper frequency cutoff in the waves.

Radiation belt electrons precipitated by controlled injection of VLF signals from a ground based transmitter have been directly observed for the first time. These observations were part of the SEEP (Stimulated Emission of Energetic Particles) experiment conducted during May ‐ December 1982. Key elements of SEEP were the controlled modulation of VLF transmitters and a sensitive low altitude satellite payload to detect the precipitation. An outstanding example of time‐correlated wave and particle data occurred from 8680 to 8740 seconds U.T. on 17 August 1982 when the satellite passed near the VLF transmitter at Cutler, Maine (NAA) as it was being modulated with a repeated ON (3‐s)/OFF (2‐s) pattern. During each of twelve consecutive pulses from the transmitter the electron counting rate increased significantly after start of the ON period and reached a maximum about 2 seconds later. The measured energy spectra revealed that approximately 15 to 50 percent of the enhanced electron flux was concentrated near the resonant energies for first order cyclotron interactions occurring close to the magnetic equator with the nearly monochromatic waves emitted from the transmitter.

Cyclotron resonance between trapped energetic electrons and VLF waves produces pitch angle scattering which leads to electron precipitation in the inner magnetosphere. Previous experiments have shown that in the drift loss cone at L values from 1.5 to 1.8 the energy spectrum of the electrons above 50 keV is often dominated by a single narrow peak. The center energy of this peak varies with L in a manner characteristic of cyclotron resonance between the electrons and monochromatic VLF waves in the vicinity of the geomagnetic equator. The source of the waves is probably VLF communication or navigation transmitters. We report here the results of a study of 680 occurrences of these peaks detected by the low altitude polar orbiting satellite S81-1. The present data, from altitudes between 170 and 270 km, show the resonance peaks only in two restricted longitude zones centered at 110øE and 300øE; this result contrasts with the previous measurements at higher altitudes, which detected peaks over a wide range of longitudes. The majority of nighttime events were near transmitter locations where the cyclotron resonance condition was met by electrons with pitch angles near the edge of the loss cone. Detailed study of the events occurring near 110øE reveals a 27-day periodicity in the frequency of occurrence; peaks are most often observed during minima in solar/geomagnetic activity. The peaks observed at nighttime in both longitude zones have a possible seasonal variation, being most frequent when it is wintertime at the site of the VLF transmitters responsible for the precipitation.