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Highly relativistic electron events (HREs) are periods of intense, long-lived, energetic electron fluxes in the outer radiation zone. We are using measurements from the High Energy Particle Spectrometer (HEPS) on the Upper Atmosphere Research Satellite (UARS) to develop a database of the pitch-angle and energy-resolved electron fluxes with energies between 30 keV and 4.5 MeV. The data acquired by HEPS have overlapped the declining phase of solar cycle 22 making these data very important, since HREs are thought to peak in frequency and intensity during this phase of the solar cycle. We find a consistent scenario of electrons being injected into the radiation belts by a magnetic storm (deduced from Dst) and being slowly accelerated to ever higher energies over days to weeks. The energy dependence of the flux is an essential part of the analysis. Above 300 keV the most energetic electrons are the last to appear and the slowest to fade following an injection event.

The High Energy Particle Spectrometer (HEPS) instrument on the Upper Atmosphere Research Satellite (UARS) provides a database of electron intensities well resolved in energy and pitch-angle. Because of its 57 deg. orbital inclination, UARS encounters with magnetic shells L greater than 2 occur quite far off-equator (B/B (sub 0) greater than 9), corresponding to equatorial pitch angle alpha (sub 0) greater than 20 deg. Data acquired by HEPS (October 1991 through September 1994) span the declining phase of Solar Cycle 22. To reveal the storm-associated time dependence of relativistic electron intensities over the wide range of energies (50 keV to 5 MeV) covered by HEPS, we divide the daily average of the measured spectrum at a given L value (bin width = 0.25) by the corresponding 500-day average and plot the results with a color scale that spans only 2.5 decades. The data show that our off-equatorial electron intensities typically increase with time after the end of recovery phase (not during main phase or recovery phase) of each geomagnetic storm. The delay in off-equatorial energetic electron response and the subsequent lifetime of the corresponding electron flux enhancement seem to increase with particle energy above 300 keV. The trend below 300 keV seems to be opposite, such that the delay varies inversely with electron energy. Our working hypothesis for interpretation is that stormtime radial transport tends to increase the phase-space densities of trapped relativistic electrons but typically leads to a flux increases at specified energies only as the current (as indicated by Dst) decays. Flux enhancements in early recovery phase are greatest for equatorially mirroring electrons, and to pitch-angle anisotropies are initially large. Subsequent pitch-angle diffusion broadens the flux enhancement to particles that mirror off equator, thus gradually increasing low-altitude electron intensities (as detected by HEPS/UARS) on time scales equal to about 20% of corresponding lifetimes against diffusion into the loss cone. Alternative interpretations will also be examined.

Enhancements in the fluxes of relativistic electrons trapped within the Earth's magnetosphere have been measured by the high-energy particle spectrometer, part of the particle environment monitor on the upper atmosphere research satellite (UARS). The largest increase in the electron fluxes with energies greater than 1 MeV observed on UARS from October 1991 through July 1994 was in early May 1992. The fluxes of trapped electrons in the drift loss cone and locally precipitating electrons showed differing buildup and decay rates as a function of invariant latitude. Increases of more than 2 orders of magnitude were observed in drift loss cone fluxes at magnetic latitudes of 40 deg-66 deg and in precipitating fluxes from 48 deg to 66 deg. The energy flux contained in the most intense local precipitation observed was approximately 0.1 erg/sq cm/s, entering the atmosphere and creating up to 1000 ion pairs/cu cm/s at 55-km altitude. The daily averaged energy flux from directly precipitating electrons with energies greater than 1 MeV deposited greater than 10(exp 20) erg/d worldwide into the atmosphere for the period May 12-21, 1992, producing greater than 10(exp 31) odd nitrogen molecules below 60-km altitude.

Analyses are presented for the first high-time resolution multisatellite study of the spatial and temporal characteristics of a relativistic electron enhancement event with a rapid onset. Measurements of MeV electrons were made from two low-altitude polar orbiting satellites and three spacecraft at synchronous altitude. The electron fluxes observed by the low-altitude satellites include precipitating electrons in both the bounce and drift loss cones as well as electrons that are stably trapped, whereas the observations at geosynchronous altitude are dominated by the trapped population. The fluxes of greater than 1 MeV electrons at low-satellite altitude over a wide range of L shells tracked very well the fluxes greater than 0.93 MeV at synchronous altitude.

In this paper we present observations made by the Particle Environment Monitor (PEM) instruments during the geomagnetic storm of 8-9 November, 1991. Ionization and energy deposition rates as functions of altitude in the middle and upper atmosphere by incident electrons and positive ions in the storm interval are computed. The suite of PEM instruments provides a systematic measurement of energetic particles and their associated X-rays over an energy range not fully covered by previous satellite missions.

Narrow L-dependent peaks commonly occur in the energy spectra of electrons precipitating from the inner radiation belt at L approximately equal to 1.5-1.85, and the cause of the peaks has been attributed to cyclotron resonance interactions with waves generated by VLF transmitters. In the slot region, L-dependent peaks have also been reported at L approximately equal to 2-3.5, but these have been predominately wider and consistent with their origin being cyclotron resonance interactions involving naturally occurring hiss. The infrequent occurrence of narrow peaks in electrons precipitating at L greater than or approximately equal to 2 is investigated. From coordinated wave and plasma density measurements it is found that if the narrow peaks are formed by first-order cyclotron resonant interactions occurring close to the magnetic equator between narrow band waves and the trapped electron population, then the equatorial plasma density gradients are unusually steep. This finding is consistent with the evidence previously obtained by other techniques for structured plasma density profiles in that L shell region of space.

Simultaneous measurements have been made of slot region precipitating electron spectra, wave frequency distributions and plasma density profiles using the P78-1 low altitude satellite and ISEE 1 spacecraft. Broad bands are often observed in the electron energy spectra with a well-defined low energy cutoff which decreases in energy with increasing L value. This is consistent with calculations for first-order cyclotron resonance with waves at the high frequency cutoff of the band. On the basis of electron spectra and plasma densities observed from upper hybrid resonance frequencies, hiss cutoff frequencies are calculated for an assumed first-order cyclotron resonance interaction and compared with wave spectral density profiles. Plasma wave and electron measurements are generally consistent, supporting the precipitation mechanism hypothesis.

The launch of the P78-1 low-altitude satellite containing a high-resolution electron spectrometer and the ISEE spacecraft with a plasma wave experiment have made it possible to perform simultaneous measurements of the waves, the plasma densities, and the precipitating electrons. It was found that nearly monochromatic waves in the range from 10 kHz to 25 kHz were frequently present in the near equatorial regions, their frequencies, narrow bandwidth, and geographic locations being consistent with various ground-based VLF transmitters as the sources. In the energy spectra of the precipitating electrons observed at low altitudes, narrow and at times multiple peaks often appear with central energies that decrease with increasing L shell. The narrow widths suggest that the interactions take place within a restricted range of latitudes which, based on other considerations, are likely equatorial.

The role that energetic particles in the substorm plasma have on the charging and discharging of typical dielectric layers used on spacecraft was investigated using spectra and pitch angle distributions measured in situ on the SCATHA spacecraft prior to and during a few kilovolt differential charging event in eclipse conditions. The particle spectra was input to deposition codes that determine the dose rate as a function of depth in Kapton and Teflon layers used in an experiment on SCATHA. The calculated ambient dose rates of a few rads/sec throughout the bulk of the samples are sufficiently high that radiation damage levels are reached on the time scale of 1 year. Surface dose is a factor of 100 higher. Bulk conductivity profiles were obtained from the dose rates using empirical relationships. The radiation induced bulk conductivities calculated at the peak charging time are found to be smaller than the intrinsic dark conductivity range of solar conditioned Kapton but higher than the corresponding value for Teflon.

The energetic electron environment at the geosynchronous orbit is responsible for a variety of adverse charging effects on spacecraft components. The most serious of these is the degradation and failure of a complementary-metal-oxide-semiconductor (CMOS) electronic components as a result of internal charge-buildup induced by the energetic electrons. Efforts to accurately determine the expected lifetime of these components in this orbit are hampered by the lack of detailed knowledge of the electron spectrum and intensity, particularly of the more penetrating energies greater than 1.5 MeV. This problem is illustrated through the calculation of the dose received by a CMOS device from the energetic electrons and associated bremsstrahlung as a function of aluminum shielding thickness using the NASA AE-6 and the Aerospace measured electron environments. Two computational codes which were found to be in good agreement were used to perform the calculations. For a given shielding thickness the dose received with the two radiation environments differ by as much as a factor of seven with a corresponding variation in lifetime of the CMOS.

Relativistic electron precipitation spikes were observed in the quasi-trapped population at midlatitudes by an array of electron detectors aboard the low-altitude polar-orbiting satellite 1972-076B. The fluxes of electrons of energies not less than 4 MeV may undergo pronounced enhancements, while the intensities of electrons in the few hundred keV range may not be significantly affected. Energy spectra and pitch angle distributions were measured over the energy range 160 keV to 2.4 MeV with a combination of three spectrometers. Fluxes of energetic electrons (greater than 4 MeV) were measured with high sensitivity in each of two large plastic scintillator anticoincidence counters surrounding a germanium spectrometer.

High energy electromagnetic radiation and energetic particles from sun, galaxies, and nuclear explosions in spacecraft environment