[1] We examine signatures of two types of waves that may be involved in the acceleration of energetic electrons in Earth's outer radiation belts. We have compiled a database of ULF wave power from SAMNET and IMAGE ground magnetometer stations for 1987-2001. Long-duration, comprehensive, in situ VLF/ELF chorus wave observations are not available, so we infer chorus wave activity from low-altitude SAMPEX observations of MeV electron microbursts for 1996-2001 since microbursts are thought to be caused by interactions between chorus and trapped electrons. We compare the ULF and microburst observations to in situ trapped electrons observed by high-altitude satellites from 1989-2001. We find that electron acceleration at low L shells is closely associated with both ULF activity and MeV microbursts and thereby probably also with chorus activity. Electron flux enhancements across the outer radiation belt are, in general, related to both ULF and VLF/ELF activity. However, we suggest that electron flux peaks observed at L $ 4.5 are likely caused by VLF/ELF wave acceleration, while ULF activity probably produces the dominant electron acceleration at geosynchronous orbit and beyond.

Abstract.The Solar, Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite frequently observes relativistic (> 1 MeV) electron precipitation in the radiation belts at oe shells of 4-6 with bursty temporal structure lasting < 1 s. This phenomenon can occur at all local times but is most often seen between 0200 and 1000 magnetic local time. VLF chorus is also observed to occur preferentially at these same local times. Using electron observations from the SAMPEX satellite Heavy Ion Large Telescope and data from the Polar satellite plasma wave instrument, we show correlation between observations of relativistic electron microbursts and VLF chorus with frequencies <2 kHz. In addition, the duration of the individual rising frequency chorus elements is comparable to the duration of the relativistic electron microbursts. It has been speculated that relativistic electron microbursts are caused by wave-particle interactions, which strongly scatter electrons into the loss cone for a short period. Lower-energy electron microbursts in the range from tens to hundreds of keV have long been associated with chorus waves, since these lower-energy electrons can resonate at the equator with whistler-mode waves at chorus frequencies. Electrons of MeV energies do not satisfy the first-order cyclotron resonance condition with chorus wave frequencies at the equator. However, MeV electrons may interact with chorus through higher-order resonances or off-equatorial interactions.

[1] The high-resolution germanium detector aboard the MAXIS (MeV Auroral X-ray Imaging and Spectroscopy) balloon payload detected nine X-ray bursts with significant flux extending above 0.5 MeV during an 18 day flight over Antarctica. These minutes-to-hours-long events are characterized by an extremely flat spectrum ($E À2) similar to the first MeV event discovered in 1996, indicating that the bulk of parent precipitating electrons is at relativistic energies. The MeV bursts were detected between magnetic latitudes 58°-68°(L-values of 3.8-6.7) but only in the late afternoon/dusk sectors (14:30 -00:00 MLT), suggesting scattering by EMIC (electromagnetic ion cyclotron) waves as a precipitation mechanism. We estimate the average flux of precipitating E ! 0.5 MeV electrons to be $360 cm À2 s À1 , corresponding to about 5 Â 10 25 such electrons precipitated during the eight days at L = 3.8-6.7, compared to $2 Â 10 25 trapped 0.5-3.6 MeV electrons estimated from dosimeter measurements on a GPS spacecraft. These observations show that MeV electron precipitation events are a primary loss mechanism for outer zone relativistic electrons.

Abstract. Observations of relativistic (>1 MeV) electronmicrobursts by the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite are frequently associated with geomagnetic storms. We examine the characteristics of these microbursts during 1997 and 1998, paying particular attention to the three storms selected by

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Abstract. We report the fixst detection of a terrestrial X-ray burst extending up to MeV energies, made by a liquid-nitrogen-cooled germanium detector (~ 2 keV FWHM resolution) on a high-altitude balloon at 65.5 ø magnetic latitude (L=5.7) in the late afternoon (1815 MLT) during low geomagnetic activity. The burst occurred at 1532-1554 UT on August 20, 1996, and consisted of seven peaks of -60-90 s duration, spaced by ~100-200 s, with quasi-periodic (~10-20 s) modulation of the peak count rates. The very hard X-ray spectrum extends to the instrumental limit of 1.4 MeV, and is consistent with bremsstrahlung emission from monoenergetic, ~1.7 MeV, precipitating electrons. Since the trapped relativistic electrons showed a steeply falling energy spectrum from 0.6 to 4 MeV (at L=6.6), the precipitation mechanism appears to be highly energy selective. The modulation frequencies suggest scattering of the MeV electrons due to gyro-resonance with Doppler-shifted electromagnetic ion cyclotron waves, but either equatorial proton densities a factor of ~102 higher than typical for the plasmasphere or significant O + densities would be Here we present high spectral resolution balloon observations of the most energetic terrestrial hard X-ray burst ever detected. The nearly stationary balloon platform allows the temporal evolution of the entire precipitation event to be observed.

We report here on the formation of new ion radiation belts observed in connection with several solar energetic particle events and large geomagnetic storms in 1998 and 2000. We use observations from the Polar spacecraft, the highly elliptical orbit (HEO) 1997‐068, and the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), to study details of the inner zone radiation belt at high and low altitudes. We focus specifically on the four International Solar Terrestrial Physics events of August and September 1998 and April and July 2000. In several events we find new 2–15 MeV proton belts at various locations between L = 2.0 and L = 3.5. The low‐altitude SAMPEX observations revealed features not visible at high altitudes, such as radiation belts with multiple peaks in L shell. During the July 2000 event, energetic helium and iron were observed at L ∼ 2, suggesting a solar energetic particle source for these injected ions. We compare observations of these new belts and remark on the significant differences from event to event.

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