X-ray observations of MeV electron precipitation with a balloon-borne germanium spectrometer

The MAXIS (MeV Auroral X-ray Imaging and Spectroscopy) balloon payload was launched on a long duration balloon from McMurdo, Antarctica on Jan. 12, 2000. The high spectral resolution germanium spectrometer aboard MAXIS detected nine X-ray bursts with significant flux extending above 0.5 MeV during the 18 day flight. The X-rays are bremsstrahlung produced by precipitating electrons and the events are characterized by an extremely flat X-ray spectrum (∼E−2) similar to the first MeV event discovered in 1996, indicating relativistic parent electrons. The bursts were detected between magnetic latitudes 58°–68° (IGRF L-values of 3.8–6.7) with durations varying from several minutes to several hours and occurred only in the late afternoon/dusk sectors (14:30–00:00 MLT) while softer precipitation was detected at all magnetic local times. In addition, the MeV events were associated with substorm activity and several events showed Ultra Low Frequency (mHz) modulation of the X-ray count rate. Spacecraft and ground-based observations indicate the presence of ULF wave activity near the time and location of the events which may be causing the modulation by some mechanism that is not understood.; The MeV events are well modeled by a very flat exponential precipitating electron distribution and the average flux of precipitating electrons with E ≥ 0.5 MeV is estimated to be ∼360 cm−2 s−1. This corresponds to about 5 × 1025 precipitated electrons in this latitude range during the eight day observing interval compared to ∼2 × 10 25 trapped 0.5–3.6 MeV electrons estimated using GPS electron measurements. The MAXIS observations indicate that these electron precipitation events are common and may be the primary loss mechanism for outer zone relativistic electrons. Individual events were also compared with measurements of the trapped electrons from which it is estimated that only a few percent of the loss cone is being filled by the mechanism acting to precipitate the particles. This indicates that if the mechanism is wave-particle interaction, it is not in the strong diffusion regime.; Looking towards the future, a Compton telescope for imaging MeV X-ray bursts at high energies is also investigated. Such an instrument may also be used to measure polarization and thus may provide a method for inferring the precipitating electron pitch angle distribution.