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



[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−2) 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−2s−1, corresponding to about 5 × 1025 such electrons precipitated during the eight days at L = 3.8–6.7, compared to ∼2 × 1025 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.