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

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\textdegree\textendash68\textdegree (L-values of 3.8\textendash6.7) but only in the late afternoon/dusk sectors (14:30\textendash00:00 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 \texttimes 1025 such electrons precipitated during the eight days at L = 3.8\textendash6.7, compared to \~2 \texttimes 1025 trapped 0.5\textendash3.6 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.