HIGH-RESOLUTION SPECTRA OF 20-300-KEV HARD X-RAYS FROM ELECTRON-PRECIPITATION OVER ANTARCTICA

In December 1990, a set of liquid-nitrogen-cooled germanium hard X-ray and gamma-ray spectrometers was flown aboard a high-altitude balloon from McMurdo, Antarctica, for solar, astrophysical, and terrestrial observations, This flight was the first circumnavigation (similar to 9-day duration) of the Antarctic continent by a large (800,000-cubic-meter) balloon. Bremsstrahlung hard X-ray emission extending up to similar to 300 keV, from the precipitation of high-energy electrons, was observed on six separate occasions over the auroral zone, all during low geomagnetic activity (K-p less than or equal to 2+). All events were consistent with emission at the trapping boundary; observations over the polar cap showed no precipitation. We present the first high-resolution Delta E similar to 2 keV/full width at half maximum (FWHM) spectra of this hard X-ray emission in the energy range 20-300 keV. The observed count spectra are deconvolved by model-independent techniques to photon spectra and then to the precipitating electron spectra. The spectral harness shows an inverse relation with L as expected. Our results suggest that high-resolution spectroscopy could be extremely effective in characterizing electron precipitation if coupled with imaging capability.