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 ob servations, This flight was the first circumnavigation (similar to 9-d ay duration) of the Antarctic continent by a large (800,000-cubic-mete r) balloon. Bremsstrahlung hard X-ray emission extending up to similar to 300 keV, from the precipitation of high-energy electrons, was obse rved on six separate occasions over the auroral zone, all during low g eomagnetic activity (K-p less than or equal to 2+). All events were co nsistent with emission at the trapping boundary; observations over the polar cap showed no precipitation. We present the first high-resoluti on 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 observ ed count spectra are deconvolved by model-independent techniques to ph oton spectra and then to the precipitating electron spectra. The spect ral 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 capa bility.