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 (~9-day duration) of the Antarctic continent by a large (800,000-cubic-meter) balloon. Bremsstrahlung hard X-ray emission extending up 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 (Kp≤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 (ΔE~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 the precipitating electron spectra. The spectral hardness shows an inverse relation with L as extremely effective in characterizing electron precipitation if coupled with imaging capability. ¿American Geophysical Union 1995