FIRST DETECTION OF A TERRESTRIAL MEV X-RAY BURST

We report the first detection of a terrestrial X-ray burst extending u p to MeV energies, made by a liquid-nitrogen-cooled germanium detector (similar to 2 keV FWHM resolution) on a high-altitude balloon at 65.5 degrees magnetic latitude (L=5.7) in the late afternoon (1815 MLT) du ring low geomagnetic activity. The burst occurred at 1532-1554 UT on A ugust 20, 1996, and consisted of seven peaks of similar to 60-90 s dur ation, spaced by similar to 100-200 s, with quasi-periodic (similar to 10-20 s) modulation of the peak count rates. The very hard X-ray spec trum extends to the instrumental limit of 1.4 MeV, and is consistent w ith bremsstrahlung emission from monoenergetic, similar to 1.7 MeV, pr ecipitating electrons. Since the trapped relativistic electrons showed a steeply falling energy spectrum from 0.6 to 4 MeV (at L=6.6), the p recipitation mechanism appears to be highly energy selective. The modu lation frequencies suggest scattering of the MeV electrons due to gyro -resonance with Doppler-shifted electromagnetic ion cyclotron waves, b ut either equatorial proton densities a factor of similar to 10(2) hig her than typical for the plasmasphere or significant O+ densities woul d be required.