First detection of a terrestrial MeV X-ray burst

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Abstract

We report the first detection of a terrestrial X-ray burst extending up to MeV energies, made by a liquid-nitrogen-cooled germanium detector (~ 2 keV FWHM resolution) on a high-altitude balloon at 65.5°magnetic latitude (L=5.7) in the late afternoon (1815 MLT) during low geomagnetic activity. The burst occurred at 1532-1554 UT on August 20, 1996, and consisted of seven peaks of ~60-90 s duration, spaced by ~100-200 s, with quasi-periodic (~10-20 s) modulation of the peak count rates. The very hard X-ray spectrum extends to the instrumental limit of 1.4 MeV, and is consistent with bremsstrahlung emission from monoenergetic, ~1.7 MeV, precipitating electrons. Since the trapped relativistic electrons showed a steeply falling energy spectrum from 0.6 to 4 MeV (at L=6.6), the precipitation mechanism appears to be highly energy selective. The modulation frequencies suggest scattering of the MeV electrons due to gyro-resonance with Doppler-shifted electromagnetic ion cyclotron waves, but either equatorial proton densities a factor of ~102 higher than typical for the plasmasphere or significant O+ densities would be required.

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Foat, J. E., Lin, R. P., Smith, D. M., Fenrich, F., Millan, R., Roth, I., … Treilhou, J. P. (1998). First detection of a terrestrial MeV X-ray burst. Geophysical Research Letters, 25(22), 4109–4112. https://doi.org/10.1029/1998GL900134

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