High‐resolution spectra of 20‐300 Kev hard X‐rays from electron precipitation over Antarctica

Abstract

In December 1990, • set of liquid-nitrogen-cooled germ•nium hard X-ray •nd g•mm•-r•y spectrometers w•s flown •bo•rd • high-altitude balloon from McMurdo, Antarctica, for solar, •strophysical, •nd terrestrial observations. This flight w•s the first circumnavigation (•9-d•y duration) of the Antarctic continent by • l•rge (800,000-cubic-meter) balloon. Bremsstrahlung h•rd X-ray emission extending up to •300 keV, from the precipitation of high-energy electrons, w•s observed on six separate occasions over the auroral zone, all during low geomagnetic •ctivity (K r _< 2+). All events were consistent with emission •t the tr•pping boundary; obserwtions over the polar c•p showed no precipitation. We present the first high-resolution (AE keV/full width hMf 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 hardness 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. •> 50% of the time at invariant latitude 65 ø. The time-? averaged count rate for this common X-ray "drizzle" is about 15 (cma/sec) -1 integrated above 25 keV lAnderson, 1965]. Below this energy the atmosphere above the balloon absorbs most of the X-rays. In intense precipitation events, fluxes ranging up to ~200 (cma/sec) -1 have been seen [Anderson and Eriemark, 1960; Brown and Barcus, 1963]. The hard X-ray spectrum drops off rapidly in energy, generally disappearing below background by ~300 keV. The energetic electrons which create these hard X-rays Paper mtrnber 95JA01472. 0148-0227/95/95 JA-01472505.00 usually contain much less total energy than the softer (order of 1-10 keV) population of electrons responsible for visible aurorae.

First detection of a terrestrial MeV X‐ray burst

“…The observations were made with a liquid nitrogen cooled, 5.5 cm diam x 5.5 cm coaxial germanium detector (GeD), which had flown previously on an Antarctic long duration (~9 day) balloon flight [Smith et al, 1995]. The GeD inner bore is electrically segmented to form two detectors: DO, which occupies a few cm 3 at the center of the top surface, and D1 with the rest of the ~130 cm 3 volume.…”

“…The energy spectrum of the X-rays typically drops off rapidly between 20-200 keV with e-folding energies of ~20-30 keV (see, for example, Smith et al, 1995)…”

Understanding relativistic electron losses with BARREL

“…1. Higher energy resolution is necessary for inverting the X-ray observations to determine the energy and flux of precipitating electrons (e.g., Smith et al, 1995). This is provided with binned X-ray spectra, acquired every 4 s in 48 logarithmically spaced energy channels between 20 keV and 7 MeV.…”

Observation of relativistic electron precipitation during a rapid decrease of trapped relativistic electron flux

“…Figure 3 shows the average background‐subtracted X‐ray photon spectrum from 2120–2345 UT on January 19 (crosses). A model X‐ray spectrum, assuming the source fills the instrument field of view [ Smith et al , 1995] (Figure 3, dashed line) based on the exponential fit (Figure 2, bottom) of the LANL data (electron e‐folding energy of 290 keV and low‐energy cutoff of 610 keV) is not consistent with the observed spectrum. A low‐energy cutoff of 610 keV produces a harder (flatter) spectrum than observed.…”