Solar Flare Hard X-Ray Emission from the High Corona

Abstract

One of the largest solar hard X-ray (HXR) flares and solar energetic particle (SEP) events recorded by the Mars Odyssey mission while orbiting Mars occurred on 2002 October 27 and is related to a very fast (~2300 km s-1) coronal mass ejection (CME). From the Earth, the flare site is 40.4° ± 3.5° behind the solar limb and only emissions from the high corona at least 1.5 × 105 km radially above the main flare site can be seen. Nevertheless, the Earth-orbiting Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed HXR emission up to 60 keV with a relatively flat, nonthermal spectrum (γ between 3 and 3.5) that has an onset simultaneous with the main HXR emission observed above 60 keV by the Gamma-Ray Spectrometer (GRS) orbiting Mars. While GRS records several smaller enhancements after the main peak, the high coronal source observed by RHESSI shows a long exponential decay (τ = 135 ± 5 s) with progressive spectral hardening. The emissions from the high corona originate from an extended source (~1.5 × 105 km in diameter) that expands (390 ± 70 km s-1) and moves upwards (750 ± 80 km s-1) in the same direction as the CME. These observations reveal the existence of energetic electrons in the high corona in closed magnetic structures related to the CME that are accelerated at the same time as the main energy release in the flare. Although the number of energetic electrons in the high corona is only a small fraction of the total accelerated electrons, about 10% of all electrons in the high coronal source are nonthermal (>10 keV).