IMPULSIVE PHASE CORONAL HARD X-RAY SOURCES IN AN X3.9 CLASS SOLAR FLARE

Abstract

[Abridged]We present analysis of a pair of unusually energetic coronal hard\nX-ray (HXR) sources detected by RHESSI during the impulsive phase of an X3.9\nclass solar flare on 2003 November 3, which simultaneously shows two intense\nfootpoint (FP) sources. A distinct loop top (LT) coronal source is detected up\nto ~150 keV and a second (upper) coronal source up to ~80 keV. These photon\nenergies are much higher than commonly observed in coronal sources and pose\ngrave modeling challenges. The LT source in general appears higher in altitude\nwith increasing energy and exhibits a more limited motion compared to the\nexpansion of the thermal loop. The high energy LT source shows an impulsive\ntime profile and its nonthermal power law spectrum exhibits soft-hard-soft\nevolution during the impulsive phase, similar to the FP sources. The upper\ncoronal source exhibits an opposite spatial gradient and a similar spectral\nslope compared to the LT source. These properties are consistent with the model\nof stochastic acceleration of electrons by plasma waves or turbulence. However,\nthe LT and FP spectral index difference (varying from ~0-1) is much smaller\nthan commonly measured and than that expected from a simple stochastic\nacceleration model. Additional confinement or trapping mechanisms of high\nenergy electrons in the corona are required. Comprehensive modeling including\nboth kinetic effects and the macroscopic flare structure may shed light on this\nbehavior. These results highlight the importance of imaging spectroscopic\nobservations of the LT and FP sources up to high energies in understanding\nelectron acceleration in solar flares. Finally, we show that the electrons\nproducing the upper coronal HXR source may very likely be responsible for the\ntype III radio bursts at the decimetric/metric wavelength observed during the\nimpulsive phase of this flare.\n