X-ray Emission from the Base of a Current Sheet in the Wake of a Coronal Mass Ejection

Abstract

Following a coronal mass ejection (CME) which started on 2002 November 26, RHESSI observed for 12 hr an X-ray source above the solar limb, at altitudes between 0.1 and 0.3 R_S above the photosphere. The Geostationary Operational Environmental Satellite baseline was remarkably high throughout this event. The X-ray source's temperature peaked around 10-11 MK, and its emission measure increased throughout this time interval. Higher up, at 0.7 R_S , hot (initially >8 MK) plasma has been observed by Ultraviolet Coronograph Spectrometer on Solar and Heliospheric Observatory for 2.3 days. This hot plasma was interpreted as the signature of a current sheet (CS) trailing the CME. The thermal energy content of the X-ray source is more than an order of magnitude larger than in the CS. Hence, it could be the source of the hot plasma in the CS, although CS heating by magnetic reconnection within it cannot be discounted. To better characterize the X-ray spectrum, we have used novel techniques (back-projection-based and visibility-based) for long-integration (several hours) imaging spectroscopy. There is no observed nonthermal hard X-ray bremsstrahlung emission, leading to the conclusion that there is either very little particle acceleration occurring in the vicinity of this postflare X-ray source, or that either the photon spectral index would have had to be uncharacteristically (in flare parlance) high (γ gsim 8) and/or the low-energy cutoff very low (E_c lsim 6 keV).