Plasma heating in a post eruption Current Sheet: a case study based on ultraviolet, soft, and hard X-ray data

Abstract: Off-limb observations of the solar corona after Coronal Mass Ejections (CMEs) often show strong, compact, and persistent UV sources behind the eruption. They are primarily observed by the SOHO/UVCS instrument in the "hot" Fe XVIII {\lambda}974 {\AA} line and are usually interpreted as a signature of plasma heating due to magnetic reconnection in the post-CME Current Sheet (CS). Nevertheless, the physical process itself and the altitude of the main energy release are currently not fully understood. In this work, we studied the evolution of plasma heating after the CME of 2004 July 28 by comparing UV spectra acquired by UVCS with soft X-ray (SXR) and hard X-ray (HXR)images of the post-flare loops taken by GOES/SXI and RHESSI. The X-ray data show a long-lasting extended source that is rising upwards, toward the high-temperature source detected by UVCS. UVCS data show the presence of significant non-thermal broadening in the CS (signature of turbulent motions) and a strong density gradient across the CS region. The thermal energy released in the HXR source is on the order of about 10^32 erg, a factor about 2-5 larger than the energy required to explain the high-temperature plasma sampled by UVCS. Nevertheless, the very different time evolutions of SXR and HXR sources compared to the UV emission suggest that reconnection occurring above the post-eruption arcades are not directly responsible for the high-temperature plasma sampled higher up by UVCS. We conclude that an additional plasma heating mechanism (such as turbulent reconnection) in the CS is likely required.