Direct observation of the energy release site in a solar flare by SDO/AIA, Hinode/EIS, and RHESSI

Abstract

Aims. We present direct evidence of the detection of the main energy release site in a non-eruptive solar flare, SOL2013-11-09T06:38 UT. This GOES class C2.6 event was characterised by two flaring ribbons and a compact, bright coronal source located between them, which is the focus of our study. Methods. We use imaging from SDO/AIA, and imaging spectroscopy from RHESSI to characterise the thermal and non-thermal emission from the coronal source, and EUV spectroscopy from the Hinode/Extreme ultraviolet Imaging Spectrometer, which scanned the coronal source during the impulsive peak, to analyse Doppler shifts in Fe xii (log T = 6.2) and Fe xxiv (log T = 7.2) emission lines, and determine the source density. Results. The coronal source exhibited an impulsive emission lightcurve in all SDO/AIA filters during the impulsive phase. RHESSI hard X-ray images indicate both thermal and non-thermal emission at the coronal source, and its plasma temperature derived from RHESSI imaging spectroscopy shows an impulsive rise, reaching a maximum at 12-13 MK about 10 s prior to the hard X-ray peak. High red-shifts associated with this bright source indicate downflows of 40-250 km s -1 at a broad range of temperatures, interpreted as loop shrinkage and/or outflows along the magnetic field. Outflows from the coronal source towards each ribbon are also observed by SDO/AIA images at 171, 193, 211, 304, and 1600 Å. The electron density of the source obtained from a Fe xiv line pair is 10 11.50 cm -3 which is collisionally thick to electrons with energy up to 45-65 keV, responsible for the source's non-thermal X-ray emission. Conclusions. Given the rich observational evidence, we conclude that the bright coronal source is the location of the main release of magnetic energy in this flare, with a geometry consistent with component reconnection between crossing, current-carrying loops. We argue that the energy that can be released via reconnection, based on observational estimates, can plausibly account for the non-thermal energetics of the flare.