Coronal Hard X-Ray Sources in Solar Flares Observed with Yohkoh/HXT

Abstract

In solar flares the Yohkoh Hard X-ray Telescope (HXT) has revealed, in addition to the most prominent double-footpoint sources, several types of coronal hard X-ray sources. (1) A compact source that appears only during the impulsive phase. It is located above the soft X-ray loop and has a relatively hard spectrum. This strongly suggests that magnetic reconnection takes place above (outside of) the bright soft X-ray loop. It is maybe that the reconnection outflow impinges upon the underlying loop, forms a shock, and energized electrons resulting in a hard X-ray source above the loop. (2) A gradual source that begins in the impulsive phase but becomes dominant later in the gradual phase with a characteristic time scale of 5–10 min. It is located near the soft X-ray loop apex and is characterized by a very soft spectrum, which is interpreted as originating from an isothermal plasma with the electron temperatures of 30–40 MK. The plasma seems to be directly heated in situ to those super-hot temperatures. (3) Another gradual source seen in the so-called super-hot thermal flares (Type A flares). This source is quite similar to the gradual source mentioned above, but appears with quite weak, if any, impulsive signatures. (4) A coronal source seen in Long Duration Event (LDE) flares. LDE flares usually show a clear cusp- shaped loop structure in soft X-rays, in which a soft-spectrum, relatively large-sized (1–2 arcmin), long-lasting (typically 30 min) hard X-ray source appears. It is located below the soft X-ray cusp but slightly above the densest portion of the bright soft X-ray loop.KeywordsSolar FlareMagnetic ReconnectionImpulsive PhaseCoronal SourceGradual PhaseThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.