Loop-Like Hard X-Ray Emission in a 2005 January 20 Flare

We present analyses of hard X-ray (HXR) emission observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) of an X7.1 flare on 2005 January 20. Generally, this flare shows three HXR sources: two footpoint sources and a looptop one. During some period of the flare evolution, HXR emission from the legs of the loop was evident, and a loop-like HXR structure appeared, especially at 25–50 keV, which is unusual, and has been rarely reported before. We consider this phenomenon to be observational evidence for chromospheric evaporation. To confirm this point, we calculated the height distribution of the energy deposition by an electron beam using different atmospheric models. The results suggest that, in order to engender the observed loop-like HXR emission in this flare, the coronal mass density and the electron-beam flux should be extremely high, which is a possible reason why loop-like HXR emission is rarely observed.