Abstract

Chromospheric evaporation implies the mass flow from chromosphere to corona along the loop legs in the solar flares, indicating the targets of hard X-ray emissions movement in the flare loops. From observations, hard X-rays tend to rise up the double footpoint sources along the loop legs and finally merge together around the top at the same position as the loop top source. After Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), the evidence of chromospheric evaporation in hard X-rays are studied and analyzed in several events. The source motions caused by the chromospheric evaporation are not observed at the whole energy band of hard X-rays, but favorably seen at 10-30 keV. Hard X-ray sources at the high energy (i.e. above 50 keV) would never show motions and stay around the footpoint during the evaporation. The observational results show that the typical velocity of hard X-ray source motions ranges from 200 kms^-1 to 500 kms^-1, the typical timescale of source merger is around 60 s (between 30 and 80 s).