Understanding the relationship among different emission components plays an essential role in the study of particle acceleration and energy conversion in solar flares. In flares where gradual and impulsive emission components can be readily identified, the impulsive emission has been attributed to non-thermal particles. A group led by Dr. Siming Liu at the Purple Mountain Observatory carried out detailed analysis of H-alpha and X-ray observations of a GOES class B micro-flare loop on the solar disk and found evidence to the contrary. The impulsive hard X-ray emission is shown to be consistent with a hot, quasi-thermal origin, and there is little evidence of emission from chromospheric footpoints, which challenges conventional models of flares and reveals a class of micro-flares associated with dense loops. H-alpha observations indicate that the loop lies very low in the solar corona or even in the chromosphere and both emission and absorption materials evolve during the flare. The enhanced H-alpha emission may very well originate from the photosphere when the low-lying flare loop heats up the underlying chromosphere and reduces the corresponding H-alpha opacity. These observations may be compared with detailed modeling of flare loops with the internal kink instability, where the mode remains confined in space without apparent change in the global field shape, to uncover the underlying physical processes and to probe the structure of solar atmosphere.

     This work is supported by the National Natural Science Foundation of China via the grants 11143007, 11173063, 11173064, 11233008; the EU’s SOLAIRE Research and Training Network at the University of Glasgow (MTRN-CT-2006-035484); STFC Grant ST/1001808/1; the EC-funded FP7 project HESPE (FP-2010-SPACE-1-263086); and was published on Astrophysical Physical Journal on June 1st 2013 (769, 135).