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Impulsive Thermal X-ray Emission from a Low-lying Coronal Loop

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. We carry out detailed analysis of Hα and X-ray observations of a GOES class B microflare loop on the solar disk. The impulsive hard X-ray emission, however, is found 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 microflares associated with dense loops. Hα 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 Hα emission may very well originate from the photosphere when the low-lying flare loop heats up the underlying chromosphere and reduces the corresponding Hα 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.