Ly alpha Emission Enhancement Associated with Soft X-Ray Microflares

Ly alpha (Ly alpha, 1216 angstrom) is the strongest emission line in the solar ultraviolet spectrum. In the present work, we obtained a Ly alpha enhancement catalog covering flares larger than B1 class from the GOES/EUVS data during 2010-2016. We focused on the 242 B-class events which are less investigated, however, show non-negligible Ly alpha emission enhancement. We found that on average the Ly alpha peak of B-class flares is 0.85% stronger than the background. For the flare energetics, it is found that the weaker the soft X-ray (SXR) flare, the larger the ratio of the radiated energy in Ly alpha to SXR. Using the RHESSI data and multi-wavelength observations taken by SDO-AIA, we diagnose the thermal and non-thermal properties of several flares. Three case studies show that the coincidence of the Ly alpha peak with the SXR time-derivative peak is not a sufficient condition of the nonthermal property of a Ly alpha microflare. The Ly alpha enhancement in the microflares may be caused by the nonthermal electron beams or/and thermal conduction. However for type III events, we found that the delay of the Ly alpha peak with respect to the SXR peak can be attributed to either the Ly alpha emission from a filament erupted or the cooling of the thermal plasma in flare loops. Furthermore, interestingly the Ly alpha emission from filaments can not only occur in the decay phase of the flare, but also in the pre-flare phase. In this case, the Ly alpha emission was originated from an erupted filament which probably initiated the flare.