Statistical Studies of Solar White-light Flares and Comparisons with Superflares on Solar-type Stars

Abstract

Recently, many superflares on solar-type stars have been discovered as white-light flares (WLFs). The statistical study found a correlation between their energies (E) and durations (τ): τ \propto {E}^0.39, similar to those of solar hard/soft X-ray flares, τ \propto {E}^0.2{--0.33}. This indicates a universal mechanism of energy release on solar and stellar flares, i.e., magnetic reconnection. We here carried out statistical research on 50 solar WLFs observed with Solar Dynamics Observatory/HMI and examined the correlation between the energies and durations. As a result, the E-τ relation on solar WLFs (τ \propto {E}^0.38) is quite similar to that on stellar superflares (τ \propto {E}^0.39). However, the durations of stellar superflares are one order of magnitude shorter than those expected from solar WLFs. We present the following two interpretations for the discrepancy: (1) in solar flares, the cooling timescale of WLFs may be longer than the reconnection one, and the decay time of solar WLFs can be elongated by the cooling effect; (2) the distribution can be understood by applying a scaling law (τ \propto {E}^1/3{B}^-5/3) derived from the magnetic reconnection theory. In the latter case, the observed superflares are expected to have 2-4 times stronger magnetic field strength than solar flares.