Energetic Protons Inferred from White-light Flares

In association with strong solar flares, we sometimes observe enhancements of visible continuum radiation, which is known as a "white-light flare". As most white-light (WL) events show close correlations in time and location with hard X-rays and/or radio emission, there is some consensus that WL emission originates from accelerated particles, especially non-thermal electrons. One model proposes that WL is emitted near the photosphere; however, non-thermal electrons are thermalized in the chromosphere and cannot reach the photosphere. Thus, there is a problem: how can the energy of non-thermal electrons - and/or other accelerated particles such as high-energy protons - propagate to the photosphere and produce WL emission? In this study, we investigate the possibility that accelerated protons may produce the WL emission of solar flares. We found 51 WL events observed by Hinode/SOT and/or SDO/HMI. Among them, gamma-rays with energies greater than 1 MeV were observed by Fermi/GBM and/or RHESSI only in the X1.8-class flare on October 23, 2012 and in the M7.9-class flare on June 25, 2015. Focusing on these flare events, we compare the energetics of WL emission and of accelerated ion fluxes. We estimated the total energy of accelerated protons for each event using the gamma-ray emission around 2.2 MeV and the Hua et al. (2002) magnetic-loop transport and interaction model. On the other hand, the total energy of WL emission can be estimated by fitting the three-color bands of Hinode/SOT and the intensity ratio of peak emission. As a result, the WL emission energy for the June 25, 2015 event cannot be explained by the accelerated-ion energy. We find that it is difficult for accelerated ions to provide sufficient energy to account for WL emission. This fact also indicates that WL emission does not originate mainly from accelerated ions.