Enhanced black body radiation as a generating mechanism for white light solar flares

Abstract

A white light flare is a rare impulsive event observed in the visible continuum radiating from localized regions during the early explosive stage of a few solar flares. A self-consistent model is developed which attempts to explain the white light flare in terms of an enhanced local black body radiation due to a temperature perturbation at about optical depth unity in the photosphere. Solar cosmic ray and radio observations indicate that energetic protons and electrons are generated during the early phase of a solar proton flare. The model assumes a blast of energetic electrons and protons in equal numbers in the 10 to 1000 MeV range, incident on the photosphere from above, releasing most of its energy to the ambient gas at about the depth one sees the normal photospheric continuum radiation. This is interpreted by the study in terms of a temperature perturbation of the layer and a reradiation of the energy from an optically thin medium with a radiative relaxation time of several seconds and a radiation temperature of several hundred degrees above the normal. For the purpose of analysis, an inverse power law energy distribution for the energetic particles is assumed, and the analysis is applied to the May 23, 1967 white light flare event. The number of energetic particles required to account for the enhanced continuum radiation by this mechanism is set between 10^31 and 10^32. No other process seems to account as efficiently for the observed emission except possibly the synchrotron process, but then only with the most favorable geometry. Since the Planck function at 6000 0 K predicts negligible radio and x-radiation, the model allows for the generation of these radiations by other mechanisms, for example, the synchrotron and bremsstrahlung mechanisms, respectively. Assuming that the bremsstrahlung mechanism is responsible for the hard x-radiation, the inverse power law distribution is truncated at the low energy end of the spectrum. The final result is a differential energy spectrum, dN/dE = 4 x 10^32 , 0.50 MeV < E < 1 MeV, = 4 X 10^32E^-(5/3), 1 MeV < E < 1 BeV, which would account for the radiations during the impulsive phase of the May 23, 1967 event. Although the predictions of the model agree well with the observations, new observations, particularly of white light polarization, are needed to test the model.