PHYSICAL PROPERTIES OF WHITE-LIGHT SOURCES IN THE 2011 FEBRUARY 15 SOLAR FLARE

Abstract

White light flares (WLFs) are observational rarities, making them\nunderstudied events. However, optical emission is a significant contribution to\nflare energy budgets and the emission mechanisms responsible could have\nimportant implications for flare models. Using Hinode SOT optical continuum\ndata taken in broadband red, green and blue filters, we investigate white-light\nemission from the X2.2 flare SOL2011-02-15T01:56:00. We develop a technique to\nrobustly identify enhanced flare pixels and, using a knowledge of the RGB\nfilter transmissions, determined the source color temperature and effective\ntemperature. We investigated two idealized models of WL emission - an optically\nthick photospheric source, and an optically thin chromospheric slab. Under the\noptically thick assumption, the color temperature and effective temperature of\nflare sources in sunspot umbra and penumbra were determined as a function of\ntime and position. Values in the range of 5000-6000K were found, corresponding\nto a blackbody temperature increase of a few hundred kelvin. The power emitted\nin the optical was estimated at $\\sim 10^{26}$ergs s$^{-1}$. In some of the\nwhite-light sources the color and blackbody temperatures are the same within\nuncertainties, consistent with a blackbody emitter. In other regions this is\nnot the case, suggesting that some other continuum emission process is\ncontributing. An optically thin slab model producing hydrogen recombination\nradiation is also discussed as a potential source of WL emission; it requires\ntemperatures in the range 5,500 - 25,000K, and total energies of $\\sim\n10^{27}$ergs s$^{-1}$.\n