Flare ribbons observed with G-band and Fe I 6302 Å filters of the solar optical telescope on board Hinode

The Solar Optical Telescope (SOT) on board the Hinode satellite observed an X3.4 class flare on 2006 December 13. A typical two-ribbon structure was observed, not only in the chromospheric Ca II H line, but also in the G-band and Fe I 6302 Å line. The high-resolution, seeing-free images achieved by SOT revealed, for the first time, sub-arcsec fine structures of the "white light" flare. The G-band flare ribbons on sunspot umbrae showed a sharp leading edge, followed by a diffuse inside, as well as a previously known core-halo structure. The underlying structures, such as umbral dots, penumbral filaments, and granules, were visible in the flare ribbons. Assuming that the sharp leading edge was directly heated by a particle beam and the diffuse parts were heated by radiative back-warming, we estimated the depth of the diffuse flare emission using an intensity profile of the flare ribbon. We found that the depth of the diffuse emission was about 100 km or less from the height of the source of radiative back-warming. The flare ribbons were also visible in the Stokes-V images of Fe I 6302 Å, as a transient polarity reversal. This is probably related to a "magnetic transient" reported in the literature. The intensity increase in Stokes-I images indicates that the Fe I 6302 Å line was significantly deformed by the flare, which may cause such a magnetic transient.