Hα Impact Polarization Observed in a Gradual Flare

Linear polarization of the Hα line was detected in kernels of a gradual flare, which was accompanied by a filament eruption. The maximum degree of polarization of this flare exceeds 1 %, and the orientation of the polarization is approximately perpendicular to the flare ribbons. These results can be explained with the assumption that the observed polarization is impact polarization produced by accelerated protons. The hard X-rays observed by the RHESSI show a thermal component, but there is no detectable power-law component due to accelerated electrons. Therefore, protons are presumed to be not only the cause of the polarization, but also a possible candidate of the principal heating agent in this flare. Further study of the impact polarization with a higher polarimetric precision will be realized by the SolarB SOT, because the SOT has a polarimetric capacity in some chromospheric lines.