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. Downward motion of the Hα emitting layer of the flare kernels, which is generally accepted as the reaction to the chromospheric evaporation, was also observed. Both the polarization and the downward motion show the tendency to exhibit a significant signal during the rise phase of kernel brightenings. This fact suggests that not only the downward motion but also the polarization is closely related to the heating agent of the chromosphere. The hard X-rays observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (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, which lacks high-energy electrons.