H α impact polarization observed in solar flares as a diagnostic of energy transport mechanisms

Abstract

Various mechanisms of energy transport and energy release in solar flares create a population of energetic particles with an anisotropic velocity distribution function. By collisional exciation, these particles may produce linearly polarized line emission. The characteristics of the impact polarization associated with diverse mechanisms of energy transport are reviewed. Observations of linear polarization at some locations of H α filtergrams, taken at the time of three solar flares, are reported. The polarized electric vector is directed in the flare-to-disk-center direction. This polarization indicates that energetic particles with anisotropic velocity distributions are present in the high chromosphere during solar flares. They could be either deca-eV electrons coming from the transition zone or produced in local acceleration in weak double-layers or deca-keV protons accelerated in the corona.