Polarization of the hydrogen H alpha line in solar flares - Contribution of the local polarized radiation field and effect of the spectral index of the proton energy distribution

Linear polarization of the hydrogen H alpha line was observed during solar flares. The polarization vector is directed towards disk center and its deg ree is of the order of 5%. The best explanation for this polarization is an isotropic collisional excitation of the n = 3 level of hydrogen by vertical beams of protons with an energy greater than a few keV. However, previous calculations gave an expected polarization degree of 2.5% or less, a factor of two below the observations. In this paper, the theoretical model for th e formation of the line polarization has been refined, including the effect of polarization in the local radiation field that is created by hydrogen p roton anisotropic excitation. We have also increased the spectral index of the proton energy distribution from 4 to 5, giving more weight to the low e nergy protons which are the most efficient for impact polarization, without ionizing the atmosphere too much. It is found that the inclusion of the po larization of the local radiation field does not increase the H alpha polar ization very significantly; however, going from a spectral index of 4 to 5 results in an expected polarization degree of 4.5%, compatible with the obs ervations.