POLARIZATION OF THE HYDROGEN H-ALPHA LINE IN THE SOLAR-FLARES - CONTRIBUTION OF THE DIFFERENT RADIATIVE AND COLLISIONAL PROCESSES

Linear polarization of the H-alpha line of hydrogen has been observed in solar flares. The observed polarization degree can be as high as 10 % for a flare located near the limb and a one minute integration time . This polarization is currently explained as anisotropic collisional excitation of the n = 3 level by vertical beams of protons with an ene rgy greater than a few keV. Transfer of population between the Zeeman excited states by the local protons with an isotropic velocity distrib ution may reduce the polarization expected from the beam bombardment o nly. The amplitude of this effect has been computed by solving the sta tistical equilibrium equations for a 9-level hydrogen atom (all the le vels of n = 1,2,3 including fine structure) for three different atmosp heric models. The different collisional and radiative processes for po pulating and depopulating the Zeeman sublevels have been taken into ac count: excitation and deexcitation by collisions with the protons of t he beam and with the local electrons and protons, excitation by the lo cal Ly(alpha), Ly(beta) and H-alpha radiation, and deexcitation by spo ntaneous emission. It has been found that the polarization is not dest royed but reduced by a factor two to ten. Beams of non-thermal particl es can be, as assumed previously, at the origin of the observed polari zation, however, to get a significant degree of polarization, the flux of these particles must be significantly higher than that originally expected.