IMPACT POLARIZATION AND CHARGE-EXCHANGE AS DIAGNOSTICS OF PROTON-BEAMS

Linear polarization of the hydrogen H alpha line has been observed in solar flares. This polarization is actually explained as induced by an isotropic collisional excitation of the n = 3 level by vertical proton beams with an energy of a few keV. At such energies, charge exchange between the proton beam and the local H atoms is important and must be taken into account in the statistical equilibrium equations. Accurate cross sections for direct excitation and charge exchange of n = 2 and 3 levels by 1-100 keV protons are calculated in a two-centre atomic o rbital close-coupling method and compared to the available experimenta l data. The comparision shows a good agreement. Cross sections for exc itation of the sublevels gives the alignment of the excited states and the polarization fraction of the Lyman alpha, Lyman beta and Balmer a lpha lines. The profiles of the CaII K line and the H alpha line obser ved during the flares are extremely broad and their interpretation req uires the introduction of a macroturbulence velocity. From the calcula ted differential cross sections, we show that, for the collisional ene rgies considered, the recoil velocity of the H atom after direct excit ation gives rise to a macroscopic Doppler broadening that can partiall y explain the observations. Combining observations of the wing profile s of the Lyman alpha and beta lines, and observations of the polarizat ion of the H alpha line would provide complementary diagnostics of the velocity distribution function of the proton beams.