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Astron. Astrophys. 334, 1136-1144 (1998)

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Impact polarization and charge exchange as diagnostics of proton beams

C. Balança and N. Feautrier

DAMAP and URA 812 du CNRS, Observatoire de Paris, F-92195 Meudon Cedex, France

Received 19 November 1997 / Accepted 2 March 1998

Abstract

Linear polarization of the hydrogen H [FORMULA] line has been observed in solar flares. This polarization is actually explained as induced by anisotropic collisional excitation of the [FORMULA] 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 [FORMULA] and 3 levels by 1-100 keV protons are calculated in a two-centre atomic orbital close-coupling method and compared to the available experimental data. The comparision shows a good agreement. Cross sections for excitation of the sublevels gives the alignment of the excited states and the polarization fraction of the Lyman [FORMULA], Lyman [FORMULA] and Balmer [FORMULA] lines. The profiles of the CaII K line and the H [FORMULA] line observed during the flares are extremely broad and their interpretation requires the introduction of a macroturbulence velocity. From the calculated differential cross sections, we show that, for the collisional energies considered, the recoil velocity of the H atom after direct excitation gives rise to a macroscopic Doppler broadening that can partially explain the observations.

Combining observations of the wing profiles of the Lyman [FORMULA] and [FORMULA] lines, and observations of the polarization of the H [FORMULA] line would provide complementary diagnostics of the velocity distribution function of the proton beams.

Key words: atomic processes – Line: profiles – polarization – Sun: flares

Send offprint requests to: C. Balanca@obspm.fr

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998

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