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Astron. Astrophys. 324, 1211-1221 (1997)

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Polarization of the hydrogen H line in solar flares: contribution of the different radiative and collisional processes

E. Vogt 1, S. Sahal-Bréchot 2 and J.C. Hénoux 3

1 Observatoire de Paris-Meudon, DASOP, CNRS URA 2080 (Laboratoire de Physique du Soleil et de l'Héliosphère), 5, place Jules Janssen, F-92195 Meudon Cedex, France (Etienne.Vogt@obspm.fr)
2 Observatoire de Paris-Meudon, DAMAp, CNRS URA 812 (Laboratoire Atomes et Molécules en Astrophysique), 5, place Jules Janssen, F-92195 Meudon Cedex, France (Sylvie.Sahal-Brechot@obspm.fr)
3 Observatoire de Paris-Meudon, DASOP, CNRS URA 2080 (Laboratoire de Physique du Soleil et de l'Héliosphère), 5, place Jules Janssen, F-92195 Meudon Cedex, France (Jean-claude.Henoux@obspm.fr)

Received 30 May 1996 / Accepted 19 February 1997

Abstract

Linear polarization of the [FORMULA] 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 [FORMULA] level by vertical beams of protons with an energy greater than a few keV. Transfer of population between the Zeeman excited states by the local protons with an isotropic velocity distribution may reduce the polarization expected from the beam bombardment only. The amplitude of this effect has been computed by solving the statistical equilibrium equations for a 9-level hydrogen atom (all the levels of [FORMULA] including fine structure) for three different atmospheric models. The different collisional and radiative processes for populating and depopulating the Zeeman sublevels have been taken into account: excitation and deexcitation by collisions with the protons of the beam and with the local electrons and protons, excitation by the local [FOR__mark__MULA], [FORMULA] and [FORMULA] radiation, and deexcitation by spontaneous emission. It has been found that the polarization is not destroyed but reduced by a factor two to ten. Beams of non-thermal particles can be, as assumed previously, at the origin of the observed polarization, however, to get a significant degree of polarization, the flux of these particles must be significantly higher than that originally expected.

Key words: atomic processes – line: formation – polarization – Sun: flares

Send offprint requests to: E. Vogt

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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