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

Details Polarization of the hydrogen Halpha_ line in solar flares: contribution of the different radiative and collisional processes. Polarization of the hydrogen Halpha_ line in solar flares: contribution of the different radiative and collisional processes.

Aug 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...324.1211v&link_type=abstract

Astronomy and Astrophysics, v.324, p.1211-1221

Astronomy and Astrophysics

Astrophysics

27

Atomic Processes, Line: Formation, Polarization, Sun: Flares

Scientific paper

Linear polarization of the Halpha_ 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 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 n=1, 2,3 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 Lyalpha_, Lybeta_ and Halpha_ 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.

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