Polarization of the hydrogen H line in solar flares

Contribution of the local polarized radiation field and effect of the spectral index of the proton energy distribution

¹ Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy e-mail: vogt@na.astro.it
² Observatoire de Paris-Meudon, DAMAp, CNRS UMR 8588 (Département Atomes et Molécules en Astrophysique), 5 Place Jules Janssen, 92195 Meudon Cedex, France e-mail: Sylvie.Sahal-Brechot@obspm.fr, V.Bommier@obspm.fr

Corresponding author: E. Vogt, etienne@astro.gla.ac.uk

Received: 3 April 2001
Accepted: 1 June 2001

Abstract

Linear polarization of the hydrogen Hα line was observed during solar flares. The polarization vector is directed towards disk center and its degree is of the order of 5% . The best explanation for this polarization is anisotropic collisional excitation of the level of hydrogen by vertical beams of protons with an energy greater than a few keV. However, previous calculations gave an expected polarization degree of 2.5% or less, a factor of two below the observations. In this paper, the theoretical model for the formation of the line polarization has been refined, including the effect of polarization in the local radiation field that is created by hydrogen proton anisotropic excitation. We have also increased the spectral index of the proton energy distribution from 4 to 5, giving more weight to the low energy protons which are the most efficient for impact polarization, without ionizing the atmosphere too much. It is found that the inclusion of the polarization of the local radiation field does not increase the Hα polarization very significantly; however, going from a spectral index of 4 to 5 results in an expected polarization degree of 4.5% , compatible with the observations.