Hydrogen Hα line polarization in solar flares

Theoretical investigation of atomic polarization by proton beams considering self-consistent NLTE polarized radiative transfer

¹ Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 298, 251 65 Ondřejov, Czech Republic e-mail: [stepan;pheinzel]@asu.cas.cz
² LERMA, Observatoire de Paris – Meudon, CNRS UMR 8112, 5 place Jules Janssen, 92195 Meudon Cedex, France e-mail: [jiri.stepan;sylvie.sahal-brechot]@obspm.fr

Received: 31 August 2006
Accepted: 20 December 2006

Abstract

Context. We present a theoretical review of the effect of impact polarization of a hydrogen Hα line due to an expected proton beam bombardment in solar flares.

Aims. Several observations indicate the presence of the linear polarization of the hydrogen Hα line observed near the solar limb above and preferentially in the radial direction. We theoretically review the problem of deceleration of the beam originating in the coronal reconnection site due to its interaction with the chromospheric plasma, and describe the formalism of the density matrix used in our description of the atomic processes and the treatment of collisional rates.

Methods. We solve the self-consistent NLTE radiation transfer problem for the particular semiempirical chromosphere models for both intensity and linear polarization components of the radiation field.

Results. In contrast to recent calculations, our results show that the energy distribution of the proton beam at Hα formation levels and depolarizing collisions by background electrons and protons cause a significant reduction of the effect below 0.1%. The radiation transfer solution shows that tangential resonance-scattering polarization dominates over the impact polarization effect in all considered models.

Conclusions. In the models studied, proton beams are unlikely to be a satisfying explanation for the observed linear polarization of the Hα line.