Hydrogen H$α$ line polarization in solar flares. Theoretical investigation of atomic polarization by proton beams considering self-consistent NLTE polarized radiative transfer

Details Hydrogen H$α$ line polarization in solar flares. Theoretical investigation of atomic polarization by proton beams considering self-consistent NLTE polarized radiative transfer Hydrogen H$α$ line polarization in solar flares. Theoretical investigation of atomic polarization by proton beams considering self-consistent NLTE polarized radiative transfer

2007-01-22

arxiv.org/abs/astro-ph/0701617v1

Astron.Astrophys.465:621-631,2007

Astronomy and Astrophysics

Astrophysics

11 pages, 11 figures, accepted for publication in A&A

Scientific paper

10.1051/0004-6361:20066329

Context. We present a theoretical review of the effect of impact polarization of a hydrogen H$\alpha$ line due to an expected proton beam bombardment in solar flares. Aims. Several observations indicate the presence of the linear polarization of the hydrogen H$\alpha$ line observed near the solar limb above 5% 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 H$\alpha$ 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 H$\alpha$ line.

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