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Impact polarization and charge exchange as diagnostics of proton beams
C. Balança and
Received 19 November 1997 / Accepted 2 March 1998
Linear polarization of the hydrogen H line has been observed in solar flares. This polarization is actually explained as induced by anisotropic collisional excitation of the level by vertical proton beams with an energy of a few keV. At such energies, charge exchange between the proton beam and the local H atoms is important and must be taken into account in the statistical equilibrium equations.
Accurate cross sections for direct excitation and charge exchange of and 3 levels by 1-100 keV protons are calculated in a two-centre atomic orbital close-coupling method and compared to the available experimental data. The comparision shows a good agreement. Cross sections for excitation of the sublevels gives the alignment of the excited states and the polarization fraction of the Lyman , Lyman and Balmer lines. The profiles of the CaII K line and the H line observed during the flares are extremely broad and their interpretation requires the introduction of a macroturbulence velocity. From the calculated differential cross sections, we show that, for the collisional energies considered, the recoil velocity of the H atom after direct excitation gives rise to a macroscopic Doppler broadening that can partially explain the observations.
Combining observations of the wing profiles of the Lyman and lines, and observations of the polarization of the H line would provide complementary diagnostics of the velocity distribution function of the proton beams.
Key words: atomic processes Line: profiles polarization Sun: flares
Send offprint requests to: C. Balanca@obspm.fr
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998