Impact polarization and charge exchange as diagnostics of proton beams

Details Impact polarization and charge exchange as diagnostics of proton beams Impact polarization and charge exchange as diagnostics of proton beams

Jun 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...334.1136b&link_type=abstract

Astronomy and Astrophysics, v.334, p.1136-1144 (1998)

Astronomy and Astrophysics

Astrophysics

11

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

Scientific paper

Linear polarization of the hydrogen Hα line has been observed in solar flares. This polarization is actually explained as induced by anisotropic collisional excitation of the n=3 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 n=2 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 alpha , Lyman beta and Balmer alpha lines. The profiles of the CaII K line and the Hα 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 alpha and beta lines, and observations of the polarization of the Hα line would provide complementary diagnostics of the velocity distribution function of the proton beams.

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