Acceleration Of Photoelectrons In A Return-Current Electric Field And Ha Polarization

Details Acceleration Of Photoelectrons In A Return-Current Electric Field And Ha Polarization Acceleration Of Photoelectrons In A Return-Current Electric Field And Ha Polarization

Oct 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999esasp.446..369k&link_type=abstract

8th SOHO Workshop: Plasma Dynamics and Diagnostics in the Solar Transition Region and Corona. Proceedings of the Conference hel

Astronomy and Astrophysics

Astrophysics

Scientific paper

Chromospheric hydrogen Balmer Ha and Hb lines have been found to be linearly polarized in the gradual phase of some solar flares. This impact polarization is believed to be caused by low energy protons (E< 200 keV). However, we still must examine if low energy electrons (E< 200 eV) with an anisotropic velocity distribution function, if presents, could also generate impact polarisation . In a recent paper by H noux and Karlicky (1999), the evolution, in chromospheric layers, of the ve-locity distribution function of high energy photoelectrons produced by flare X-ray irradiation, was studied. A significant anisotropy was found. However, due to the dominance of local electrons in the line formation process, the resulting impact polarization was one order of magnitude lower than ob-served. In this paper, we look for the change of the photoelectrons anisotropy that could result from the pre-sence of a return-current electric field. A 1-D test particle code is used for computating both the re-turn-current electric field generated by monoenergetic electron beams in flaring chromospheric layers and the evolution of the photoelectron distribution function. Using a 100 keV electron beam with a density of 2 and the MAVH flare atmosphere, it is found that, at chromospheric level, low-energy photoelectrons (E at 150 eV), produced by flare X-ray irradiation, can be accelerated by the return-current electric field. This acceleration is very sensi-tive to the exact density and temperature profile of the solar atmosphere and the resulting effect as generation of impact Ha polarization is discussed.

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