Impact Polarization of the Hα Line during Solar Flares
The chromospheric Hα line of hydrogen shows linear polarization in solar flares. This phenomenon was discovered at Meudon Observatory with a monochromatic polarimeter. The observed polarization is interpreted as impact polarization, resulting from the excitation of hydrogen by directed beams of protons with an energy of a few hundreds keV at their origin in the corona.
To support this interpretation, it has been necessary to evaluate the role and the importance of collisions between hydrogen atoms and thermal background particles — electrons and protons — as these collisions lead to population exchange between the sublevels of the hydrogen atom that may reduce significantly the resulting polarization. Therefore, we have computed the statistical equilibrium of the hydrogen atom taking into account all the different excitation and deexcitation processes.
A more sophisticated analysis of existing observations has also been performed in order to clearly separate the solar polarization signal from the artifacts caused by atmospheric turbulence and to get a better evaluation of the degree of polarization. We have done this extended data processing on two flares observed at Meudon Observatory on July 17, 1982 and June 20, 1989.
Also, a more powerful polarimeter has been developped, using a CCD camera, to provide better data during the next maximum of solar activity. This instrument is briefly described.
Key wordsImpact Polarization Solar Flares Polarimeters
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