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Astron. Astrophys. 349, 283-294 (1999)

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Observations of linear polarization in the [FORMULA] line during two solar flares

Etienne Vogt and Jean-Claude Hénoux

Observatoire de Paris-Meudon, DASOP, CNRS URA 2080 (Laboratoire de Physique du Soleil et de l'Héliosphère), 5 Place Jules Janssen, F-92195 Meudon Cedex, France

Received 19 June 1998 / Accepted 9 March 1999


Two chromospheric flares were observed in Meudon using an [FORMULA] flare polarimeter on 17th July 1982 and 20th June 1989. A careful reduction of the data sets has been done in order to remove the false polarization signals induced by atmospheric turbulence. In each event, linear polarization directed towards the disk center with a polarization degree of the order of 5% is detected. This polarization is correlated in time with the maximum of soft X-ray emission. One event also shows polarization parallel to the solar limb at the moment of maximum hard X-ray emission but with a poor signal to noise ratio.

We interpret this polarization as impact polarization, resulting from the excitation of hydrogen atoms by anisotropic particles. The tangential polarization could be due to a beam of energetic electrons moving vertically. However, the best candidates to explain both tangential and radial polarization are beams of energetic protons with an energy below 100 keV entering into the chromosphere. The variation of the polarization direction could result from a modification of the magnetic topology during the flare.

Key words: Sun: flares – polarization – instrumentation: polarimeters

Send offprint requests to: E. Vogt

© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999