Flare line impact polarization

Na D2 589 nm line polarization in the 2001 June 15 flare^⋆

¹ Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris, 5 place Jules Janssen, 92190 Meudon, France
e-meil: jc.henoux@orange.fr
² Astronomical Institute, Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic
e-meil: karlicky@asu.cas.cz

Received: 25 April 2012
Accepted: 9 June 2013

Abstract

Context. The impact polarization of optical chromospheric lines in solar flares is still being debated. For this reason, additional observations and improved flare atmosphere models are needed still.

Aims. The polarization-free telescope THEMIS used in multiline 2 MulTiRaies (MTR) mode allows accurate simultaneous linear polarization measurements in various spectral lines.

Methods. In the 2001 June 15 flare, Hα, Hβ, and Mg D2 lines linear impact polarization was reported as present in THEMIS 2 MTR observations. In this paper, THEMIS data analysis was extended to the Na D2 line. Sets of I ± U and I ± Q flare Stokes S 2D-spectra were corrected from dark-current, spectral-line curvature and from transmission differences. Then, we derived the linear polarization degree P and polarization orientation angle α 2D-spectra. No change in relative positioning could be found that would reduce the Stokes parameters U and Q values. No V and I crosstalks could explain our results either.

Results. The Na D2 line is linearly polarized with a polarization degree exceeding 5% at some locations. The polarization was found to be radial at outer ribbons edges, and tangential at their inner edges. This orientation change may be due to differences in electron distribution functions on the opposite borders of flare chromospheric ribbons. Electron beams propagating along magnetic field lines, together with return currents, could explain both radial and tangential polarization. At the inner ribbon edges, intensity profile-width enlargements and blueshifts in polarization profiles are observed. This suggests chromospheric evaporation.