EDP Sciences
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Volume 412, Number 1, December II 2003
Page(s) 271 - 280
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20031407

A&A 412, 271-280 (2003)
DOI: 10.1051/0004-6361:20031407

Effect of anisotropic velocity distribution on the linear polarization of coronal lines

Does the ion cyclotron exist in the inner corona?
N.-E. Raouafi and S. K. Solanki

Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
(Received 16 September 2002 / Accepted 3 September 2003 )

The effect of an anisotropic velocity field distribution of scattering ions on the polarization parameters of a spectral line emitted by resonance scattering is considered. The anisotropy of the velocity field distribution can be interpreted in terms of the ion-cyclotron effect that is believed to influence some heavy ions in the solar corona. We present a theoretical study of the Stokes parameters of a spectral line emitted by atoms or ions in the presence of a bi-Maxwellian velocity field distribution. It is found from test calculations that such a distribution measurably changes the polarization properties of the $\ion{O}{vi}$ D 2 coronal line. Consequently, measurements of the linear polarization of this line may serve as a new diagnostic of a possible bi-Maxwellian velocity distribution. As a preliminary application, the obtained theoretical results are used to interpret the polarization parameters of the $\ion{O}{vi}$ D 2 coronal line ( $\lambda1031.92$) measured using SUMER/SoHO observations. The obtained results are compatible with SUMER's observations for more reasonable solar wind parameters than for an isotropic velocity field distribution of the scattering ions. Thus, the outflow velocity field vector of the emitting ions is less inclined with respect to the polar axis of the Sun (the minimal value of the polar angle  $\eta$ is  $8^{\circ}$ for an anisotropic velocity distribution versus  $17^{\circ}$ for an isotropic velocity field distribution). These results are obtained assuming that the re-emitted photons come from a small area in the center of the coronal polar hole, with zero magnetic field. Since SUMER/SoHO observations integrate over the line of sight, the results of the current analysis must be considered preliminary pending computations including an integration along the line of sight.

Key words: polarization -- scattering -- line: profiles -- Sun: corona -- Sun: solar wind -- Sun: UV radiation

Offprint request: N.-E. Raouafi, raouafi@linmpi.mpg.de

© ESO 2003