Evidence of collisional depolarization of the Ba II 4554 line in the low chromosphere

Institut d'Astrophysique Spatiale, CNRS-Université Paris-Sud 11, 91405 Orsay Cedex, France e-mail: moncef.derouich@ias.u-psud.fr

Received: 21 October 2007
Accepted: 18 January 2008

Abstract

Context. Rigorous modeling of the formation is potentially interesting since this strongly polarized line forms in the solar chromosphere where the magnetic field is not very well known.

Aims. We investigate the role of isotropic collisions with neutral hydrogen in the formation of the polarized line and, thus, in the determination of the magnetic field.

Methods. Multipole relaxation and transfer rates of the d- and p-states of by isotropic collisions with neutral hydrogen are calculated. We consider a plane-parallel layer of that is situated at the low chromosphere and that is anisotropically illuminated from below, which produces linear polarization in the line by scattering processes. To compute that polarization, we solve the statistical equilibrium equations for levels including collisions, radiation, and magnetic-field effects.

Results. Variation laws of the relaxation and transfer rates with hydrogen number density n_H and temperature were deduced. The polarization of the line is clearly affected by isotropic collisions with neutral hydrogen, although the collisional depolarization of its upper level is negligible. This is because the alignment of the metastable levels and of the are vulnerable to collisions. At the height of formation of the line where cm^-3, we find that neglecting the collisions induces ~25% inaccuracy in the calculation of the polarization and ~35% inaccuracy on microturbulent magnetic field determination.

Conclusions. The polarization of the line decreases due to collisions with hydrogen atoms. In addition, during scattering processes collisions could change the frequency of the photons. To quantitatively study this line, one should deal with the problem of development of general theory treating partial redistribution of frequencies and including transfer and relaxation rates by collisions for a multilevel atom with hyperfine structure.