Anisotropic collisions and impact circular polarization

Application to the solar Halpha line

Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain e-mail: moncef@iac.es

Received: 23 August 2006
Accepted: 5 November 2006

Abstract

Aims.In light of recent contradictory observational results concerning the atomic polarization of solar lines, our purpose is to present certain collisional effects that might contribute towards a possible explanation. In particular, we aim to draw attention to the possibility of creating the “impact circular polarization”.

Methods.A general theoretical formulation of the problem of anisotropic collisions in the tensorial representation is obtained in an arbitrary symmetry of the relative velocity distribution. To try to understand this in concrete terms and estimate the effect of these collisions for creating and increasing the atomic orientation, we determine an explicit expression of the alignment-to-orientation transfer rates between two hydrogen levels in a particular case of symmetry.

Results. The anisotropic collisions could play a role in creating and increasing the atomic orientation by an alignment-to-orientation conversion mechanism (impact circular polarization). Physically, this is due to coherence transfer by anisotropic collisions. This transfer, and hence the creation of atomic circular polarization, can be achieved in different ways, which we describe in the case of the H_α line. However, for given solar conditions, the alignment-to-orientation transfer rates seem to be 4 to 5 times lower than the population-to-alignment transfer rates which are basically responsible for generating the impact linear polarization.