Polarized line formation with angle-dependent partial frequency redistribution

Abstract

The linear polarization of spectral lines seen in the solar limb observations is created by the scattering of the anisotropic radiation field by atoms and molecules. The partial frequency redistribution (PRD) effects in line scattering are necessary ingredients for interpreting the linear polarization observed in strong resonance lines. This polarization is sensitive to the form of the PRD function used in the polarized line transfer equation. The use of angle-averaged PRD function is quite common in the literature on polarized transfer, as it greatly reduces the computing efforts. In this paper we present our recent work on the importance of the angle-dependent PRD in polarized line transfer. First we present a brief historical background on polarized transfer with angle-dependent PRD. To simplify the numerical work needed to handle angledependent PRD function a Stokes vector decomposition technique was developed by Frisch (2009, 2010). After briefly recalling this technique, we present two numerical methods developed to solve the polarized line transfer with angle-dependent PRD. These are (1) polarized accelerated lambda iteration method and (2) the scattering expansion method. Through illustrative examples, we show that while angle-dependent effects are somewhat less important for scattering polarization in the absence of magnetic fields, they play an important role in the presence of a weak magnetic field.