Simulations of Zeeman - split Ca II K - line Stokes profiles with angle - dependent partial redistribution

The formulation of the polarized radiative transfer equations for Zeeman-split spectral lines is still incomplete for cases with frequency-dependent line source function, e.g. when partial frequency distribution (PRD) of line photons occurs (strong resonance lines). Under the well-founded assumption that the field-free approximation works equally well for lines with partial redistribution as for lines with complete redistribution (CRD), we decouple the computation of the level populations (accounting for PRD effects) from the formal Stokes vector solution. The level populations are obtained with the Hubený & Lites (1995) version of Carlsson’s (1986) MULTI code, and the formal solution follows by means of a modified version of the Murphy & Rees (1990) SPSR code. Due to lack of the appropriate equations concerning the combination of partial redistribution and Zeeman splitting of spectral lines, an ad hoc partial redistribution approach (basically allowing forwavelength dependence of the line source function) is implemented in the SPSR code. This combination of codes is used to study the relevance of partial redistribution to the Ca iiK-line diagnostics of solar plage regions using grids of flux-tube models. In addition to magnetic fields, velocity fields play an important role in the formation of the K-line profiles, but these invalidate the commonly-used angle-averaged PRD formalism. We therefore extended the Hubený & Lites (1995) angle-averaged PRDversion ofMULTI code to the angle-dependent case,which allows line profiles to be computed under PRD conditions for arbitrary (but not too large) velocity fields. This code is subsequently used to perform the non-magnetic non-LTE statistical equilibrium computations for a few schematic velocity structures in plage flux tubes and in the surrounding non-magnetic atmosphere. The SPSR code is again used to obtain the K-line Stokes profiles. We investigate in particular the asymmetry of the K line intensity and circularly polarized profiles produced by the velocity fields. Send offprint requests to: J. Bruls