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Astron. Astrophys. 325, 1179-1198 (1997)

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

J.H.M.J. Bruls 1, 2 and S.K. Solanki 3

1 Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, D-79104 Freiburg, Germany
2 Instituto de Astrofísica de Canarias, C./ Via Láctea s/n, E-38200 La Laguna, Tenerife, Spain
3 Institut für Astronomie, ETH-Zentrum, CH-8092 Zürich, Switzerland

Received 8 October 1996 / Accepted 6 March 1997

Abstract

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 Hubeny & 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 for wavelength 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 ii K-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 Hubeny & Lites 1995 angle-averaged PRD version of MULTI 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.

Key words: line: formation – line: profiles – radiative transfer – Sun: chromosphere – Sun: faculae, plages – Sun: magnetic fields

Send offprint requests to: J. Bruls

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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