Astron. Astrophys. 325, 1179-1198
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Simulations of Zeeman-split Ca II K-line Stokes
with angle-dependent partial redistribution
J.H.M.J. Bruls1,2 and S.K. Solanki3
1Kiepenheuer-Institut für Sonnenphysik,
Schöneckstr. 6, D-79104 Freiburg, Germany
2Instituto de Astrofísica de Canarias,
C./ Via Láctea s/n, E-38200 La Laguna, Tenerife,
3Institut für Astronomie,
ETH-Zentrum, CH-8092 Zürich, Switzerland
Received 8 October 1996 / Accepted 6 March 1997
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
version of Carlsson's (1986)
MULTI code, and
the formal solution follows by means of a modified
version of the
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
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
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)
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
© European Southern Observatory (ESO) 1997
Online publication: September 9, 1997
Last change: April 28, 1998