89 research outputs found
Polarized line formation with J-state interference in the presence of magnetic fields: A heuristic treatment of collisional frequency redistribution
An expression for the partial frequency redistribution (PRD) matrix for line
scattering in a two-term atom, which includes the J-state interference between
its fine structure line components is derived. The influence of collisions
(both elastic and inelastic) and an external magnetic field on the scattering
process is taken into account. The lower term is assumed to be unpolarized and
infinitely sharp. The linear Zeeman regime in which the Zeeman splitting is
much smaller than the fine structure splitting is considered. The inelastic
collision rates between the different levels are included in our treatment. We
account for the depolarization caused by the collisions coupling the fine
structure states of the upper term, but neglect the polarization transfer
between the fine structure states. When the fine structure splitting goes to
zero, we recover the redistribution matrix that represents the scattering on a
two-level atom (which exhibits only m-state interference --- namely the Hanle
effect). The way in which the multipolar index of the scattering atom enters
into the expression for the redistribution matrix through the collisional
branching ratios is discussed. The properties of the redistribution matrix are
explored for a single scattering process for an L=0 to 1 to 0 scattering
transition with S=1/2 (a hypothetical doublet centered at 5000 A and 5001 A).
Further, a method for solving the Hanle radiative transfer equation for a
two-term atom in the presence of collisions, PRD, and J-state interference is
developed. The Stokes profiles emerging from an isothermal constant property
medium are computed.Comment: Accepted for publication in Journal of Quantitative Spectroscopy and
Radiative Transfer (JQSRT
Origin of spatial variations of scattering polarization in the wings of the Ca {\sc i} 4227 \AA line
Polarization that is produced by coherent scattering can be modified by
magnetic fields via the Hanle effect. According to standard theory the Hanle
effect should only be operating in the Doppler core of spectral lines but not
in the wings. In contrast, our observations of the scattering polarization in
the Ca {\sc i} 4227 \AA line reveals the existence of spatial variations of the
scattering polarization throughout the far line wings. This raises the question
whether the observed spatial variations in wing polarization have a magnetic or
non-magnetic origin. A magnetic origin may be possible if elastic collisions
are able to cause sufficient frequency redistribution to make the Hanle effect
effective in the wings without causing excessive collisional depolarization, as
suggested by recent theories for partial frequency redistribution with coherent
scattering in magnetic fields. To model the wing polarization we apply an
extended version of the technique based on the "last scattering approximation".
This model is highly successful in reproducing the observed Stokes
polarization (linear polarization parallel to the nearest solar limb),
including the location of the wing polarization maxima and the minima around
the Doppler core, but it fails to reproduce the observed spatial variations of
the wing polarization in terms of magnetic field effects with frequency
redistribution. This null result points in the direction of a non-magnetic
origin in terms of local inhomogeneities (varying collisional depolarization,
radiation-field anisotropies, and deviations from a plane-parallel atmospheric
stratification).Comment: Accepted in May 2009 for publication in The Astrophysical Journa
On the Sensitivity of Partial Redistribution Scattering Polarization Profiles to Various Atmospheric Parameters
This paper presents a detailed study of the scattering polarization profiles
formed under partial frequency redistribution (PRD) in two thermal models of
the solar atmosphere. Particular attention is given to understanding the
influence of several atmospheric parameters on the emergent fractional linear
polarization profiles. The shapes of these profiles are interpreted in
terms of the anisotropy of the radiation field, which in turn depends on the
source function gradient that sets the angular variation of the specific
intensity. We define a suitable frequency integrated anisotropy factor for PRD
that can be directly related to the emergent linear polarization. We show that
complete frequency redistribution is a good approximation to model weak
resonance lines. We also show that the emergent linear polarization profiles
can be very sensitive to the thermal structure of the solar atmosphere and, in
particular, to spatial variations of the damping parameter.Comment: 45 pages, 16 figures, accepted for publication in the Astrophysical
Journal (2010
Full non-LTE spectral line formation II. Two-distribution radiation transfer with coherent scattering in the atom's frame
In the present article, we discuss a numerical method of solution for the
so-called "full non-LTE" radiation transfer problem, basic formalism of which
was revisited by Paletou & Peymirat (2021; see also Oxenius 1986). More
specifically, usual numerical iterative methods for non-LTE radiation transfer
are coupled with the above-mentioned formalism. New numerical additions are
explained in detail. We benchmark the whole process with the standard non-LTE
transfer problem for a two-level atom with Hummer's (1962, 1969)
partial frequency redistribution function. We finally display new quantities
such as the spatial distribution of the velocity distribution function of
excited atoms, that can only be accessed to by adopting this more general frame
for non-LTE radiation transfer.Comment: 7 pages, 5 figures, accepted A&
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