92 research outputs found
The transfer of resonance line polarization with partial frequency redistribution and J-state interference
The linear polarization signals produced by scattering processes in strong
resonance lines are rich in information on the magnetic and thermal structure
of the chromosphere and transition region of the Sun and of other stars. A
correct modeling of these signals requires accounting for partial frequency
redistribution effects, as well as for the impact of quantum interference
between different fine structure levels (J-state interference). In this paper,
we present a theoretical approach suitable for modeling the transfer of
resonance line polarization when taking these effects into account, along with
an accurate numerical method of solution of the problem's equations. We
consider a two-term atom with unpolarized lower term and infinitely sharp lower
levels, in the absence of magnetic fields. We show that by making simple formal
substitutions on the quantum numbers, the theoretical approach derived here for
a two-term atom can also be applied to describe a two-level atom with hyperfine
structure. An illustrative application to the MgII doublet around 2800A is
presented.Comment: Accepted for publication in Astronomy & Astrophysic
PORTA: A three-dimensional multilevel radiative transfer code for modeling the intensity and polarization of spectral lines with massively parallel computers
The interpretation of the intensity and polarization of the spectral line
radiation produced in the atmosphere of the Sun and of other stars requires
solving a radiative transfer problem that can be very complex, especially when
the main interest lies in modeling the spectral line polarization produced by
scattering processes and the Hanle and Zeeman effects. One of the difficulties
is that the plasma of a stellar atmosphere can be highly inhomogeneous and
dynamic, which implies the need to solve the non-equilibrium problem of the
generation and transfer of polarized radiation in realistic three-dimensional
(3D) stellar atmospheric models. Here we present PORTA, an efficient multilevel
radiative transfer code we have developed for the simulation of the spectral
line polarization caused by scattering processes and the Hanle and Zeeman
effects in 3D models of stellar atmospheres. The numerical method of solution
is based on the non-linear multigrid iterative method and on a novel
short-characteristics formal solver of the Stokes-vector transfer equation
which uses monotonic B\'ezier interpolation. Therefore, with PORTA the
computing time needed to obtain at each spatial grid point the self-consistent
values of the atomic density matrix (which quantifies the excitation state of
the atomic system) scales linearly with the total number of grid points.
Another crucial feature of PORTA is its parallelization strategy, which allows
us to speed up the numerical solution of complicated 3D problems by several
orders of magnitude with respect to sequential radiative transfer approaches,
given its excellent linear scaling with the number of available processors. The
PORTA code can also be conveniently applied to solve the simpler 3D radiative
transfer problem of unpolarized radiation in multilevel systems.Comment: 15 pages, 15 figures, to appear in Astronomy and Astrophysic
On the magnetic field of off-limb spicules
Determining the magnetic field related to solar spicules is vital for
developing adequate models of these plasma jets, which are thought to play a
key role in the thermal, dynamic and magnetic structure of the Chromosphere.
Here we report on the magnetic properties of off-limb spicules in a very quiet
region of the solar atmosphere, as inferred from new spectropolarimetric
observations in the He I 10830 A triplet obtained with the Tenerife Infrared
Polarimeter. We have used a novel inversion code for Stokes profiles caused by
the joint action of atomic level polarization and the Hanle and Zeeman effects
(HAZEL) to interpret the observations. Magnetic fields as strong as ~40G were
detected in a very localized area of the slit, which could represent a possible
lower value of the field strength of organized network spicules.Comment: Accepted for publication in ApJ, 24 pages, 5 Figure
Influence of atomic polarization and horizontal illumination on the Stokes profiles of the He I 10830 multiplet
The polarization observed in the spectral lines of the He I 10830 multiplet
carries valuable information on the dynamical and magnetic properties of plasma
structures in the solar chromosphere and corona, such as spicules, prominences,
filaments, emerging magnetic flux regions, etc. Here we investigate the
influence of atomic level polarization on the emergent Stokes profiles for a
broad range of magnetic field strengths, in both 90 degree and forward
scattering geometry. We show that, contrary to a widespread belief, the
selective emission and absorption processes caused by the presence of atomic
level polarization may have an important influence on the emergent linear
polarization, even for magnetic field strengths as large as 1000 G.
Consequently, the modeling of the Stokes Q and U profiles should not be done by
taking only into account the contribution of the transverse Zeeman effect
within the framework of the Paschen-Back effect theory, unless the magnetic
field intensity of the observed plasma structure is sensibly larger than 1000
G. We point out also that in low-lying optically thick plasma structures, such
as those of active region filaments, the (horizontal) radiation field generated
by the structure itself may substantially reduce the positive contribution to
the anisotropy factor caused by the (vertical) radiation field coming from the
underlying solar photosphere, so that the amount of atomic level polarization
may turn out to be negligible. Only under such circumstances may the emergent
linear polarization of the He I 10830 multiplet in such regions of the solar
atmosphere be dominated by the contribution caused by the transverse Zeeman
effect.Comment: Accepted for publication in The Astrophysical Journal (It is
tentatively scheduled for the ApJ January 20, 2007 issue
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