30 research outputs found
The Hanle effect of Ly in an MHD model of the Solar Transition Region
In order to understand the heating of the solar corona it is crucial to
obtain empirical information on the magnetic field in its lower boundary (the
transition region). To this end, we need to measure and model the linear
polarization produced by scattering processes in strong UV lines, such as the
hydrogen Ly line. The interpretation of the observed Stokes profiles
will require taking into account that the outer solar atmosphere is highly
structured and dynamic, and that the height of the transition region may well
vary from one place in the atmosphere to another. Here we report on the
Ly scattering polarization signals we have calculated in a realistic
model of an enhanced network region, resulting from a state-of-the-art
radiation MHD simulation. This model is characterized by spatially complex
variations of the physical quantities at transition region heights. The results
of our investigation lead us to emphasize that scattering processes in the
upper solar chromosphere should indeed produce measurable linear polarization
in Ly. More importantly, we show that via the Hanle effect the model's
magnetic field produces significant changes in the emergent and
profiles. Therefore, we argue that by measuring the polarization signals
produced by scattering processes and the Hanle effect in Ly and
contrasting them with those computed in increasingly realistic atmospheric
models, we should be able to decipher the magnetic, thermal and dynamic
structure of the upper chromosphere and transition region of the Sun.Comment: Accepted for publication in The Astrophysical Journal Letter
A conjugate gradient method for the solution of the non-LTE line radiation transfer problem
This study concerns the fast and accurate solution of the line radiation
transfer problem, under non-LTE conditions. We propose and evaluate an
alternative iterative scheme to the classical ALI-Jacobi method, and to the
more recently proposed Gauss-Seidel and Successive Over-Relaxation (GS/SOR)
schemes. Our study is indeed based on the application of a preconditioned
bi-conjugate gradient method (BiCG-P). Standard tests, in 1D plane parallel
geometry and in the frame of the two-level atom model, with monochromatic
scattering, are discussed. Rates of convergence between the previously
mentioned iterative schemes are compared, as well as their respective timing
properties. The smoothing capability of the BiCG-P method is also demonstrated.Comment: Research note: 4 pages, 5 figures, accepted to A&
A 3D radiative transfer framework: I. non-local operator splitting and continuum scattering problems
We describe a highly flexible framework to solve 3D radiation transfer
problems in scattering dominated environments based on a long characteristics
piece-wise parabolic formal solution and an operator splitting method. We find
that the linear systems are efficiently solved with iterative solvers such as
Gauss-Seidel and Jordan techniques. We use a sphere-in-a-box test model to
compare the 3D results to 1D solutions in order to assess the accuracy of the
method. We have implemented the method for static media, however, it can be
used to solve problems in the Eulerian-frame for media with low velocity
fields.Comment: A&A, in press. 14 pages, 19 figures. Full resolution figures
available at ftp://phoenix.hs.uni-hamburg.de/preprints/3DRT_paper1.pdf HTML
version (low res figures) at
http://hobbes.hs.uni-hamburg.de/~yeti/PAPERS/3drt_paper1/index.htm
A 3D radiative transfer framework: II. line transfer problems
Higher resolution telescopes as well as 3D numerical simulations will require
the development of detailed 3D radiative transfer calculations. Building upon
our previous work we extend our method to include both continuum and line
transfer. We present a general method to calculate radiative transfer including
scattering in the continuum as well as in lines in 3D static atmospheres. The
scattering problem for line transfer is solved via means of an operator
splitting (OS) technique. The formal solution is based on a
long-characteristics method. The approximate operator is constructed
considering nearest neighbors {\em exactly}. The code is parallelized over both
wavelength and solid angle using the MPI library. We present the results of
several test cases with different values of the thermalization parameter and
two choices for the temperature structure. The results are directly compared to
1D spherical tests. With our current grid setup the interior resolution is much
lower in 3D than in 1D, nevertheless the 3D results agree very well with the
well-tested 1D calculations. We show that with relatively simple
parallelization that the code scales to very large number of processors which
is mandatory for practical applications. Advances in modern computers will make
realistic 3D radiative transfer calculations possible in the near future. Our
current code scales to very large numbers of processors, but requires larger
memory per processor at high spatial resolution.Comment: A&A, in press, 9 pages, 9 Figures. Full resolution version available
at ftp://phoenix.hs.uni-hamburg.de/preprints/3DRT_paper2.pd
Center-to-Limb Variation of Solar Line Profiles as a Test of NLTE Line Formation Calculations
We present new observations of the center-to-limb variation of spectral lines
in the quiet Sun. Our long-slit spectra are corrected for scattered light,
which amounts to 4-8 % of the continuum intensity, by comparison with a Fourier
transform spectrum of the disk center. We examine the effect of inelastic
collisions with neutral hydrogen in NLTE line formation calculations of the
oxygen infrared triplet, and the Na I 6160.8 A line. Adopting a classical
one-dimensional theoretical model atmosphere, we find that the sodium
transition, formed in higher layers, is much more effectively thermalized by
hydrogen collisions than the high-excitation oxygen lines. This result appears
as a simple consequence of the decrease of the ratio NH/Ne with depth in the
solar photosphere. The center-to-limb variation of the selected lines is
studied both under LTE and NLTE conditions. In the NLTE analysis, inelastic
collisions with hydrogen atoms are considered with a simple approximation or
neglected, in an attempt to test the validity of such approximation. For the
sodium line studied, the best agreement between theory and observation happens
when NLTE is considered and inelastic collisions with hydrogen are neglected in
the rate equations. The analysis of the oxygen triplet benefits from a very
detailed calculation using an LTE three-dimensional model atmosphere and NLTE
line formation. The chi**2 statistics favors including hydrogen collisions with
the approximation adopted, but the oxygen abundance derived in that case is
significantly higher than the value derived from OH infrared transitions.Comment: 10 pages, 8 figures, to appear in A&
Hydrogen H line polarization in solar flares. Theoretical investigation of atomic polarization by proton beams considering self-consistent NLTE polarized radiative transfer
Context. We present a theoretical review of the effect of impact polarization
of a hydrogen H line due to an expected proton beam bombardment in
solar flares. Aims. Several observations indicate the presence of the linear
polarization of the hydrogen H line observed near the solar limb above
5% and preferentially in the radial direction. We theoretically review the
problem of deceleration of the beam originating in the coronal reconnection
site due to its interaction with the chromospheric plasma, and describe the
formalism of the density matrix used in our description of the atomic processes
and the treatment of collisional rates. Methods. We solve the self-consistent
NLTE radiation transfer problem for the particular semiempirical chromosphere
models for both intensity and linear polarization components of the radiation
field. Results. In contrast to recent calculations, our results show that the
energy distribution of the proton beam at H formation levels and
depolarizing collisions by background electrons and protons cause a significant
reduction of the effect below 0.1%. The radiation transfer solution shows that
tangential resonance-scattering polarization dominates over the impact
polarization effect in all considered models. Conclusions. In the models
studied, proton beams are unlikely to be a satisfying explanation for the
observed linear polarization of the H line.Comment: 11 pages, 11 figures, accepted for publication in A&
Photometric properties of resolved and unresolved magnetic elements
We investigate the photometric signature of magnetic flux tubes in the solar
photosphere. We developed two dimensional, static numerical models of isolated
and clustered magnetic flux tubes. We investigated the emergent intensity
profiles at different lines-of-sight for various spatial resolutions and
opacity models. We found that both geometric and photometric properties of
bright magnetic features are determined not only by the physical properties of
the tube and its surroundings, but also by the particularities of the
observations, including the line/continuum formation height, the spatial
resolution and the image analyses techniques applied. We show that some
observational results presented in the literature can be interpreted by
considering bright magnetic features to be clusters of smaller elements, rather
than a monolithic flux tube.Comment: 12 page
Scattering Polarization and Hanle Effect in Stellar Atmospheres with Horizontal Inhomogeneities
Scattering of light from an anisotropic source produces linear polarization
in spectral lines and the continuum. In the outer layers of a stellar
atmosphere the anisotropy of the radiation field is typically dominated by the
radiation escaping away, but local horizontal fluctuations of the physical
conditions may also contribute, distorting the illumination and hence, the
polarization pattern. Additionally, a magnetic field may perturb and modify the
line scattering polarization signals through the Hanle effect. Here, we study
such symmetry-breaking effects. We develop a method to solve the transfer of
polarized radiation in a scattering atmosphere with weak horizontal
fluctuations of the opacity and source functions. It comprises linearization
(small opacity fluctuations are assumed), reduction to a quasi-planeparallel
problem through harmonic analysis, and numerical solution by generalized
standard techniques. We apply this method to study scattering polarization in
atmospheres with horizontal fluctuations in the Planck function and opacity. We
derive several very general results and constraints from considerations on the
symmetries and dimensionality of the problem, and we give explicit solutions of
a few illustrative problems of especial interest. For example, we show (a) how
the amplitudes of the fractional linear polarization signals change when
considering increasingly smaller horizontal atmospheric inhomogeneities, (b)
that in the presence of such inhomogeneities even a vertical magnetic field may
modify the scattering line polarization, and (c) that forward scattering
polarization may be produced without the need of an inclined magnetic field.
These results are important to understand the physics of the problem and as
benchmarks for multidimensional radiative transfer codes.Comment: 27 pages, 13 figures, to appear in Ap
2D non-LTE radiative modelling of He I spectral lines formed in solar prominences
The diagnosis of new high-resolution spectropolarimetric observations of
solar prominences made in the visible and near-infrared mainly, requires a
radiative modelling taking into account for both multi-dimensional geometry and
complex atomic models. Hereafter we contribute to the improvement of the
diagnosis based on the observation of He I multiplets, by considering 2D
non-LTE unpolarized radiation transfer, and taking also into account the atomic
fine structure of helium. It is an improvement and a direct application of the
multi-grid Gauss-Seidel/SOR iterative scheme in 2D cartesian geometry developed
by us. It allows us to compute realistic emergent intensity profiles for the He
I 10830 A and D3 multiplets, which can be directly compared to the simultaneous
and high-resolution observations made at THeMIS. A preliminary 2D multi-thread
modelling is also discussed.Comment: 6 pages, 9 figures, A&