525 research outputs found
Scattering line polarization in rotating, optically thick disks
To interpret observations of astrophysical disks it is essential to
understand the formation process of the emitted light. If the disk is optically
thick, scattering dominated and permeated by a Keplerian velocity field,
Non-Local Thermodynamic Equilibrium radiative transfer modeling must be done to
compute the emergent spectrum from a given disk model. We investigate Non-local
thermodynamic equilibrium polarized line formation in different simple disk
models and aim to demonstrate the importance of both radiative transfer effects
and scattering as well as the effects of velocity fields. We self-consistently
solve the coupled equations of radiative transfer and statistical equilibrium
for a two level atom model by means of Jacobi iteration. We compute scattering
polarization, that is Q/I and U/I line profiles. The degree of scattering
polarization is significantly influenced by the inclination of the disk with
respect to observer, but also by the optical thickness of the disk and the
presence of rotation. Stokes U shows double-lobed profiles with amplitude which
increases with the disk rotation. Our results suggest that the line profiles,
especially the polarized ones, emerging from gaseous disks differ significantly
from the profiles predicted by simple approximations. The profiles are diverse
in shape, but typically symmetric in Stokes Q and antisymmetric in Stokes U. A
clear indicator of disk rotation is the presence of Stokes U, which might prove
to be a useful diagnostic tool. We also demonstrate that, for moderate
rotational velocities, an approximate treatment can be used, where non-local
thermodynamic equilibrium radiative transfer is done in the velocity field-free
approximation and Doppler shift is applied in the process of spatial
integration over the whole emitting surface.Comment: 16 pages; 12 figures; Accepted with revision for A&A. This is the
version after first round of referee's suggestion
Polarized Line Formation in Multi-Dimensional Media.III. Hanle Effect with Partial Frequency Redistribution
In the previous two papers, namely, \citet{anuknn11} and \citet{anuetal11} we
solved the polarized radiative transfer (RT) equation in multi-dimensional
(multi-D) geometries, with partial frequency redistribution (PRD) as the
scattering mechanism. We assumed Rayleigh scattering as the only source of
linear polarization () in both these papers. In this paper we extend
these previous works to include the effect of weak oriented magnetic fields
(Hanle effect) on line scattering. We generalize the technique of Stokes vector
decomposition in terms of the irreducible spherical tensors ,
developed in \citet{anuknn11}, to the case of RT with Hanle effect. A fast
iterative method of solution (based on the Stabilized Preconditioned
Bi-Conjugate-Gradient technique), developed in \citet{anuetal11}, is now
generalized to the case of RT in magnetized three-dimensional media. We use the
efficient short-characteristics formal solution method for multi-D media,
generalized appropriately to the present context. The main results of this
paper are the following: (1) A comparison of emergent profiles
formed in one-dimensional (1D) media, with the corresponding emergent,
spatially averaged profiles formed in multi-D media, shows that in the
spatially resolved structures, the assumption of 1D may lead to large errors in
linear polarization, especially in the line wings. (2) The multi-D RT in
semi-infinite non-magnetic media causes a strong spatial variation of the
emergent profiles, which is more pronounced in the line wings. (3)
The presence of a weak magnetic field modifies the spatial variation of the
emergent profiles in the line core, by producing significant
changes in their magnitudes.Comment: 31 pages, 14 figures, Submitted to ApJ, Under revie
Inference of magnetic fields in inhomogeneous prominences
Most of the quantitative information about the magnetic field vector in solar
prominences comes from the analysis of the Hanle effect acting on lines formed
by scattering. As these lines can be of non-negligible optical thickness, it is
of interest to study the line formation process further. We investigate the
multidimensional effects on the interpretation of spectropolarimetric
observations, particularly on the inference of the magnetic field vector. We do
this by analyzing the differences between multidimensional models, which
involve fully self-consistent radiative transfer computations in the presence
of spatial inhomogeneities and velocity fields, and those which rely on simple
one-dimensional geometry. We study the formation of a prototype line in ad hoc
inhomogeneous, isothermal 2D prominence models. We solve the NLTE polarized
line formation problem in the presence of a large-scale oriented magnetic
field. The resulting polarized line profiles are then interpreted (i.e.
inverted) assuming a simple 1D slab model. We find that differences between
input and the inferred magnetic field vector are non-negligible. Namely, we
almost universally find that the inferred field is weaker and more horizontal
than the input field. Spatial inhomogeneities and radiative transfer have a
strong effect on scattering line polarization in the optically thick lines. In
real-life situations, ignoring these effects could lead to a serious
misinterpretation of spectropolarimetric observations of chromospheric objects
such as prominences.Comment: 11 pages, 9 figure
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
Hanle effect in the solar Ba II D2 line: a diagnostic tool for chromospheric weak magnetic fields
The physics of the solar chromosphere depends in a crucial way on its
magnetic structure. However there are presently very few direct magnetic field
diagnostics available for this region. Here we investigate the diagnostic
potential of the Hanle effect on the Ba II D2 line resonance polarization for
the determination of weak chromospheric turbulent magnetic fields......Comment: In press in astronomy and astrophysic
Hanle effect in the CN violet system with LTE modeling
Weak entangled magnetic fields with mixed polarity occupy the main part of
the quiet Sun. The Zeeman effect diagnostics fails to measure such fields
because of cancellation in circular polarization. However, the Hanle effect
diagnostics, accessible through the second solar spectrum, provides us with a
very sensitive tool for studying the distribution of weak magnetic fields on
the Sun. Molecular lines are very strong and even dominate in some regions of
the second solar spectrum. The CN system is
one of the richest and most promising systems for molecular diagnostics and
well suited for the application of the differential Hanle effect method. The
aim is to interpret observations of the CN
system using the Hanle effect and to obtain an estimation of the magnetic field
strength. We assume that the CN molecular layer is situated above the region
where the continuum radiation is formed and employ the single-scattering
approximation. Together with the Hanle effect theory this provides us with a
model that can diagnose turbulent magnetic fields. We have succeeded in fitting
modeled CN lines in several regions of the second solar spectrum to
observations and obtained a magnetic field strength in the range from 10--30 G
in the upper solar photosphere depending on the considered lines.Comment: Accepted for publication in Astronomy and Astrophysic
Steady-state signatures of radiation trapping by cold multilevel atoms
In this paper, we use steady-state measurements to obtain evidence of
radiation trapping in an optically thick a cloud of cold rubidium atoms. We
investigate the fluorescence properties of our sample, pumped on opened
transitions. The intensity of fluorescence exhibits a non trivial dependence on
the optical thickness of the media. A simplified model, based on rate equations
self-consistently coupled to a diffusive model of light transport, is used to
explain the experimental observations in terms of incoherent radiation trapping
on one spectral line. Measurements of atomic populations and fluorescence
spectrum qualitatively agree with this interpretation.Comment: 8 pages, 5 figure
A new view on exoplanet transits: Transit of Venus described using three-dimensional solar atmosphere Stagger-grid simulations
Stellar activity and, in particular, convection-related surface structures,
potentially cause fluctuations that can affect the transit light curves.
Surface convection simulations can help the interpretation of ToV. We used
realistic three-dimensional radiative hydrodynamical simulation of the Sun from
the Stagger-grid and synthetic images computed with the radiative transfer code
Optim3D to provide predictions for the transit of Venus in 2004 observed by the
satellite ACRIMSAT. We computed intensity maps from RHD simulation of the Sun
and produced synthetic stellar disk image. We computed the light curve and
compared it to the ACRIMSAT observations and also to the light curves obtained
with solar surface representations carried out using radial profiles with
different limb-darkening laws. We also applied the same spherical tile imaging
method to the observations of center-to-limb Sun granulation with HINODE. We
managed to explain ACRIMSAT observations of 2004 ToV and showed that the
granulation pattern causes fluctuations in the transit light curve. We
evaluated the contribution of the granulation to the ToV. We showed that the
granulation pattern can partially explain the observed discrepancies between
models and data. This confirms that the limb-darkening and the granulation
pattern simulated in 3D RHD Sun represent well what is imaged by HINODE. In the
end, we found that the Venus's aureole contribution during ToV is less intense
than the solar photosphere, and thus negligible. Being able to explain
consistently the data of 2004 ToV is a new step forward for 3D RHD simulations
that are becoming essential for the detection and characterization of
exoplanets. They show that the granulation have to be considered as an
intrinsic incertitude, due to the stellar variability, on precise measurements
of exoplanet transits of, most likely, planets with small diameters.Comment: Accepted for publication in Astronomy and Astrophysic
Determining the Magnetization of the Quiet Sun Photosphere from the Hanle Effect and Surface Dynamo Simulations
The bulk of the quiet solar photosphere is thought to be significantly
magnetized, due to the ubiquitous presence of a tangled magnetic field at
subresolution scales with an average strength ~ 100 G. This conclusion was
reached through detailed three-dimensional (3D) radiative transfer modeling of
the Hanle effect in the Sr I 4607 line, using the microturbulent field
approximation and assuming that the shape of the probability density function
of the magnetic field strength is exponential. Here we relax both
approximations by modeling the observed scattering polarization in terms of the
Hanle effect produced by the magnetic field of a 3D photospheric model
resulting from a (state-of-the-art) magneto-convection simulation with surface
dynamo action. We show that the scattering polarization amplitudes observed in
the Sr I 4607 line can be explained only after enhancing the magnetic strength
of the photospheric model by a sizable scaling factor, F=10, which implies
= 130 G in the upper photosphere. We argue also that in order to explain both
the Hanle depolarization of the Sr I 4607 line and the Zeeman signals observed
in Fe I lines we need to introduce a height-dependent scaling factor, such that
the ensuing = 160 G in the low photosphere and = 130 G in the upper
photosphere.Comment: To appear in ApJ Letter
Magnetic flux in the inter-network quiet Sun from comparison with numerical simulations
Khomenko et al. estimate the mean magnetic field strength of the quiet Sun to
be 20 G. The figure is smaller than several existing estimates, and it comes
from the comparison between observed Zeeman polarization signals and synthetic
signals from numerical simulations of magneto-convection. The numerical
simulations require an artificially large magnetic diffusivity, which smears
out magnetic structures smaller than the grid scale. Assuming a turbulent
cascade for the unresolved artificially smeared magnetic fields, we find that
their unsigned magnetic flux is at least as important as that explicitly shown
in the simulation. The unresolved fields do not produce Zeeman polarization but
contribute to the unsigned flux.Since they are not considered by Khomenko et
al., their mean magnetic field strength has to be regarded as a lower limit.
This kind of bias is not specific of a particular numerical simulation or a
spectral line. It is to be expected when observed quiet Sun Zeeman signals are
compared with synthetic signals from simulations.Comment: Accepted A&A. 4 pages, 0 figure
- âŠ