64 research outputs found
3-D non-LTE radiative transfer effects in Fe I lines: III. Line formation in magneto-hydrodynamic atmospheres
Non-local thermodynamic equilibrium (NLTE) effects in diagnostically
important solar Fe I lines are important due to the strong sensitivity of Fe I
to ionizing UV radiation, which may lead to a considerable under-population of
the Fe I levels in the solar atmosphere and, therefore, to a sizeable weakening
of Fe I lines. Such NLTE effects may be intensified or weakened by horizontal
radiative transfer (RT) in a three-dimensionally (3-D) structured atmosphere.
We analyze the influence of horizontal RT on commonly used Fe I lines in a
snapshot of a 3-D radiation magneto-hydrodynamic (MHD) simulation of a plage
region. NLTE- and horizontal RT effects occur with considerable strength (up to
50% in line depth or equivalent width) in the analyzed snapshot. As they may
have either sign and both signs occur with approximately the same frequency and
strength, the net effects are small when considering spatially averaged
quantities. The situation in the plage atmosphere turns out to be rather
complex. Horizontal transfer leads to line-weakening relative to 1-D NLTE
transfer near the boundaries of kG magnetic elements. Around the centers of
these elements, however, we find an often significant line-strengthening. This
behavior is in contrast to that expected from previous 3-D RT computations in
idealized flux-tube models, which display only a line weakening. The origin of
this unexpected behavior lies in the fact that magnetic elements are surrounded
by dense and relatively cool down-flowing gas, which forms the walls of the
magnetic elements. The continuum in these dense walls is often formed in colder
gas than in the central part of the magnetic elements. Consequently, the
central parts of the magnetic element experience a sub-average UV-irradiation
leading to the observed 3-D NLTE line strengthening.Comment: 13 pages, 11 figures, accepted for publication in A&
On the physical origin of the second solar spectrum of the Sc II line at 4247 A
The peculiar three-peak structure of the linear polarization profile shown in
the second solar spectrum by the Ba II line at 4554 A has been interpreted as
the result of the different contributions coming from the barium isotopes with
and without hyperfine structure (HFS). In the same spectrum, a triple peak
polarization signal is also observed in the Sc II line at 4247 A. Scandium has
a single stable isotope (^{45}Sc), which shows HFS due to a nuclear spin I=7/2.
We investigate the possibility of interpreting the linear polarization profile
shown in the second solar spectrum by this Sc II line in terms of HFS. A
two-level model atom with HFS is assumed. Adopting an optically thin slab
model, the role of atomic polarization and of HFS is investigated, avoiding the
complications caused by radiative transfer effects. The slab is assumed to be
illuminated from below by the photospheric continuum, and the polarization of
the radiation scattered at 90 degrees is investigated. The three-peak structure
of the scattering polarization profile observed in this Sc II line cannot be
fully explained in terms of HFS. Given the similarities between the Sc II line
at 4247 A and the Ba II line at 4554 A, it is not clear why, within the same
modeling assumptions, only the three-peak Q/I profile of the barium line can be
fully interpreted in terms of HFS. The failure to interpret this Sc II
polarization signal raises important questions, whose resolution might lead to
significant improvements in our understanding of the second solar spectrum. In
particular, if the three-peak structure of the Sc II signal is actually
produced by a physical mechanism neglected within the approach considered here,
it will be extremely interesting not only to identify this mechanism, but also
to understand why it seems to be less important in the case of the barium line.Comment: 8 pages, 8 figures, and 1 table. Accepted for publication in
Astronomy and Astrophysic
NLTE modeling of Stokes vector center-to-limb variations in the CN violet system
The solar surface magnetic field is connected with and even controls most of
the solar activity phenomena. Zeeman effect diagnostics allow for measuring
only a small fraction of the fractal-like structured magnetic field. The
remaining hidden magnetic fields can only be accessed with the Hanle effect.
Molecular lines are very convenient for applying the Hanle effect diagnostics
thanks to the broad range of magnetic sensitivities in a narrow spectral
region. With the UV version of the Zurich Imaging Polarimeter ZIMPOL II
installed at the 45 cm telescope of the Istituto Ricerche Solari Locarno
(IRSOL), we simultaneously observed intensity and linear polarization
center-to-limb variations in two spectral regions containing the (0,0) and
(1,1) bandheads of the CN B 2 {\Sigma} - X 2 {\Sigma} system. Here we present
an analysis of these observations. We have implemented coherent scattering in
molecular lines into a NLTE radiative transfer code. A two-step approach was
used. First, we separately solved the statistical equilibrium equations and
compute opacities and intensity while neglecting polariza- tion. Then we used
these quantities as input for calculating scattering polarization and the Hanle
effect. We have found that it is impossible to fit the intensity and
polarization simultaneously at different limb angles in the frame- work of
standard 1D modeling. The atmosphere models that provide correct intensity
center-to-limb variations fail to fit linear polar- ization center-to-limb
variations due to lacking radiation field anisotropy. We had to increase the
anisotropy by means of a specially introduced free parameter. This allows us to
successfully interpret our observations. We discuss possible reasons for
underestimating the anisotropy in the 1D modeling.Comment: 15 pages, 10 figures, accepted for publication in
Astronomy&Astrophysic
Influence of NLTE effects in Fe I lines on inverted atmosphere II. 6301 A and 6302 A lines formed in 3DNLTE
This paper forms the second part of our study on how the neglect of NLTE
conditions in the formation of Fe I 6301.5 A and the 6302.5 A lines influences
the atmosphere obtained by inverting their profiles in LTE. The main cause of
NLTE effects is the line opacity deficit due to the excess ionization of the Fe
I atoms by the UV photons in the Sun. In the first paper, the above
photospheric lines were assumed to have formed in 1DNLTE and the effects of
horizontal radiation transfer (RT) were neglected. In the present paper, the
iron lines are computed in 3DNLTE. We investigate the influence of horizontal
RT on the inverted atmosphere and how it can enhance or reduce the errors due
to the neglect of 1DNLTE effects. The iron lines are computed in LTE, 1DNLTE
and 3DNLTE. They all are inverted using an LTE inversion code. The atmosphere
from the inversion of LTE profiles is taken as the reference model. The test
atmospheres from the inversion of 1DNLTE and 3DNLTE profiles are compared with
it. The differences between models are analysed and correspondingly attributed
to NLTE and 3D effects. The effects of horizontal RT are evident in regions
surrounded by strong horizontal gradients in temperature. In some regions, the
3D effects enhance the 1DNLTE effects while in some, they weaken. The errors
due to neglecting the 3D effects are less than 5% in temperature while the
errors are mostly less than 20% in both velocity and magnetic field strength.
These errors are found to survive spatial and spectral degradation. The neglect
of horizontal RT is found to introduce errors in the derived atmosphere. How
large the errors are depends on how strong the local horizontal gradients are
in temperature. Compared to the 1DNLTE effect, the 3D effects are more
localised to specific regions in the atmosphere and overall less dominant.Comment: Accepted for publication in A&A. Abstract abridged for arxiv
submissio
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
Are collisions with neutral hydrogen important for modelling the Second Solar Spectrum of Ti I and Ca II ?
The physical interpretation of scattering line polarization offers a novel
diagnostic window for exploring the thermal and magnetic structure of the quiet
regions of the solar atmosphere. Here we evaluate the impact of isotropic
collisions with neutral hydrogen atoms on the scattering polarization signals
of the 13 lines of multiplet 42 of Ti I and on those of the K line and of the
IR triplet of Ca II, with emphasis on the collisional transfer rates between
nearby J-levels. To this end, we calculate the linear polarization produced by
scattering processes considering realistic multilevel models and solving the
statistical equilibrium equations for the multipolar components of the atomic
density matrix. We confirm that the lower levels of the 13 lines of multiplet
42 of Ti I are completely depolarized by elastic collisions. We find that
upper-level collisional depolarization turns out to have an unnoticeable impact
on the emergent linear polarization amplitudes, except for the {\lambda 4536
line for which it is possible to notice a rather small depolarization caused by
the collisional transfer rates. Concerning the Ca II lines, we show that the
collisional rates play no role on the polarization of the upper level of the K
line, while they have a rather small depolarizing effect on the atomic
polarization of the metastable lower levels of the Ca II IR triplet.Comment: Accepted for publication in Astronomy and Astrophysic
Bright fibrils in Ca II K
Context: Except for the Ca II resonance lines, fibrils are ubiquitously
present in most high-resolution observations of chromospheric lines. Aims: We
show that fibrils are also a prevailing feature in Ca II K, provided the
spatial-resolution is sufficiently high. Methods: We present high spatial
resolution observations of an active region in the Ca I} K line from the
Swedish Solar Telescope. Through a comparison between photospheric intensity
and magnetic field data, we study the connection between bright chromospheric
fibrils and photospheric structures. Additionally, using Fourier analysis we
study how the fibrils are linked to the observed dynamics. Results: We find
that very narrow, bright fibrils are a prevailing feature over large portions
of the observed field. We also find a clear connection between the fibril
footpoints and photospheric magnetic features. We show that the fibrils play
two distinct roles in the observed dynamics: depending on their location they
can act as a canopy suppressing oscillations or they can channel low-frequency
oscillations into the chromosphere. Conclusions: The Ca II K fibrils share many
characteristics with fibrils observed in other chromospheric lines, but some
features, such as the very small widths, are unique to these observations.Comment: To be published in A&A. High resolution version can be downloaded
from: http://www.mps.mpg.de/homes/pietarila/fibrils.pd
Three-dimensional non-LTE radiative transfer effects in Fe I lines I. Flux sheet and flux tube geometries
In network and active region plages, the magnetic field is concentrated into
structures often described as flux tubes (FTs) and sheets (FSs). 3-D radiative
transfer (RT) is important for energy transport in these concentrations. It is
also expected to be important for diagnostic purposes but has rarely been
applied for that purpose. Using true 3-D, non-LTE (NLTE) RT in FT/FS models, we
compute Fe line profiles commonly used to diagnose the Sun's magnetic field by
comparing the results with those obtained from LTE/1-D (1.5-D) NLTE
calculations. Employing a multilevel iron atom, we study the influence of basic
parameters such as Wilson depression, wall thickness, radius/width, thermal
stratification or magnetic field strength on all Stokes parameters in the
thin-tube approximation. The use of different levels of approximations of RT
may lead to considerable differences in profile shapes, intensity contrasts,
equivalent widths, and the determination of magnetic field strengths. In
particular, LTE, which often provides a good approach in planar 1-D
atmospheres, is a poor approximation in our flux sheet model for some of the
most important diagnostic Fe I lines (524.7nm, 525.0nm, 630.1nm, and 630.2nm).
The observed effects depend on parameters such as the height of line formation,
field strength, and internal temperature stratification. Differences between
the profile shapes may lead to errors in the determination of magnetic fields
on the order of 10 to 20%, while errors in the determined temperature can reach
300-400K. The empirical FT models NET and PLA turn out to minimize the effects
of 3D RT, so that results obtained with these models by applying LTE may also
remain valid for 3-D NLTE calculations. Finally, horizontal RT is found to only
insignificantly smear out structures such as the optically thick walls of FTs
and FSs, allowing features as narrow as 10km to remain visible.Comment: 20 pages, 21 figures, accepted for publication to "Astronomy and
Astrophysics
Recent Advances in Chromospheric and Coronal Polarization Diagnostics
I review some recent advances in methods to diagnose polarized radiation with
which we may hope to explore the magnetism of the solar chromosphere and
corona. These methods are based on the remarkable signatures that the
radiatively induced quantum coherences produce in the emergent spectral line
polarization and on the joint action of the Hanle and Zeeman effects. Some
applications to spicules, prominences, active region filaments, emerging flux
regions and the quiet chromosphere are discussed.Comment: Review paper to appear in "Magnetic Coupling between the Interior and
the Atmosphere of the Sun", eds. S. S. Hasan and R. J. Rutten, Astrophysics
and Space Science Proceedings, Springer-Verlag, 200
The energy of waves in the photosphere and lower chromosphere: III. Inversion setup for Ca II H spectra in local thermal equilibrium
The Ca II H line is one of the strongest lines in the solar spectrum and
provides continuous information on the solar atmosphere from the photosphere to
the lower chromosphere. We describe an inversion approach that reproduces
observed Ca II H spectra assuming LTE. We developed an inversion strategy based
on the SIR code. The approach uses a two-step procedure with an archive of
pre-calculated spectra to fit the line core and a subsequent iterative
modification to improve the fit in the line wing. Simultaneous spectra in the
630nm range can optionally be used to fix the continuum temperature. The method
retrieves 1D temperature stratifications neglecting lateral radiative
transport. LOS velocities are included by an empirical approach. An archive of
about 300.000 pre-calculated spectra is more than sufficient to reproduce the
line core of observed Ca II H spectra both in quiet Sun and in active regions.
The final thermodynamical stratifications match observed and best-fit spectra
to a level of about 0.5 (1) % of Ic in the line wing (core). Inversion schemes
based on pre-calculated spectra allow one a reliable and relatively fast
retrieval of solar properties from observed chromospheric spectra. The approach
can be easily extended to an 1D NLTE case by a simple exchange of the
pre-calculated archive spectra.Comment: 15 pages, 15 figures, accepted for publication in A&A. The animation
will only be provided in the A&A online sectio
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