130 research outputs found
Multi-line Stokes inversion for prominence magnetic-field diagnostics
We present test results on the simultaneous inversion of the Stokes profiles
of the He I lines at 587.6 nm (D_3) and 1083.0 nm in prominences (90-deg
scattering). We created datasets of synthetic Stokes profiles for the case of
quiescent prominences (B<200 G), assuming a conservative value of 10^-3 of the
peak intensity for the polarimetric sensitivity of the simulated observations.
In this work, we focus on the error analysis for the inference of the magnetic
field vector, under the usual assumption that the prominence can be assimilated
to a slab of finite optical thickness with uniform magnetic and thermodynamic
properties. We find that the simultaneous inversion of the two lines
significantly reduces the errors on the inference of the magnetic field vector,
with respect to the case of single-line inversion. These results provide a
solid justification for current and future instrumental efforts with multi-line
capabilities for the observations of solar prominences and filaments.Comment: 14 pages, 5 figures, 1 tabl
Near-IR internetwork spectro-polarimetry at different heliocentric angles
The analysis of near infrared spectropolarimetric data at the internetwork at
different regions on the solar surface could offer constraints to reject
current modeling of these quiet areas.
We present spectro-polarimetric observations of very quiet regions for
different values of the heliocentric angle for the Fe I lines at 1.56 micron,
from disc centre to positions close to the limb. The spatial resolution of the
data is 0.7-1". We analyze direct observable properties of the Stokes profiles
as the amplitude of circular and linear polarization as well as the total
degree of polarization. Also the area and amplitude asymmetries are studied.
We do not find any significant variation of the properties of the
polarimetric signals with the heliocentric angle. This means that the magnetism
of the solar internetwork remains the same regardless of the position on the
solar disc. This observational fact discards the possibility of modeling the
internetwork as a Network-like scenario. The magnetic elements of internetwork
areas seem to be isotropically distributed when observed at our spatial
resolution.Comment: Sorry, this is the version with the correct bibliography. Some
figures had to be compressed. Accepted for publication in A&
Tangled Magnetic Fields in Solar Prominences
Solar prominences are an important tool for studying the structure and
evolution of the coronal magnetic field. Here we consider so-called "hedgerow"
prominences, which consist of thin vertical threads. We explore the possibility
that such prominences are supported by tangled magnetic fields. A variety of
different approaches are used. First, the dynamics of plasma within a tangled
field is considered. We find that the contorted shape of the flux tubes
significantly reduces the flow velocity compared to the supersonic free fall
that would occur in a straight vertical tube. Second, linear force-free models
of tangled fields are developed, and the elastic response of such fields to
gravitational forces is considered. We demonstrate that the prominence plasma
can be supported by the magnetic pressure of a tangled field that pervades not
only the observed dense threads but also their local surroundings. Tangled
fields with field strengths of about 10 G are able to support prominence
threads with observed hydrogen density of the order of 10^(11) cm^(-3).
Finally, we suggest that the observed vertical threads are the result of
Rayleigh-Taylor instability. Simulations of the density distribution within a
prominence thread indicate that the peak density is much larger than the
average density. We conclude that tangled fields provide a viable mechanism for
magnetic support of hedgerow prominences.Comment: 14 pages (emulateapj style), 10 figures, ApJ, in pres
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Daytime sky polarization calibration limitations
The daytime sky has recently been demonstrated as a useful calibration tool for deriving polarization cross-talk properties of large astronomical telescopes. The Daniel K. Inouye Solar Telescope and other large telescopes under construction can benefit from precise polarimetric calibration of large mirrors. Several atmospheric phenomena and instrumental errors potentially limit the technique’s accuracy. At the 3.67-m AEOS telescope on Haleakala, we performed a large observing campaign with the HiVIS spectropolarimeter to identify limitations and develop algorithms for extracting consistent calibrations. Effective sampling of the telescope optical configurations and filtering of data for several derived parameters provide robustness to the derived Mueller matrix calibrations. Second-order scattering models of the sky show that this method is relatively insensitive to multiple-scattering in the sky, provided calibration observations are done in regions of high polarization degree. The technique is also insensitive to assumptions about telescope-induced polarization, provided the mirror coatings are highly reflective. Zemax-derived polarization models show agreement between the functional dependence of polarization predictions and the corresponding on-sky calibrations
Removal of Spectro-Polarimetric Fringes by 2D Pattern Recognition
We present a pattern-recognition based approach to the problem of removal of
polarized fringes from spectro-polarimetric data. We demonstrate that 2D
Principal Component Analysis can be trained on a given spectro-polarimetric map
in order to identify and isolate fringe structures from the spectra. This
allows us in principle to reconstruct the data without the fringe component,
providing an effective and clean solution to the problem. The results presented
in this paper point in the direction of revising the way that science and
calibration data should be planned for a typical spectro-polarimetric observing
run.Comment: ApJ, in pres
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
Time-Dependent Behavior of Linear Polarization in Unresolved Photospheres, With Applications for The Hanle Effect
Aims: This paper extends previous studies in modeling time varying linear
polarization due to axisymmetric magnetic fields in rotating stars. We use the
Hanle effect to predict variations in net line polarization, and use geometric
arguments to generalize these results to linear polarization due to other
mechanisms. Methods: Building on the work of Lopez Ariste et al., we use simple
analytic models of rotating stars that are symmetric except for an axisymmetric
magnetic field to predict the polarization lightcurve due to the Hanle effect.
We highlight the effects for the variable line polarization as a function of
viewing inclination and field axis obliquity. Finally, we use geometric
arguments to generalize our results to linear polarization from the weak
transverse Zeeman effect. Results: We derive analytic expressions to
demonstrate that the variable polarization lightcurve for an oblique magnetic
rotator is symmetric. This holds for any axisymmetric field distribution and
arbitrary viewing inclination to the rotation axis. Conclusions: For the
situation under consideration, the amplitude of the polarization variation is
set by the Hanle effect, but the shape of the variation in polarization with
phase depends largely on geometrical projection effects. Our work generalizes
the applicability of results described in Lopez Ariste et al., inasmuch as the
assumptions of a spherical star and an axisymmetric field are true, and
provides a strategy for separating the effects of perspective from the Hanle
effect itself for interpreting polarimetric lightcurves.Comment: 6 pages; 4 figures. Includes an extra figure found only in this
preprint versio
On the Magnetic Field Strength of Active Region Filaments
We study the vector magnetic field of a filament observed over a compact
Active Region Neutral Line. Spectropolarimetric data acquired with TIP-II (VTT,
Tenerife, Spain) of the 10830 \AA spectral region provide full Stokes vectors
which were analyzed using three different methods: magnetograph analysis,
Milne-Eddington inversions and PCA-based atomic polarization inversions. The
inferred magnetic field strengths in the filament are of the order of 600 - 700
G by all these three methods. Longitudinal fields are found in the range of 100
- 200 G whereas the transverse components become dominant, with fields as large
as 500 - 600 G. We find strong transverse fields near the Neutral Line also at
photospheric levels. Our analysis indicates that strong (higher than 500 G, but
below kG) transverse magnetic fields are present in Active Region filaments.
This corresponds to the highest field strengths reliably measured in these
structures. The profiles of the Helium 10830 \AA lines observed in this Active
Region filament are dominated by the Zeeman effect.Comment: Accepted for publication in Astronomy and Astrophysics, 9 pages, 4
figure
Quiet Sun magnetic fields from simultaneous inversions of visible and infrared spectropolarimetric observations
We study the quiet Sun magnetic fields using spectropolarimetric observations
of the infrared and visible Fe I lines at 6301.5, 6302.5, 15648 and 15653 A.
Magnetic field strengths and filling factors are inferred by the simultaneous
fit of the observed Stokes profiles under the MISMA hypothesis. The
observations cover an intra-network region at the solar disk center. We analyze
2280 Stokes profiles whose polarization signals are above noise in the two
spectral ranges, which correspond to 40% of the field of view. Most of these
profiles can be reproduced only with a model atmosphere including 3 magnetic
components with very different field strengths, which indicates the
co-existence of kG and sub-kG fields in our 1.5" resolution elements. We
measure an unsigned magnetic flux density of 9.6 G considering the full field
of view. Half of the pixels present magnetic fields with mixed polarities in
the resolution element. The fraction of mixed polarities increases as the
polarization weakens. We compute the probability density function of finding
each magnetic field strength. It has a significant contribution of kG field
strengths, which concentrates most of the observed magnetic flux and energy.
This kG contribution has a preferred magnetic polarity, while the polarity of
the weak fields is balanced.Comment: 16 pages and 14 figure
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