1,051 research outputs found
Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects
A big challenge in solar and stellar physics in the coming years will be to
decipher the magnetism of the solar outer atmosphere (chromosphere and corona)
along with its dynamic coupling with the magnetic fields of the underlying
photosphere. To this end, it is important to develop rigorous diagnostic tools
for the physical interpretation of spectropolarimetric observations in suitably
chosen spectral lines. Here we present a computer program for the synthesis and
inversion of Stokes profiles caused by the joint action of atomic level
polarization and the Hanle and Zeeman effects in some spectral lines of
diagnostic interest, such as those of the He I 10830 A and D_3 multiplets. It
is based on the quantum theory of spectral line polarization, which takes into
account all the relevant physical mechanisms and ingredients (optical pumping,
atomic level polarization, Zeeman, Paschen-Back and Hanle effects). The
influence of radiative transfer on the emergent spectral line radiation is
taken into account through a suitable slab model. The user can either calculate
the emergent intensity and polarization for any given magnetic field vector or
infer the dynamical and magnetic properties from the observed Stokes profiles
via an efficient inversion algorithm based on global optimization methods. The
reliability of the forward modeling and inversion code presented here is
demonstrated through several applications, which range from the inference of
the magnetic field vector in solar active regions to determining whether or not
it is canopy-like in quiet chromospheric regions. This user-friendly diagnostic
tool called "HAZEL" (from HAnle and ZEeman Light) is offered to the
astrophysical community, with the hope that it will facilitate new advances in
solar and stellar physics.Comment: 62 pages, 19 figures, 3 tables. Accepted for publication in Ap
Scattering polarization in the CaII Infrared Triplet with Velocity Gradients
Magnetic field topology, thermal structure and plasma motions are the three
main factors affecting the polarization signals used to understand our star. In
this theoretical investigation, we focus on the effect that gradients in the
macroscopic vertical velocity field have on the non-magnetic scattering
polarization signals, establishing the basis for general cases. We demonstrate
that the solar plasma velocity gradients have a significant effect on the
linear polarization produced by scattering in chromospheric spectral lines. In
particular, we show the impact of velocity gradients on the anisotropy of the
radiation field and on the ensuing fractional alignment of the CaII levels, and
how they can lead to an enhancement of the zero-field linear polarization
signals. This investigation remarks the importance of knowing the dynamical
state of the solar atmosphere in order to correctly interpret
spectropolarimetric measurements, which is important, among other things, for
establishing a suitable zero field reference case to infer magnetic fields via
the Hanle effect.Comment: 14 pages, 10 figures, 3 appendixes, accepted for publication in Ap
Dichroic Masers due to Radiation Anisotropy and the Influence of the Hanle Effect on the Circumstellar SiO Polarization
The theory of the generation and transfer of polarized radiation, mainly
developed for interpreting solar spectropolarimetric observations, allows to
reconsider, in a more rigorous and elegant way, a physical mechanism that has
been suggested some years ago to interpret the high degree of polarization
often observed in astronomical masers. This mechanism, for which the name of
'dichroic maser' is proposed, can operate when a low density molecular cloud is
illuminated by an anisotropic source of radiation (like for instance a nearby
star). Here we investigate completely unsaturated masers and show that
selective stimulated emission processes are capable of producing highly
polarized maser radiation in a non-magnetic environment. The polarization of
the maser radiation is linear and is directed tangentially to a ring
equidistant to the central star. We show that the Hanle effect due to the
presence of a magnetic field can produce a rotation (from the tangential
direction) of the polarization by more that 45 degrees for some selected
combinations of the strength, inclination and azimuth of the magnetic field
vector. However, these very same conditions produce a drastic inhibition of the
maser effect. The rotations of about 90 degrees observed in SiO masers in the
evolved stars TX Cam by Kemball & Diamond (1997) and IRC+10011 by Desmurs et al
(2000) may then be explainedby a local modification of the anisotropy of the
radiation field, being transformed from mainly radial to mainly tangential.Comment: Accepted for publication on Ap
A Substantial Amount of Hidden Magnetic Energy in the Quiet Sun
Deciphering and understanding the small-scale magnetic activity of the quiet
solar photosphere should help to solve many of the key problems of solar and
stellar physics, such as the magnetic coupling to the outer atmosphere and the
coronal heating. At present, we can see only of the complex
magnetism of the quiet Sun, which highlights the need to develop a reliable way
to investigate the remaining 99%. Here we report three-dimensional radiative
tranfer modelling of scattering polarization in atomic and molecular lines that
indicates the presence of hidden, mixed-polarity fields on subresolution
scales. Combining this modelling with recent observational data we find a
ubiquitous tangled magnetic field with an average strength of G,
which is much stronger in the intergranular regions of solar surface convection
than in the granular regions. So the average magnetic energy density in the
quiet solar photosphere is at least two orders of magnitude greater than that
derived from simplistic one-dimensional investigations, and sufficient to
balance radiative energy losses from the solar chromosphere.Comment: 21 pages and 2 figures (letter published in Nature on July 15, 2004
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 Zeeman effect in the G band
We investigate the possibility of measuring magnetic field strength in G-band
bright points through the analysis of Zeeman polarization in molecular CH
lines. To this end we solve the equations of polarized radiative transfer in
the G band through a standard plane-parallel model of the solar atmosphere with
an imposed magnetic field, and through a more realistic snapshot from a
simulation of solar magneto-convection. This region of the spectrum is crowded
with many atomic and molecular lines. Nevertheless, we find several instances
of isolated groups of CH lines that are predicted to produce a measurable
Stokes V signal in the presence of magnetic fields. In part this is possible
because the effective Land\'{e} factors of lines in the stronger main branch of
the CH A--X transition tend to zero rather quickly for
increasing total angular momentum , resulting in a Stokes spectrum of
the G band that is less crowded than the corresponding Stokes spectrum. We
indicate that, by contrast, the effective Land\'{e} factors of the and
satellite sub-branches of this transition tend to for increasing .
However, these lines are in general considerably weaker, and do not contribute
significantly to the polarization signal. In one wavelength location near 430.4
nm the overlap of several magnetically sensitive and non-sensitive CH lines is
predicted to result in a single-lobed Stokes profile, raising the
possibility of high spatial-resolution narrow-band polarimetric imaging. In the
magneto-convection snapshot we find circular polarization signals of the order
of 1% prompting us to conclude that measuring magnetic field strength in
small-scale elements through the Zeeman effect in CH lines is a realistic
prospect.Comment: 22 pages, 6 figures. To be published in the Astrophysical Journa
G-band Spectral Synthesis in Solar Magnetic Concentrations
Narrow band imaging in the G-band is commonly used to trace the small
magnetic field concentrations of the Sun, although the mechanism that makes
them bright has remained unclear. We carry out LTE syntheses of the G-band in
an assorted set of semi-empirical model magnetic concentrations. The syntheses
include all CH lines as well as the main atomic lines within the band-pass. The
model atmospheres produce bright G-band spectra having many properties in
common with the observed G-band bright points. In particular, the contrast
referred to the quiet Sun is about twice the contrast in continuum wavelengths.
The agreement with observations does not depend on the specificities of the
model atmosphere, rather it holds from single fluxtubes to MIcro-Structured
Magnetic Atmospheres. However, the agreement requires that the real G-band
bright points are not spatially resolved, even in the best observations. Since
the predicted G-band intensities exceed by far the observed values, we foresee
a notable increase of contrast of the G-band images upon improvement of the
angular resolution. According to the LTE modeling, the G-band spectrum emerges
from the deep photosphere that produces the continuum. Our syntheses also
predict solar magnetic concentrations showing up in continuum images but not in
the G-band . Finally, we have examined the importance of the CH
photo-dissociation in setting the amount of G-band absorption. It turns out to
play a minor role.Comment: To appear in ApJ, 554 n2 Jun 20, 33 pages and 9 figure
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