95 research outputs found
Polynomial Approximants for the Calculation of Polarization Profiles in the \ion{He}{1} 10830 \AA Multiplet
The \ion{He}{1} multiplet at 10830 \AA is formed in the incomplete
Paschen-Back regime for typical conditions found in solar and stellar
atmospheres. The positions and strengths of the various components that form
the Zeeman structure of this multiplet in the Paschen-Back regime are
approximated here by polynomials. The fitting errors are smaller than
m\AA in the component positions and in the relative
strengths. The approximant polynomials allow for a very fast implementation of
the incomplete Paschen-Back regime in numerical codes for the synthesis and
inversion of polarization profiles in this important multiplet.Comment: ApJ Supplements (in press
Signatures of Incomplete Paschen-Back Splitting in the Polarization Profiles of the He I 10830 multiplet
We investigate the formation of polarization profiles induced by a magnetic
field in the He I multiplet at 1083,0 nm . Our analysis considers the Zeeman
splitting in the incomplete Paschen-Back regime. The effects turn out to be
important and produce measurable signatures on the profiles, even for fields
significantly weaker than the level-crossing field (400 G). When compared
to profiles calculated with the usual linear Zeeman effect, the incomplete
Paschen-Back profiles exhibit the following conspicuous differences: a) a
non-Doppler blueshift of the Stokes V zero-crossing wavelength of the blue
component; b) area and peak asymmetries, even in the absence of velocity and
magnetic gradients; c) a 25% reduction in the amplitude of the red
component. These features do not vanish in the weak field limit. The spectral
signatures that we analyze in this paper may be found in previous observations
published in the literature.Comment: Accepted for publication in The Astrophysical Journa
The contrast of magnetic elements in synthetic CH- and CN-band images of solar magnetoconvection
We present a comparative study of the intensity contrast in synthetic CH-band
and violet CN-band filtergrams computed from a high-resolution simulation of
solar magnetoconvection. The underlying simulation has an average vertical
magnetic field of 250 G with kG fields concentrated in its intergranular lanes,
and is representative of a plage region. To simulate filtergrams typically
obtained in CH- and CN-band observations we computed spatially resolved spectra
in both bands and integrated these spectra over 1 nm FWHM filter functions
centred at 430.5 nm and 388.3 nm, respectively. We find that the average
contrast of magnetic bright points in the simulated filtergrams is lower in the
CN-band by a factor of 0.96. This result strongly contradicts earlier
semi-empirical modeling and recent observations, which both etimated that the
bright-point contrast in the CN-band is \emph{higher} by a factor of 1.4. We
argue that the near equality of the bright-point contrast in the two bands in
the present simulation is a natural consequence of the mechanism that causes
magnetic flux elements to be particularly bright in the CN and CH filtergrams,
namely the partial evacuation of these elements and the concomitant weakening
of molecular spectral lines in the filter passbands. We find that the RMS
intensity contrast in the whole field-of-view of the filtergrams is 20.5% in
the G band and 22.0% in the CN band and conclude that this slight difference in
contrast is caused by the shorter wavelength of the latter. Both the
bright-point and RMS intensity contrast in the CN band are sensitive to the
precise choice of the central wavelength of the filter.Comment: 24 pages, 9 figures, submitted to Ap
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
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
Magnetic Field Measurement with Ground State Alignment
Observational studies of magnetic fields are crucial. We introduce a process
"ground state alignment" as a new way to determine the magnetic field direction
in diffuse medium. The alignment is due to anisotropic radiation impinging on
the atom/ion. The consequence of the process is the polarization of spectral
lines resulting from scattering and absorption from aligned atomic/ionic
species with fine or hyperfine structure. The magnetic field induces precession
and realign the atom/ion and therefore the polarization of the emitted or
absorbed radiation reflects the direction of the magnetic field. The atoms get
aligned at their low levels and, as the life-time of the atoms/ions we deal
with is long, the alignment induced by anisotropic radiation is susceptible to
extremely weak magnetic fields (G). In fact,
the effects of atomic/ionic alignment were studied in the laboratory decades
ago, mostly in relation to the maser research. Recently, the atomic effect has
been already detected in observations from circumstellar medium and this is a
harbinger of future extensive magnetic field studies. A unique feature of the
atomic realignment is that they can reveal the 3D orientation of magnetic
field. In this article, we shall review the basic physical processes involved
in atomic realignment. We shall also discuss its applications to
interplanetary, circumstellar and interstellar magnetic fields. In addition,
our research reveals that the polarization of the radiation arising from the
transitions between fine and hyperfine states of the ground level can provide a
unique diagnostics of magnetic fields in the Epoch of Reionization.Comment: 30 pages, 12 figures, chapter in Lecture Notes in Physics "Magnetic
Fields in Diffuse Media". arXiv admin note: substantial text overlap with
arXiv:1203.557
Zero-Field Dichroism in the Solar Chromosphere
We explain the linear polarization of the Ca II infrared triplet observed
close to the edge of the solar disk. In particular, we demonstrate that the
physical origin of the enigmatic polarizations of the 866.2 nm and 854.2 nm
lines lies in the existence of atomic polarization in their metastable lower
levels, which produces differential absorption of polarization components
(dichroism). To this end, we have solved the problem of the generation and
transfer of polarized radiation by taking fully into account all the relevant
optical pumping mechanisms in multilevel atomic models. We argue that
`zero-field' dichroism may be of great diagnostic value in astrophysics.Comment: 10 pages, 3 figure
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
Measuring the Hidden Aspects of Solar Magnetism
2008 marks the 100th anniversary of the discovery of astrophysical magnetic
fields, when George Ellery Hale recorded the Zeeman splitting of spectral lines
in sunspots. With the introduction of Babcock's photoelectric magnetograph it
soon became clear that the Sun's magnetic field outside sunspots is extremely
structured. The field strengths that were measured were found to get larger
when the spatial resolution was improved. It was therefore necessary to come up
with methods to go beyond the spatial resolution limit and diagnose the
intrinsic magnetic-field properties without dependence on the quality of the
telescope used. The line-ratio technique that was developed in the early 1970s
revealed a picture where most flux that we see in magnetograms originates in
highly bundled, kG fields with a tiny volume filling factor. This led to
interpretations in terms of discrete, strong-field magnetic flux tubes embedded
in a rather field-free medium, and a whole industry of flux tube models at
increasing levels of sophistication. This magnetic-field paradigm has now been
shattered with the advent of high-precision imaging polarimeters that allow us
to apply the so-called "Second Solar Spectrum" to diagnose aspects of solar
magnetism that have been hidden to Zeeman diagnostics. It is found that the
bulk of the photospheric volume is seething with intermediately strong, tangled
fields. In the new paradigm the field behaves like a fractal with a high degree
of self-similarity, spanning about 8 orders of magnitude in scale size, down to
scales of order 10 m.Comment: 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, Heidelberg, Berlin, 200
Polarization of photospheric lines from turbulent dynamo simulations
We employ the magnetic and velocity fields from turbulent dynamo simulations
to synthesize the polarization of a typical photospheric line. The synthetic
Stokes profiles have properties in common with those observed in the quiet Sun.
The simulated magnetograms present a level of signal similar to that of the
Inter-Network regions. Asymmetric Stokes V profiles with two, three and more
lobes appear in a natural way. The intensity profiles are broadened by the
magnetic fields in fair agreement with observational limits. Furthermore, the
Hanle depolarization signals of the Sr I 4607 A line turn out to be within the
solar values. Differences between synthetic and observed polarized spectra can
also be found. There is a shortage of Stokes V asymmetries, that we attribute
to a deficit of structuring in the magnetic and velocity fields from the
simulations as compared to the Sun This deficit may reflect the fact that the
Reynolds numbers of the numerical data are still far from solar values. We
consider the possibility that intense and tangled magnetic fields, like those
in the simulations, exist in the Sun. This scenario has several important
consequences. For example, less than 10% of the existing unsigned magnetic flux
would be detected in present magnetograms. The existing flux would exceed by
far that carried by active regions during the maximum of the solar cycle.
Detecting these magnetic fields would involve improving the angular resolution,
the techniques to interpret the polarization signals, and to a less extent, the
polarimetric sensitivity.Comment: Accepted for publication in ApJ. 20 pag. 11 fig
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