58,559 research outputs found
The scattering polarization of the Ly-alpha lines of H I and He II taking into account PRD and J-state interference effects
Recent theoretical investigations have pointed out that the cores of the
Ly-alpha lines of H I and He II should show measurable scattering polarization
signals when observing the solar disk, and that the magnetic sensitivity,
through the Hanle effect, of such linear polarization signals is suitable for
exploring the magnetism of the solar transition region. Such investigations
were carried out in the limit of complete frequency redistribution (CRD) and
neglecting quantum interference between the two upper J-levels of each line.
Here we relax both approximations and show that the joint action of partial
frequency redistribution (PRD) and J-state interference produces much more
complex fractional linear polarization (Q/I) profiles, with large amplitudes in
their wings. Such wing polarization signals turn out to be very sensitive to
the temperature structure of the atmospheric model, so that they can be
exploited for constraining the thermal properties of the solar chromosphere.
Finally, we show that the approximation of CRD without J-state interference is
however suitable for estimating the amplitude of the linear polarization
signals in the core of the lines, where the Hanle effect operates.Comment: Accepted for publication in The Astrophysical Journal Letter
Scattering Polarization of Hydrogen Lines in Weakly Magnetized Stellar Atmospheres I. Formulation and Application to Isothermal Models
Although the spectral lines of hydrogen contain valuable information on the
physical properties of a variety of astrophysical plasmas, including the upper
solar chromosphere, relatively little is known about their scattering
polarization signals whose modification via the Hanle effect may be exploited
for magnetic field diagnostics. Here we report on a basic theoretical
investigation of the linear polarization produced by scattering processes and
the Hanle effect in Ly-a, Ly-b and H-a taking into account multilevel radiative
transfer effects in an isothermal stellar atmosphere model, the fine-structure
of the hydrogen levels, as well as the impact of collisions with electrons and
protons. The main aim of this first paper is to elucidate the physical
mechanisms that control the linear polarization in the three lines, as well as
its sensitivity to the perturbers density and to the strength and structure of
micro-structured and deterministic magnetic fields. To this end, we apply an
efficient radiative transfer code we have developed for performing numerical
simulations of the Hanle effect in multilevel systems with overlapping line
transitions. For low density plasmas such as that of the upper solar
chromosphere collisional depolarization is caused mainly by collisional
transitions between the fine-structure levels of n=3, so that it is virtually
insignificant for Ly-a but important for Ly-b and H-a. We show the impact of
the Hanle effect on the three lines taking into account the radiative transfer
coupling between the different hydrogen line transitions. For example, we
demonstrate that the linear polarization profile of the H-a line is sensitive
to the presence of magnetic field gradients in the line core formation region
and that in solar-like chromospheres selective absorption of polarization
components does not play any significant role on the emergent scattering
polarization.Comment: 24 pages, 16 figures, 2 tables, accepted for publication in Ap
On the magnetic field of off-limb spicules
Determining the magnetic field related to solar spicules is vital for
developing adequate models of these plasma jets, which are thought to play a
key role in the thermal, dynamic and magnetic structure of the Chromosphere.
Here we report on the magnetic properties of off-limb spicules in a very quiet
region of the solar atmosphere, as inferred from new spectropolarimetric
observations in the He I 10830 A triplet obtained with the Tenerife Infrared
Polarimeter. We have used a novel inversion code for Stokes profiles caused by
the joint action of atomic level polarization and the Hanle and Zeeman effects
(HAZEL) to interpret the observations. Magnetic fields as strong as ~40G were
detected in a very localized area of the slit, which could represent a possible
lower value of the field strength of organized network spicules.Comment: Accepted for publication in ApJ, 24 pages, 5 Figure
Scattering Polarization and Hanle Effect in Stellar Atmospheres with Horizontal Inhomogeneities
Scattering of light from an anisotropic source produces linear polarization
in spectral lines and the continuum. In the outer layers of a stellar
atmosphere the anisotropy of the radiation field is typically dominated by the
radiation escaping away, but local horizontal fluctuations of the physical
conditions may also contribute, distorting the illumination and hence, the
polarization pattern. Additionally, a magnetic field may perturb and modify the
line scattering polarization signals through the Hanle effect. Here, we study
such symmetry-breaking effects. We develop a method to solve the transfer of
polarized radiation in a scattering atmosphere with weak horizontal
fluctuations of the opacity and source functions. It comprises linearization
(small opacity fluctuations are assumed), reduction to a quasi-planeparallel
problem through harmonic analysis, and numerical solution by generalized
standard techniques. We apply this method to study scattering polarization in
atmospheres with horizontal fluctuations in the Planck function and opacity. We
derive several very general results and constraints from considerations on the
symmetries and dimensionality of the problem, and we give explicit solutions of
a few illustrative problems of especial interest. For example, we show (a) how
the amplitudes of the fractional linear polarization signals change when
considering increasingly smaller horizontal atmospheric inhomogeneities, (b)
that in the presence of such inhomogeneities even a vertical magnetic field may
modify the scattering line polarization, and (c) that forward scattering
polarization may be produced without the need of an inclined magnetic field.
These results are important to understand the physics of the problem and as
benchmarks for multidimensional radiative transfer codes.Comment: 27 pages, 13 figures, to appear in Ap
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