110 research outputs found
Quiet Sun Magnetic Field Measurements Based on Lines with Hyperfine Structure
The Zeeman pattern of MnI lines is sensitive to hyperfine structure (HFS)
and, they respond to hG magnetic field strengths differently from the lines
used in solar magnetometry. This peculiarity has been employed to measure
magnetic field strengths in quiet Sun regions. However, the methods applied so
far assume the magnetic field to be constant in the resolution element. The
assumption is clearly insufficient to describe the complex quiet Sun magnetic
fields, biasing the results of the measurements. We present the first syntheses
of MnI lines in realistic quiet Sun model atmospheres. The syntheses show how
the MnI lines weaken with increasing field strength. In particular, kG magnetic
concentrations produce NnI 5538 circular polarization signals (Stokes V) which
can be up to two orders of magnitude smaller than the weak magnetic field
approximation prediction. Consequently, (1) the polarization emerging from an
atmosphere having weak and strong fields is biased towards the weak fields, and
(2) HFS features characteristic of weak fields show up even when the magnetic
flux and energy are dominated by kG fields. For the HFS feature of MnI 5538 to
disappear the filling factor of kG fields has to be larger than the filling
factor of sub-kG fields. Stokes V depends on magnetic field inclination
according to the simple consine law. Atmospheres with unresolved velocities
produce asymmetric line profiles, which cannot be reproduced by simple
one-component model atmospheres. The uncertainty of the HFS constants do not
limit the use of MnI lines for magnetometry.Comment: Accepted for publication in ApJ. 10 pages, 14 figure
Diagnostic of prominence magnetic fields through spectropolarimetric observations
In recent times there has been a strong revival of interest in spectropolarimetric observations of solar prominences. This is due to the fact that new instruments, with unprecedented spectral resolution and polarimetric accuracy, are now available to solar physicists. These instruments open for the first time the possibility of unveiling the fine structure of magnetic fields and its correlation with velocity fields and density inhomogeneities in these fascinating objects. The interpretation of spectropolarimetric observations of solar prominences still stands, however, as one of the most challenging problems of what is nowadays known as atomic astrophysics, or, in other words, modern atomic physics applied to the diagnostic of astronomical objects. In this brief review, a hystorical account of the measurements of magnetic fields in prominences is given, and the present status of the theory, which stands at the basis of the interpretation of the observations, is dicussed. Some perspectives for future investigations are also presented
Isotropic inelastic and superelastic collisional rates in a multiterm atom
The spectral line polarization of the radiation emerging from a magnetized
astrophysical plasma depends on the state of the atoms within the medium, whose
determination requires considering the interactions between the atoms and the
magnetic field, between the atoms and photons (radiative transitions), and
between the atoms and other material particles (collisional transitions). In
applications within the framework of the multiterm model atom (which accounts
for quantum interference between magnetic sublevels pertaining either to the
same J-level or to different J-levels within the same term) collisional
processes are generally neglected when solving the master equation for the
atomic density matrix. This is partly due to the lack of experimental data
and/or of approximate theoretical expressions for calculating the collisional
transfer and relaxation rates (in particular the rates for interference between
sublevels pertaining to different J-levels, and the depolarizing rates due to
elastic collisions). In this paper we formally define and investigate the
transfer and relaxation rates due to isotropic inelastic and superelastic
collisions that enter the statistical equilibrium equations of a multiterm
atom. Under the hypothesis that the atom-collider interaction can be described
by a dipolar operator, we provide expressions that relate the collisional rates
for interference between different J-levels to the usual collisional rates for
J-level populations. Finally, we apply the general equations to the case of a
two-term atom with unpolarized lower term, illustrating the impact of inelastic
and superelastic collisions on scattering polarization through radiative
transfer calculations in a slab of stellar atmospheric plasma anisotropically
illuminated by the photospheric radiation field.Comment: Accepted for publication in Astronomy & Astrophysic
Theoretical formulation of Doppler redistribution in scattering polarization within the framework of the velocity-space density matrix formalism
Within the framework of the density matrix theory for the generation and
transfer of polarized radiation, velocity density matrix correlations represent
an important physical aspect that, however, is often neglected in practical
applications by adopting the simplifying approximation of complete
redistribution on velocity. In this paper, we present an application of the
Non-LTE problem for polarized radiation taking such correlations into account
through the velocity-space density matrix formalism. We consider a two-level
atom with infinitely sharp upper and lower levels, and we derive the
corresponding statistical equilibrium equations neglecting the contribution of
velocity-changing collisions. Coupling such equations with the radiative
transfer equations for polarized radiation, we derive a set of coupled
equations for the velocity-dependent source function. This set of equations is
then particularized to the case of a plane-parallel atmosphere. The equations
presented in this paper provide a complete and solid description of the physics
of pure Doppler redistribution, a phenomenon generally described within the
framework of the redistribution matrix formalism. The redistribution matrix
corresponding to this problem (generally referred to as R_I) is derived
starting from the statistical equilibrium equations for the velocity-space
density matrix and from the radiative transfer equations for polarized
radiation, thus showing the equivalence of the two approaches.Comment: Accepted for publication in Astronomy & Astrophysic
The Hanle and Zeeman Effects in Solar Spicules: A Novel Diagnostic Window on Chromospheric Magnetism
An attractive diagnostic tool for investigating the magnetism of the solar
chromosphere is the observation and theoretical modeling of the Hanle and
Zeeman effects in spicules, as shown in this letter for the first time. Here we
report on spectropolarimetric observations of solar chromospheric spicules in
the He I 10830 \AA multiplet and on their theoretical modeling accounting for
radiative transfer effects. We find that the magnetic field in the observed
(quiet Sun) spicular material at a height of about 2000 km above the visible
solar surface has a strength of the order of 10 G and is inclined by
approximately with respect to the local vertical direction. Our
empirical finding based on full Stokes-vector spectropolarimetry should be
taken into account in future magnetohydrodynamical simulations of spicules.Comment: 12 pages and 2 figure
Modelling the incomplete Paschen-Back effect in the spectra of magnetic Ap stars
We present first results of a systematic investigation of the incomplete
Paschen-Back effect in magnetic Ap stars. A short overview of the theory is
followed by a demonstration of how level splittings and component strengths
change with magnetic field strength for some lines of special astrophysical
interest. Requirements are set out for a code which allows the calculation of
full Stokes spectra in the Paschen-Back regime and the behaviour of Stokes I
and V profiles of transitions in the multiplet 74 of FeII is discussed in some
detail. It is shown that the incomplete Paschen-Back effect can lead to
noticeable line shifts which strongly depend on total multiplet strength,
magnetic field strength and field direction. Ghost components (which violate
the normal selection rule on J) show up in strong magnetic fields but are
probably unobservable. Finally it is shown that measurements of the integrated
magnetic field modulus are not adversely affected by the Paschen-Back
effect, and that there is a potential problem in (magnetic) Doppler mapping if
lines in the Paschen-Back regime are treated in the Zeeman approximation.Comment: 8 pages, 10 figures, to appear in MNRA
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