2,287 research outputs found
Are collisions with neutral hydrogen important for modelling the Second Solar Spectrum of Ti I and Ca II ?
The physical interpretation of scattering line polarization offers a novel
diagnostic window for exploring the thermal and magnetic structure of the quiet
regions of the solar atmosphere. Here we evaluate the impact of isotropic
collisions with neutral hydrogen atoms on the scattering polarization signals
of the 13 lines of multiplet 42 of Ti I and on those of the K line and of the
IR triplet of Ca II, with emphasis on the collisional transfer rates between
nearby J-levels. To this end, we calculate the linear polarization produced by
scattering processes considering realistic multilevel models and solving the
statistical equilibrium equations for the multipolar components of the atomic
density matrix. We confirm that the lower levels of the 13 lines of multiplet
42 of Ti I are completely depolarized by elastic collisions. We find that
upper-level collisional depolarization turns out to have an unnoticeable impact
on the emergent linear polarization amplitudes, except for the {\lambda 4536
line for which it is possible to notice a rather small depolarization caused by
the collisional transfer rates. Concerning the Ca II lines, we show that the
collisional rates play no role on the polarization of the upper level of the K
line, while they have a rather small depolarizing effect on the atomic
polarization of the metastable lower levels of the Ca II IR triplet.Comment: Accepted for publication in Astronomy and Astrophysic
Scattering Polarization of the Ca II IR Triplet for Probing the Quiet Solar Chromosphere
The chromosphere of the quiet Sun is an important stellar atmospheric region
whose thermal and magnetic structure we need to decipher for unlocking new
discoveries in solar and stellar physics. To this end, we must identify and
exploit observables sensitive to weak magnetic fields (B<100 G) and to the
presence of cool and hot gas in the bulk of the solar chromosphere. Here we
report on an investigation of the Hanle effect in two semi-empirical models of
the quiet solar atmosphere with different chromospheric thermal structures. Our
study reveals that scattering polarization in the Ca II IR triplet has thermal
and magnetic sensitivities potentially of great diagnostic value. The linear
polarization in the 8498 A line shows a strong sensitivity to inclined magnetic
fields with strengths between 0.001 and 10 G, while the emergent linear
polarization in the 8542 A and 8662 A lines is mainly sensitive to magnetic
fields with strengths between 0.001 and 0.1 G. The reason for this is that the
scattering polarization of the 8542 A and 8662 A lines, unlike the 8498 A line,
is controlled mainly by the Hanle effect in their (metastable) lower levels.
Therefore, in regions with magnetic strengths sensibly larger than 1 G, their
Stokes Q and U profiles are sensitive only to the orientation of the magnetic
field vector. We also find that for given magnetic field configurations the
sign of the Q/I and U/I profiles of the 8542 A and 8662 A lines is the same in
both atmospheric models, while the sign of the linear polarization profile of
the 8498 A line turns out to be very sensitive to the thermal structure of the
lower chromosphere. We suggest that spectropolarimetric observations providing
information on the relative scattering polarization amplitudes of the Ca II IR
triplet will be very useful to improve our empirical understanding of the
thermal and magnetic structure of the quiet chromosphere.Comment: 27 pages, 16 figures, to appear 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
Estimating the magnetic field strength from magnetograms
A properly calibrated longitudinal magnetograph is an instrument that
measures circular polarization and gives an estimation of the magnetic flux
density in each observed resolution element. This usually constitutes a lower
bound of the field strength in the resolution element, given that it can be
made arbitrarily large as long as it occupies a proportionally smaller area of
the resolution element and/or becomes more transversal to the observer and
still produce the same magnetic signal. Yet, we know that arbitrarily stronger
fields are less likely --hG fields are more probable than kG fields, with
fields above several kG virtually absent-- and we may even have partial
information about its angular distribution. Based on a set of sensible
considerations, we derive simple formulae based on a Bayesian analysis to give
an improved estimation of the magnetic field strength for magnetographs.Comment: 8 pages, 7 figures, accepted for publication in A&
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