11,829 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
Equity in Latin America Since the 1990s
This paper deals with the social welfare consequences of the stagnation of Latin American growth per capita during the far-reaching economic and social changes that took place during the period 1980-2003. This period of transformation saw large-scale foreign actors gradually increase their economic and political power in Latin America, with negative consequences for domestic economies, especially in terms of increasing income inequality and rising poverty. The only major tendency mitigating these adverse trends was an increase in public expenditure in the social sector during the 1990s, which offset, but did not eliminate, the increased inequality associated with the economic transformation.Latin America, economic change, poverty, income distribution, social policy, health,education
Laboratory Frequency Redistribution Function for the Polarized -Type Three-Term Atom
We present the frequency redistribution function for the polarized three-term
atom of the -type in the collisionless regime, and we specialize it to
the case where both the initial and final terms of the three-state transition
are metastable (i.e., with infinitely sharp levels). This redistribution
function represents a generalization of the well-known function to
the case where the lower terms of the transition can be polarized and carry
atomic coherence, and it can be applied to the investigation of polarized line
formation in tenuous plasmas, where collisional rates may be low enough that
anisotropy induced atomic polarization survives even in the case of metastable
levels
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
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