3,202 research outputs found
3-D non-LTE radiative transfer effects in Fe I lines: III. Line formation in magneto-hydrodynamic atmospheres
Non-local thermodynamic equilibrium (NLTE) effects in diagnostically
important solar Fe I lines are important due to the strong sensitivity of Fe I
to ionizing UV radiation, which may lead to a considerable under-population of
the Fe I levels in the solar atmosphere and, therefore, to a sizeable weakening
of Fe I lines. Such NLTE effects may be intensified or weakened by horizontal
radiative transfer (RT) in a three-dimensionally (3-D) structured atmosphere.
We analyze the influence of horizontal RT on commonly used Fe I lines in a
snapshot of a 3-D radiation magneto-hydrodynamic (MHD) simulation of a plage
region. NLTE- and horizontal RT effects occur with considerable strength (up to
50% in line depth or equivalent width) in the analyzed snapshot. As they may
have either sign and both signs occur with approximately the same frequency and
strength, the net effects are small when considering spatially averaged
quantities. The situation in the plage atmosphere turns out to be rather
complex. Horizontal transfer leads to line-weakening relative to 1-D NLTE
transfer near the boundaries of kG magnetic elements. Around the centers of
these elements, however, we find an often significant line-strengthening. This
behavior is in contrast to that expected from previous 3-D RT computations in
idealized flux-tube models, which display only a line weakening. The origin of
this unexpected behavior lies in the fact that magnetic elements are surrounded
by dense and relatively cool down-flowing gas, which forms the walls of the
magnetic elements. The continuum in these dense walls is often formed in colder
gas than in the central part of the magnetic elements. Consequently, the
central parts of the magnetic element experience a sub-average UV-irradiation
leading to the observed 3-D NLTE line strengthening.Comment: 13 pages, 11 figures, accepted for publication in A&
A rule of thumb for riffle shuffling
We study how many riffle shuffles are required to mix n cards if only certain
features of the deck are of interest, e.g. suits disregarded or only the colors
of interest. For these features, the number of shuffles drops from 3/2 log_2(n)
to log_2(n). We derive closed formulae and an asymptotic `rule of thumb'
formula which is remarkably accurate.Comment: 27 pages, 5 table
The Magnetic Field in the Solar Atmosphere
This publication provides an overview of magnetic fields in the solar
atmosphere with the focus lying on the corona. The solar magnetic field couples
the solar interior with the visible surface of the Sun and with its atmosphere.
It is also responsible for all solar activity in its numerous manifestations.
Thus, dynamic phenomena such as coronal mass ejections and flares are
magnetically driven. In addition, the field also plays a crucial role in
heating the solar chromosphere and corona as well as in accelerating the solar
wind. Our main emphasis is the magnetic field in the upper solar atmosphere so
that photospheric and chromospheric magnetic structures are mainly discussed
where relevant for higher solar layers. Also, the discussion of the solar
atmosphere and activity is limited to those topics of direct relevance to the
magnetic field. After giving a brief overview about the solar magnetic field in
general and its global structure, we discuss in more detail the magnetic field
in active regions, the quiet Sun and coronal holes.Comment: 109 pages, 30 Figures, to be published in A&AR
Prospects of inflation in delicate D-brane cosmology
We study D-brane inflation in a warped conifold background that includes
brane-position dependent corrections for the nonperturbative superpotential.
Instead of stabilizing the volume modulus chi at instantaneous minima of the
potential and studying the inflation dynamics with an effective single field
(radial distance between a brane and an anti-brane) phi, we investigate the
multi-field inflation scenario involving these two fields. The two-field
dynamics with the potential V(phi,chi) in this model is significantly different
from the effective single-field description in terms of the field phi when the
field chi is integrated out. The latter picture underestimates the total number
of e-foldings even by one order of magnitude. We show that a correct
single-field description is provided by a field psi obtained from a rotation in
the two-field space along the background trajectory. This model can give a
large number of e-foldings required to solve flatness and horizon problems at
the expense of fine-tunings of model parameters. We also estimate the spectra
of density perturbations and show that the slow-roll parameter eta_{psi
psi}=M_{pl}^2 V_{,psi psi}/V in terms of the rotated field psi determines the
spectral index of scalar metric perturbations. We find that it is generally
difficult to satisfy, simultaneously, both constraints of the spectral index
and the COBE normalization, while the tensor to scalar ratio is sufficiently
small to match with observations.Comment: 12 pages, 8 figures, version to appear in Physical Review
ALMA detection of dark chromospheric holes in the quiet Sun
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of a quiet-Sun region at a wavelength of 3 mm, obtained during the first solar
ALMA cycle on April 27, 2017, and compare them with available chromospheric
observations in the UV and visible as well as with photospheric magnetograms.
ALMA images clearly reveal the presence of distinct particularly dark/cool
areas in the millimeter maps having temperatures as low as 60% of the normal
quiet Sun at 3 mm, which are not seen in the other data. We speculate that ALMA
is sensing cool chromospheric gas, whose presence had earlier been inferred
from infrared CO spectra.Comment: 9 pages, 3 figures, accepted for publication in ApJ
The relationship between chromospheric emissions and magnetic field strength
Aims. We analyze observational data from 4 instruments to study the
correlations between chromospheric emission, spanning the heights from the
temperature minimum region to the middle chromosphere, and photospheric
magnetic field. Methods: The data consist of radio images at 3.5 mm from the
Berkeley-Illinois-Maryland Array (BIMA), UV images at 1600 A from TRACE, Ca II
K-line filtergrams from BBSO, and MDI/SOHO longitudinal photospheric
magnetograms. For the first time interferometric millimeter data with the
highest currently available resolution are included in such an analysis. We
determine various parameters of the intensity maps and correlate the
intensities with each other and with the magnetic field. Results: The
chromospheric diagnostics studied here show a pronounced similarity in their
brightness structures and map out the underlying photospheric magnetic field
relatively well. We find a power law to be a good representation of the
relationship between photospheric magnetic field and emission from
chromospheric diagnostics at all wavelengths. The dependence of chromospheric
brightness on magnetic field is found to be different for network and
internetwork regions.Comment: 13 pages, 14 figures, 3 table
The chromosphere above sunspots at millimeter wavelengths
Aims: The aim of this paper is to demonstrate that millimeter wave data can
be used to distinguish between various atmospheric models of sunspots, whose
temperature structure in the upper photosphere and chromosphere has been the
source of some controversy. Methods: We use observations of the temperature
contrast (relative to the quiet Sun) above a sunspot umbra at 3.5 mm obtained
with the Berkeley-Illinois-Maryland Array (BIMA), complemented by submm
observations from Lindsey & Kopp (1995) and 2 cm observations with the Very
Large Array. These are compared with the umbral contrast calculated from
various atmospheric models of sunspots. Results: Current mm and submm
observational data suggest that the brightness observed at these wavelengths is
low compared to the most widely used sunspot models. These data impose strong
constraints on the temperature and density stratifications of the sunspot
umbral atmosphere, in particular on the location and depth of the temperature
minimum and the location of the transition region. Conclusions: A successful
model that is in agreement with millimeter umbral brightness should have an
extended and deep temperature minimum (below 3000 K). Better spatial resolution
as well as better wavelength coverage are needed for a more complete
determination of the chromospheric temperature stratification above sunspot
umbrae.Comment: 9 pages, 11 figures.
http://www.aanda.org/articles/aa/abs/2014/01/aa21321-13/aa21321-13.htm
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