178 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&
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
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
Three-dimensional magnetic structure of a sunspot: comparison of the photosphere and upper chromosphere
We investigate the magnetic field of a sunspot in the upper chromosphere and
compare it to the field's photospheric properties. We observed the main leading
sunspot of the active region NOAA 11124 on two days with the Tenrife Infrared
Polarimeter-2 (TIP-2) mounted at the German Vacuum Tower Telescope (VTT).
Through inversion of Stokes spectra of the He I triplet at 1083.0 nm, we
obtained the magnetic field vector of the upper chromosphere. For comparison
with the photosphere we applied height-depended inversions of the Si I 1082.71
nm and Ca I 1083.34 nm lines. We found that the umbral magnetic field strength
in the upper chromosphere is lower by a factor of 1.30-1.65 compared to the
photosphere. The magnetic field strength of the umbra decreases from the
photosphere towards the upper chromosphere by an average rate of 0.5-0.9 G
km. The difference in the magnetic field strength between both
atmospheric layers steadily decreases from the sunspot center to the outer
boundary of the sunspot, with the field (in particular its horizontal
component) being stronger in the chromopshere outside the spot, suggestive of a
magnetic canopy. The sunspot displays a twist that on average is similar in the
two layers. However, the differential twist between photosphere and
chromosphere increases rapidly towards the outer penumbral boundary. The
magnetic field vector is more horizontal with respect to the solar surface by
roughly 5-20 in the photosphere compared to the upper chromosphere.
Above a lightbridge, the chromospheric magnetic field is equally strong as that
in the umbra, whereas the lightbridge's field is weaker than its surroundings
in the photosphere by roughly 1 kG. This suggests a cusp-like magnetic field
structure above the lightbridge.Comment: 12 pages, 15 figures, accepted for publication in A&
The potential of Ca II K observations for solar activity and variability studies
Several observatories around the globe started regular full-disc imaging of
the solar atmosphere in the Ca II K line in the early decades of the 20th
century. These observations are continued today at a few sites with either old
spectroheliographs or modern telescopes equipped with narrow-band filters. The
Ca II K time series are unique in representing long-term variations of the
Sun's chromospheric magnetic field. However, meaningful results from their
analysis require accurate processing of the available data and robust merging
of the information stored in different archives. This paper provides an
overview of the historical and modern full-disc Ca II K observations, with
focus on their quality and the main results obtained from their analysis over
the last decade.Comment: 6 pages, 2 figure
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