209 research outputs found
Point spread functions for the Solar Optical Telescope onboard Hinode
The combined PSF of the BFI and the SOT onboard the Hinode spacecraft is
investigated. Observations of the Mercury transit from November 2006 and the
solar eclipse(s) from 2007 are used to determine the PSFs of SOT for the blue,
green, and red continuum channels of the BFI. For each channel large grids of
theoretical point spread functions are calculated by convolution of the ideal
diffraction-limited PSF and Voigt profiles. These PSFs are applied to
artificial images of an eclipse and a Mercury transit. The comparison of the
resulting artificial intensity profiles across the terminator and the
corresponding observed profiles yields a quality measure for each case. The
optimum PSF for each observed image is indicated by the best fit. The observed
images of the Mercury transit and the eclipses exhibit a clear proportional
relation between the residual intensity and the overall light level in the
telescope. In addition there is a anisotropic stray-light contribution. ...
BFI/SOT operate close to the diffraction limit and have only a rather small
stray-light contribution. The FWHM of the PSF is broadened by only ~1% with
respect to the diffraction-limited case, while the overall Strehl ratio is ~
0.8. In view of the large variations -- best seen in the residual intensities
of eclipse images -- and the dependence on the overall light level and position
in the FOV, a range of PSFs should be considered instead of a single PSF per
wavelength. The individual PSFs of that range allow then the determination of
error margins for the quantity under investigation. Nevertheless the
stray-light contributions are here found to be best matched with Voigt
functions with the parameters sigma = 0."008 and gamma = 0."004, 0."005, and
0."006 for the blue, green, and red continuum channels, respectively.Comment: 14 pages, 9 figures, accepted by A&
Small-scale structure and dynamics of the lower solar atmosphere
The chromosphere of the quiet Sun is a highly intermittent and dynamic
phenomenon. Three-dimensional radiation (magneto-)hydrodynamic simulations
exhibit a mesh-like pattern of hot shock fronts and cool expanding post-shock
regions in the sub-canopy part of the inter-network. This domain might be
called "fluctosphere". The pattern is produced by propagating shock waves,
which are excited at the top of the convection zone and in the photospheric
overshoot layer. New high-resolution observations reveal a ubiquitous
small-scale pattern of bright structures and dark regions in-between. Although
it qualitatively resembles the picture seen in models, more observations - e.g.
with the future ALMA - are needed for thorough comparisons with present and
future models. Quantitative comparisons demand for synthetic intensity maps and
spectra for the three-dimensional (magneto-)hydrodynamic simulations. The
necessary radiative transfer calculations, which have to take into account
deviations from local thermodynamic equilibrium, are computationally very
involved so that no reliable results have been produced so far. Until this task
becomes feasible, we have to rely on careful qualitative comparisons of
simulations and observations. Here we discuss what effects have to be
considered for such a comparison. Nevertheless we are now on the verge of
assembling a comprehensive picture of the solar chromosphere in inter-network
regions as dynamic interplay of shock waves and structuring and guiding
magnetic fields.Comment: 8 pages, 2 figures, to appear in the proceedings of the IAU Symposium
No. 247, Waves & Oscillations in the Solar Atmosphere: Heating and
Magneto-Seismology (Venezuela 2007
Dynamic Models of the Sun from the Convection Zone to the Chromosphere
The chromosphere in internetwork regions of the quiet Sun was regarded as a
static and homogeneous layer for a long time. Thanks to advances in
observations and numerical modelling, the wave nature of these atmospheric
regions received increasing attention during the last decade. Recent
three-dimensional radiation magnetohydrodynamic simulations with CO5BOLD
feature the chromosphere of internetwork regions as a dynamic and intermittent
phenomenon. It is a direct product of interacting waves that form a mesh-like
pattern of hot shock fronts and cool post-shock regions. The waves are excited
self-consistently at the top of the convection zone. In the middle chromosphere
above an average height of 1000 km, plasma beta gets larger than one and
magnetic fields become more important. The model chromosphere exhibits a
magnetic field that is much more homogeneous than in the layers below and
evolves much faster. That includes fast propagating (MHD) waves. Further
improvements of the simulations like time-dependent hydrogen ionisation are
currently in progress. This class of models is capable of explaining apparently
contradicting diagnostics such as carbon monoxide and UV emission at the same
time.Comment: 6 pages, 2 figures, to appear in proceedings of IAU symposium 239,
August 21 - 25, 2006, Pragu
Three-dimensional magnetohydrodynamic simulations of M-dwarf chromospheres
We present first results from three-dimensional radiation magnetohydrodynamic
simulations of M-type dwarf stars with CO5BOLD. The local models include the
top of the convection zone, the photosphere, and the chromosphere. The results
are illustrated for models with an effective temperature of 3240 K and a
gravitational acceleration of log g = 4.5, which represent analogues of AD Leo.
The models have different initial magnetic field strengths and field
topologies. This first generation of models demonstrates that the atmospheres
of M-dwarfs are highly dynamic and intermittent. Magnetic fields and
propagating shock waves produce a complicated fine-structure, which is clearly
visible in synthetic intensity maps in the core of the Ca II K spectral line
and also at millimeter wavelengths. The dynamic small-scale pattern cannot be
described by means of one-dimensional models, which has important implications
for the construction of semi-empirical model atmospheres and thus for the
interpretation of observations in general. Detailed three-dimensional numerical
simulations are valuable in this respect. Furthermore, such models facilitate
the analysis of small-scale processes, which cannot be observed on stars but
nevertheless might be essential for understanding M-dwarf atmospheres and their
activity. An example are so-called "magnetic tornadoes", which have recently
been found on the Sun and are presented here in M-dwarf models for the first
time.Comment: 4 pages, 4 figures; proceedings article, CoolStars 17 (June 2012);
accepted version, Astron. Nachr. 334, issue 1-2, 133-136 (2013); published
online 2013 Feb
Vortices, shocks, and heating in the solar photosphere: effect of a magnetic field
Aims: We study the differences between non-magnetic and magnetic regions in
the flow and thermal structure of the upper solar photosphere. Methods:
Radiative MHD simulations representing a quiet region and a plage region,
respectively, which extend into the layers around the temperature minimum, are
analyzed. Results: The flow structure in the upper photospheric layers of the
two simulations is considerably different: the non-magnetic simulation is
dominated by a pattern of moving shock fronts while the magnetic simulation
shows vertically extended vortices associated with magnetic flux
concentrations. Both kinds of structures induce substantial local heating. The
resulting average temperature profiles are characterized by a steep rise above
the temperature minimum due to shock heating in the non-magnetic case and by a
flat photospheric temperature gradient mainly caused by Ohmic dissipation in
the magnetic run. Conclusions: Shocks in the quiet Sun and vortices in the
strongly magnetized regions represent the dominant flow structures in the
layers around the temperature minimum. They are closely connected with
dissipation processes providing localized heating.Comment: Accepted for publicaton in A&
Non-equilibrium calcium ionisation in the solar atmosphere
Our aim is to determine the dominant processes and timescales for the
ionisation equilibrium of calcium under solar chromospheric conditions. The
study is based on numerical simulations with the RADYN code, which includes
hydrodynamics, radiative transfer, and a detailed non-equilibrium treatment of
hydrogen, calcium, and helium. The simulations are characterised by upwards
propagating shock waves, which cause strong temperature fluctuations and
variations of the ionisation degree of calcium. The passage of a hot shock
front leads to a strong net ionisation of Ca II, rapidly followed by net
recombination. The relaxation timescale of the Ca ionisation state is found to
be of the order of a few seconds at the top of the photosphere and 10 to 30 s
in the upper chromosphere. Generally, the timescales are significantly reduced
in the wakes of hot shock fronts. The timescales can be reliably determined
from a simple analysis of the eigenvalues of the transition rate matrix. The
timescales are dominated by the radiative recombination from Ca III into the
metastable Ca II energy levels of the 4d 2D term. These transitions depend
strongly on the density of free electrons and therefore on the
(non-equilibrium) ionisation degree of hydrogen, which is the main electron
donor. The ionisation/recombination timescales derived here are too long for
the assumption of an instantaneous ionisation equilibrium to be valid and, on
the other hand, are not long enough to warrant an assumption of a constant
ionisation fraction. Fortunately, the ionisation degree of Ca II remains small
in the height range, where the cores of the H, K, and the infrared triplet
lines are formed. We conclude that the difference due to a detailed treatment
of Ca ionisation has only negligible impact on the modelling of spectral lines
of Ca II and the plasma properties under the conditions in the quiet solar
chromosphere.Comment: 9 pages, 8 figures; recommended for publicatio
Carbon monoxide in the solar atmosphere II. Radiative cooling by CO lines
The role of carbon monoxide as a cooling agent for the thermal structure of
the mid-photospheric to low-chromospheric layers of the solar atmosphere in
internetwork regions is investigated. The treatment of radiative cooling via
spectral lines of carbon monoxide (CO) has been added to the radiation
chemo-hydrodynamics code CO5BOLD. [...] The CO opacity indeed causes additional
cooling at the fronts of propagating shock waves in the chromosphere. There,
the time-dependent approach results in a higher CO number density compared to
the equilibrium case and hence in a larger net radiative cooling rate. The
average gas temperature stratification of the model atmosphere, however, is
only reduced by roughly 100 K. Also the temperature fluctuations and the CO
number density are only affected to small extent. A numerical experiment
without dynamics shows that the CO cooling process works in principle and
drives the atmosphere to a cool radiative equilibrium state. At chromospheric
heights, the radiative relaxation of the atmosphere to a cool state takes
several 1000 s. The CO cooling process thus would seem to be too slow compared
to atmospheric dynamics to be responsible for the very cool temperature regions
observed in the solar atmosphere. The hydrodynamical timescales in our solar
atmosphere model are much too short to allow for the radiative relaxation to a
cool state, thus suppressing the potential thermal instability due to carbon
monoxide as a cooling agent. Apparently, the thermal structure and dynamics of
the outer model atmosphere are instead determined primarily by shock waves.Comment: 5 pages, 4 figures. A&A, accepted 06/12/200
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