4,295 research outputs found
Three-dimensional non-LTE radiative transfer computation of the Ca 8542 infrared line from a radiation-MHD simulation
Interpretation of imagery of the solar chromosphere in the widely used
\CaIIIR infrared line is hampered by its complex, three-dimensional and non-LTE
formation. Forward modelling is required to aid understanding. We use a 3D
non-LTE radiative transfer code to compute synthetic \CaIIIR images from a
radiation-MHD simulation of the solar atmosphere spanning from the convection
zone to the corona. We compare the simulation with observations obtained with
the CRISP filter at the Swedish 1--m Solar Telescope. We find that the
simulation reproduces dark patches in the blue line wing caused by Doppler
shifts, brightenings in the line core caused by upward-propagating shocks and
thin dark elongated structures in the line core that form the interface between
upward and downward gas motion in the chromosphere. The synthetic line core is
narrower than the observed one, indicating that the sun exhibits both more
vigorous large-scale dynamics as well as small scale motions that are not
resolved within the simulation, presumably owing to a lack of spatial
resolution.Comment: accepted as ApJ lette
Parallel Mapper
The construction of Mapper has emerged in the last decade as a powerful and
effective topological data analysis tool that approximates and generalizes
other topological summaries, such as the Reeb graph, the contour tree, split,
and joint trees. In this paper, we study the parallel analysis of the
construction of Mapper. We give a provably correct parallel algorithm to
execute Mapper on multiple processors and discuss the performance results that
compare our approach to a reference sequential Mapper implementation. We report
the performance experiments that demonstrate the efficiency of our method
Fast algorithms and efficient GPU implementations for the Radon transform and the back-projection operator represented as convolution operators
The Radon transform and its adjoint, the back-projection operator, can both
be expressed as convolutions in log-polar coordinates. Hence, fast algorithms
for the application of the operators can be constructed by using FFT, if data
is resampled at log-polar coordinates. Radon data is typically measured on an
equally spaced grid in polar coordinates, and reconstructions are represented
(as images) in Cartesian coordinates. Therefore, in addition to FFT, several
steps of interpolation have to be conducted in order to apply the Radon
transform and the back-projection operator by means of convolutions.
Both the interpolation and the FFT operations can be efficiently implemented
on Graphical Processor Units (GPUs). For the interpolation, it is possible to
make use of the fact that linear interpolation is hard-wired on GPUs, meaning
that it has the same computational cost as direct memory access. Cubic order
interpolation schemes can be constructed by combining linear interpolation
steps which provides important computation speedup.
We provide details about how the Radon transform and the back-projection can
be implemented efficiently as convolution operators on GPUs. For large data
sizes, speedups of about 10 times are obtained in relation to the computational
times of other software packages based on GPU implementations of the Radon
transform and the back-projection operator. Moreover, speedups of more than a
1000 times are obtained against the CPU-implementations provided in the MATLAB
image processing toolbox
Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere
The ionization of hydrogen in the solar chromosphere and transition region
does not obey LTE or instantaneous statistical equilibrium because the
timescale is long compared with important hydrodynamical timescales, especially
of magneto-acoustic shocks. We implement an algorithm to compute
non-equilibrium hydrogen ionization and its coupling into the MHD equations
within an existing radiation MHD code, and perform a two-dimensional simulation
of the solar atmosphere from the convection zone to the corona. Analysis of the
simulation results and comparison to a companion simulation assuming LTE shows
that: a) Non-equilibrium computation delivers much smaller variations of the
chromospheric hydrogen ionization than for LTE. The ionization is smaller
within shocks but subsequently remains high in the cool intershock phases. As a
result, the chromospheric temperature variations are much larger than for LTE
because in non-equilibrium, hydrogen ionization is a less effective internal
energy buffer. The actual shock temperatures are therefore higher and the
intershock temperatures lower. b) The chromospheric populations of the hydrogen
n = 2 level, which governs the opacity of Halpha, are coupled to the ion
populations. They are set by the high temperature in shocks and subsequently
remain high in the cool intershock phases. c) The temperature structure and the
hydrogen level populations differ much between the chromosphere above
photospheric magnetic elements and above quiet internetwork. d) The hydrogen n
= 2 population and column density are persistently high in dynamic fibrils,
suggesting that these obtain their visibility from being optically thick in
Halpha also at low temperature.Comment: 10 pages, 4 figure
Four-dimensional tomographic reconstruction by time domain decomposition
Since the beginnings of tomography, the requirement that the sample does not
change during the acquisition of one tomographic rotation is unchanged. We
derived and successfully implemented a tomographic reconstruction method which
relaxes this decades-old requirement of static samples. In the presented
method, dynamic tomographic data sets are decomposed in the temporal domain
using basis functions and deploying an L1 regularization technique where the
penalty factor is taken for spatial and temporal derivatives. We implemented
the iterative algorithm for solving the regularization problem on modern GPU
systems to demonstrate its practical use
Fast hyperbolic Radon transform represented as convolutions in log-polar coordinates
The hyperbolic Radon transform is a commonly used tool in seismic processing,
for instance in seismic velocity analysis, data interpolation and for multiple
removal. A direct implementation by summation of traces with different moveouts
is computationally expensive for large data sets. In this paper we present a
new method for fast computation of the hyperbolic Radon transforms. It is based
on using a log-polar sampling with which the main computational parts reduce to
computing convolutions. This allows for fast implementations by means of FFT.
In addition to the FFT operations, interpolation procedures are required for
switching between coordinates in the time-offset; Radon; and log-polar domains.
Graphical Processor Units (GPUs) are suitable to use as a computational
platform for this purpose, due to the hardware supported interpolation routines
as well as optimized routines for FFT. Performance tests show large speed-ups
of the proposed algorithm. Hence, it is suitable to use in iterative methods,
and we provide examples for data interpolation and multiple removal using this
approach.Comment: 21 pages, 10 figures, 2 table
Ellerman bombs and UV bursts: transient events in chromospheric current sheets
Ellerman bombs (EBs) and UV bursts are both brightenings related to flux
emergence regions and specifically to magnetic flux of opposite polarity that
meet in the photosphere. These two reconnection-related phenomena, nominally
formed far apart, occasionally occur in the same location and at the same time,
thus challenging our understanding of reconnection and heating of the lower
solar atmosphere. We consider the formation of an active region, including long
fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic
flux sheet, injected ~Mm below the photosphere, is studied as it pierces
the photosphere and interacts with the preexisting ambient field. Specifically,
we aim to study whether EBs and UV bursts are generated as a result of such
flux emergence and examine their physical relationship. The Bifrost radiative
magnetohydrodynamics code was used to model flux emerging into a model
atmosphere that contained a fairly strong ambient field, constraining the
emerging field to a limited volume wherein multiple reconnection events occur
as the field breaks through the photosphere and expands into the outer
atmosphere. Synthetic spectra of the different reconnection events were
computed using the D RH code and the fully 3D MULTI3D code. The formation
of UV bursts and EBs at intensities and with line profiles that are highly
reminiscent of observed spectra are understood to be a result of the
reconnection of emerging flux with itself in a long-lasting current sheet that
extends over several scale heights through the chromosphere. Synthetic
diagnostics suggest that there are no compelling reasons to assume that UV
bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally
formed at opposite ends of a long current sheet that resides in an extended
bubble of cool gas.Comment: 10 pages, 8 figures, accepted by A&
Twisted flux tube emergence from the convection zone to the corona
3D numerical simulations of a horizontal magnetic flux tube emergence with
different twist are carried out in a computational domain spanning the upper
layers of the convection zone to the lower corona. We use the Oslo Staggered
Code to solve the full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along the magnetic field lines. The emergence
of the magnetic flux tube input at the bottom boundary into a weakly magnetized
atmosphere is presented. The photospheric and chromospheric response is
described with magnetograms, synthetic images and velocity field distributions.
The emergence of a magnetic flux tube into such an atmosphere results in varied
atmospheric responses. In the photosphere the granular size increases when the
flux tube approaches from below. In the convective overshoot region some 200km
above the photosphere adiabatic expansion produces cooling, darker regions with
the structure of granulation cells. We also find collapsed granulation in the
boundaries of the rising flux tube. Once the flux tube has crossed the
photosphere, bright points related with concentrated magnetic field, vorticity,
high vertical velocities and heating by compressed material are found at
heights up to 500km above the photosphere. At greater heights in the magnetized
chromosphere, the rising flux tube produces a cool, magnetized bubble that
tends to expel the usual chromospheric oscillations. In addition the rising
flux tube dramatically increases the chromospheric scale height, pushing the
transition region and corona aside such that the chromosphere extends up to 6Mm
above the photosphere. The emergence of magnetic flux tubes through the
photosphere to the lower corona is a relatively slow process, taking of order 1
hour.Comment: 53 pages,79 figures, Submitted to Ap
The stellar atmosphere simulation code Bifrost
Context: Numerical simulations of stellar convection and photospheres have
been developed to the point where detailed shapes of observed spectral lines
can be explained. Stellar atmospheres are very complex, and very different
physical regimes are present in the convection zone, photosphere, chromosphere,
transition region and corona. To understand the details of the atmosphere it is
necessary to simulate the whole atmosphere since the different layers interact
strongly. These physical regimes are very diverse and it takes a highly
efficient massively parallel numerical code to solve the associated equations.
Aims: The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the convection
zone to the corona.
Methods: The code is subjected to a number of validation tests, among them
the Sod shock tube test, the Orzag-Tang colliding shock test, boundary
condition tests and tests of how the code treats magnetic field advection,
chromospheric radiation, radiative transfer in an isothermal scattering
atmosphere, hydrogen ionization and thermal conduction.
Results: Bifrost completes the tests with good results and shows near linear
efficiency scaling to thousands of computing cores
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