7,920 research outputs found
Sparse inversion of Stokes profiles. I. Two-dimensional Milne-Eddington inversions
Inversion codes are numerical tools used for the inference of physical
properties from the observations. Despite their success, the quality of current
spectropolarimetric observations and those expected in the near future presents
a challenge to current inversion codes. The pixel-by-pixel strategy of
inverting spectropolarimetric data that we currently utilize needs to be
surpassed and improved. The inverted physical parameters have to take into
account the spatial correlation that is present in the data and that contains
valuable physical information. We utilize the concept of sparsity or
compressibility to develop an new generation of inversion codes for the Stokes
parameters. The inversion code uses numerical optimization techniques based on
the idea of proximal algorithms to impose sparsity. In so doing, we allow for
the first time to exploit the presence of spatial correlation on the maps of
physical parameters. Sparsity also regularizes the solution by reducing the
number of unknowns. We compare the results of the new inversion code with
pixel-by-pixel inversions, demonstrating the increase in robustness of the
solution. We also show how the method can easily compensate for the effect of
the telescope point spread function, producing solutions with an enhanced
contrast.Comment: 13 pages, 8 figures, accepted for publication in A&
Real-time multiframe blind deconvolution of solar images
The quality of images of the Sun obtained from the ground are severely
limited by the perturbing effect of the turbulent Earth's atmosphere. The
post-facto correction of the images to compensate for the presence of the
atmosphere require the combination of high-order adaptive optics techniques,
fast measurements to freeze the turbulent atmosphere and very time consuming
blind deconvolution algorithms. Under mild seeing conditions, blind
deconvolution algorithms can produce images of astonishing quality. They can be
very competitive with those obtained from space, with the huge advantage of the
flexibility of the instrumentation thanks to the direct access to the
telescope. In this contribution we leverage deep learning techniques to
significantly accelerate the blind deconvolution process and produce corrected
images at a peak rate of ~100 images per second. We present two different
architectures that produce excellent image corrections with noise suppression
while maintaining the photometric properties of the images. As a consequence,
polarimetric signals can be obtained with standard polarimetric modulation
without any significant artifact. With the expected improvements in computer
hardware and algorithms, we anticipate that on-site real-time correction of
solar images will be possible in the near future.Comment: 16 pages, 12 figures, accepted for publication in A&
A Hot Downflowing Model Atmosphere For Umbral Flashes And The Physical Properties Of Their Dark Fibrils
We perform NLTE inversions in a large set of umbral flashes, including the
dark fibrils visible within them, and in the quiescent umbra by using the
inversion code NICOLE on a set of full Stokes high-resolution Ca II 8542 A
observations of a sunspot at disk center. We find that the dark structures have
Stokes profiles that are distinct from those of the quiescent and flashed
regions. They are best reproduced by atmospheres that are more similar to the
flashed atmosphere in terms of velocities, even if with reduced amplitudes. We
also find two sets of solutions that finely fit the flashed profiles: a set
that is upflowing, featuring a transition region that is deeper than in the
quiescent case and preceded by a slight dip in temperature, and a second
solution with a hotter atmosphere in the chromosphere but featuring downflows
close to the speed of sound at such heights. Such downflows may be related, or
even dependent, on the presence of coronal loops, rooted in the umbra of
sunspots, as is the case in the region analyzed. Similar loops have been
recently observed to have supersonic downflows in the transition region and are
consistent with the earlier "sunspot plumes" which were invariably found to
display strong downflows in sunspots. Finally we find, on average, a magnetic
field reduction in the flashed areas, suggesting that the shock pressure is
moving field lines in the upper layers.Comment: Accepted in June for publication at ApJ. Comments to
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Stratification of canopy magnetic fields in a plage region. Constraints from a spatially-regularized weak-field approximation method
The role of magnetic fields in the chromospheric heating problem remains
greatly unconstrained. Most theoretical predictions from numerical models rely
on a magnetic configuration, field strength and connectivity whose details have
not been well established with observational studies. High-resolution studies
of chromospheric magnetic fields in plage are very scarce or non-existent in
general. Our aim is to study the stratification of the magnetic field vector in
plage regions. We use high-spatial resolution full-Stokes observations acquired
with CRISP instrument at the Swedish 1-m Solar Telescope in the Mg I
5173, Na I 5896 and Ca II 8542 lines. We have
developed a spatially-regularized weak-field approximation (WFA) method based
on the idea of spatial regularization. This method allows for a fast
computation of magnetic field maps for an extended field of view. The fidelity
of this new technique has been assessed using a snapshot from a realistic 3D
magnetohydrodynamics simulation. We have derived the depth-stratification of
the line-of-sight component of the magnetic field from the photosphere to the
chromosphere in a plage region. The magnetic fields are concentrated in the
intergranular lanes in the photosphere and expand horizontally toward the
chromosphere, filling all the space and forming a canopy. Our results suggest
that the lower boundary of this canopy must be located around 400-600 km from
the photosphere. The mean canopy total magnetic field strength in the lower
chromosphere ( km) is 658 G. At km we estimate
G. We propose a modification to the WFA that
improves its applicability to data with worse signal-to-noise ratio. These
methods provide a quick and reliable way of studying multi-layer magnetic field
observations without the many difficulties inherent to other inversion methods.Comment: Accepted for publication on 2020-08-2
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