65 research outputs found
Spatial deconvolution of spectropolarimetric data: an application to quiet Sun magnetic elements
Observations of the Sun from the Earth are always limited by the presence of
the atmosphere, which strongly disturbs the images. A solution to this problem
is to place the telescopes in space satellites, which produce observations
without any (or limited) atmospheric aberrations. However, even though the
images from space are not affected by atmospheric seeing, the optical
properties of the instruments still limit the observations. In the case of
diffraction limited observations, the PSF establishes the maximum allowed
spatial resolution, defined as the distance between two nearby structures that
can be properly distinguished. In addition, the shape of the PSF induce a
dispersion of the light from different parts of the image, leading to what is
commonly termed as stray light or dispersed light. This effect produces that
light observed in a spatial location at the focal plane is a combination of the
light emitted in the object at relatively distant spatial locations. We aim to
correct the effect produced by the telescope's PSF using a deconvolution
method, and we decided to apply the code on Hinode/SP quiet Sun observations.
We analyze the validity of the deconvolution process with noisy data and we
infer the physical properties of quiet Sun magnetic elements after the
deconvolution process.Comment: 14 pages, 9 figure
Temporal relation between quiet-Sun transverse fields and the strong flows detected by IMaX/SUNRISE
Localized strongly Doppler-shifted Stokes V signals were detected by
IMaX/SUNRISE. These signals are related to newly emerged magnetic loops that
are observed as linear polarization features. We aim to set constraints on the
physical nature and causes of these highly Doppler-shifted signals. In
particular, the temporal relation between the appearance of transverse fields
and the strong Doppler shifts is analyzed in some detail. We calculated the
time difference between the appearance of the strong flows and the linear
polarization. We also obtained the distances from the center of various
features to the nearest neutral lines and whether they overlap or not. These
distances were compared with those obtained from randomly distributed points on
observed magnetograms. Various cases of strong flows are described in some
detail. The linear polarization signals precede the appearance of the strong
flows by on average 84+-11 seconds. The strongly Doppler-shifted signals are
closer (0.19") to magnetic neutral lines than randomly distributed points
(0.5"). Eighty percent of the strongly Doppler-shifted signals are close to a
neutral line that is located between the emerging field and pre-existing
fields. That the remaining 20% do not show a close-by pre-existing field could
be explained by a lack of sensitivity or an unfavorable geometry of the
pre-existing field, for instance, a canopy-like structure. Transverse fields
occurred before the observation of the strong Doppler shifts. The process is
most naturally explained as the emergence of a granular-scale loop that first
gives rise to the linear polarization signals, interacts with pre-existing
fields (generating new neutral line configurations), and produces the observed
strong flows. This explanation is indicative of frequent small-scale
reconnection events in the quiet Sun.Comment: 11 pages, 8 figure
Detection of emission in the Si i 1082.7 nm line core in sunspot umbrae
We analyze spectropolarimetric sunspot umbra observations taken in the
near-infrared Si i 1082.7 nm line taking NLTE effects into account. The data
were obtained with the GRIS instrument installed at the German GREGOR
telescope. A point spread function (PSF) was constructed using prior Mercury
observations with GRIS and the information provided by the adaptive optics
system of the GREGOR telescope. The data were then deconvolved from the PSF
using a principal component analysis deconvolution method and were analyzed via
the NICOLE inversion code. The Si i 1082.7 nm line seems to be in emission in
the umbra of the observed sunspot after the effects of scattered light are
removed. We show how the spectral line shape of umbral profiles changes
dramatically with the amount of scattered light. Indeed, the continuum levels
range, on average, from 44% of the quiet Sun continuum intensity to about 20%.
The inferred levels are in line with current model predictions and empirical
umbral models. Current umbral empirical models are not able to reproduce the
emission in the deconvolved umbral Stokes profiles. The results of the NLTE
inversions suggests that to obtain the emission in the Si i 1082.7 nm line, the
temperature stratification should first have a hump located at about log tau -2
and start rising at lower heights when moving into the transition region. This
is, to our knowledge, the first time the Si i 1082.7 nm line is seen in
emission in sunspot umbrae. The results show that the temperature
stratification of current umbral models may be more complex than expected with
the transition region located at lower heights above sunspot umbrae. Our
finding might provide insights into understanding why the sunspot umbra
emission in the millimeter spectral range is less than that predicted by
current empirical umbral models
The small-scale structure of photospheric convection retrieved by a deconvolution technique applied to Hinode/SP data
Solar granules are bright patterns surrounded by dark channels called
intergranular lanes in the solar photosphere and are a manifestation of
overshooting convection. Observational studies generally find stronger upflows
in granules and weaker downflows in intergranular lanes. This trend is,
however, inconsistent with the results of numerical simulations in which
downflows are stronger than upflows through the joint action of gravitational
acceleration/deceleration and pressure gradients. One cause of this discrepancy
is the image degradation caused by optical distortion and light diffraction and
scattering that takes place in an imaging instrument. We apply a deconvolution
technique to Hinode/SP data in an attempt to recover the original solar scene.
Our results show a significant enhancement in both, the convective upflows and
downflows, but particularly for the latter. After deconvolution, the up- and
downflows reach maximum amplitudes of -3.0 km/s and +3.0 km/s at an average
geometrical height of roughly 50 km, respectively. We found that the velocity
distributions after deconvolution match those derived from numerical
simulations. After deconvolution the net LOS velocity averaged over the whole
FOV lies close to zero as expected in a rough sense from mass balance.Comment: 32 pages, 13 figures, accepted for publication in Ap
Chromospheric plasma ejection above a pore
We present high spatial resolution observations of short lived transients,
ribbon and jets like events above a pore in Ca II H images where fine structure
like umbral dots, lightbridge and penumbral micro filaments are present in the
underlying photosphere. We found that current layers are formed at the edges of
convective fine structure due to the shear between their horizontal field and
the ambient vertical field. High vertical electric current density patches are
observed in the photosphere around these events which indicates the formation
of a current sheet at the reconnection site. In the framework of past studies,
low altitude reconnection could be the mechanism that produces such events. The
reconnection is caused by an opposite polarity field produced by the bending of
field lines by convective downflows at the edge of the pore fine structures.Comment: 10 pages, 5 figures, accepted for publication in MNRA
Capabilities of bisector analysis of the Si I 10827 A line for estimating line-of-sight velocities in the quiet Sun
We examine the capabilities of a fast and simple method to infer
line-of-sight (LOS) velocities from observations of the photospheric Si I 10827
A line. This spectral line is routinely observed together with the
chromospheric He I 10830 A triplet as it helps to constrain the atmospheric
parameters. We study the accuracy of bisector analysis and a line core fit of
Si I 10827 A. We employ synthetic profiles starting from the Bifrost enhanced
network simulation. The profiles are computed solving the radiative transfer
equation, including non-local thermodynamic equilibrium effects on the
determination of the atomic level populations of Si I. We found a good
correlation between the inferred velocities from bisectors taken at different
line profile intensities and the original simulation velocity at given optical
depths. This good correlation means that we can associate bisectors taken at
different line-profile percentages with atmospheric layers that linearly
increase as we scan lower spectral line intensities. We also determined that a
fit to the line-core intensity is robust and reliable, providing information
about atmospheric layers that are above those accessible through bisectors.
Therefore, by combining both methods on the Si I 10827 A line, we can
seamlessly trace the quiet-Sun LOS velocity stratification from the deep
photosphere to higher layers until around in a fast and
straightforward way. This method is ideal for generating quick-look reference
images for future missions like the Daniel K. Inoue Solar Telescope and the
European Solar Telescope, for example.Comment: Accepted for publication in Astronomy & Astrophysics, 7 pages, 7
figure
Chromospheric polarimetry through multi-line observations of the 850 nm spectral region
Future solar missions and ground-based telescopes aim to understand the
magnetism of the solar chromosphere. We performed a supporting study in
Quintero Noda et al. (2016) focused on the infrared Ca II 8542 A line and we
concluded that is one of the best candidates because it is sensitive to a large
range of atmospheric heights, from the photosphere to the middle chromosphere.
However, we believe that it is worth to try improving the results produced by
this line observing additional spectral lines. In that regard, we examined the
neighbour solar spectrum looking for spectral lines that could increase the
sensitivity to the atmospheric parameters. Interestingly, we discovered several
photospheric lines that greatly improve the photospheric sensitivity to the
magnetic field vector. Moreover, they are located close to a second
chromospheric line that also belongs to the Ca II infrared triplet, i.e. the Ca
II 8498 A line, and enhances the sensitivity to the atmospheric parameters at
chromospheric layers. We conclude that the lines in the vicinity of the Ca II
8542 A line not only increase its sensitivity to the atmospheric parameters at
all layers, but also they constitute an excellent spectral window for
chromospheric polarimetry.Comment: 11 pages, 8 figures, 1 tabl
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