delacruz-rodriguez

explanation blue bibcodes open ADS page with paths to full text

Author name code: delacruz-rodriguez
ADS astronomy entries on 2022-09-14
author:"de la Cruz Rodriguez, Jaime"

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Title: Radiative losses in the chromosphere during a C-class flare
Authors: Yadav, R.; de la Cruz Rodríguez, J.; Kerr, G. S.; Díaz Baso,
C. J.; Leenaarts, J.
2022A&A...665A..50Y Altcode: 2022arXiv220702840Y
Context. Solar flares release an enormous amount of energy
(∼1032 erg) into the corona. A substantial fraction of
this energy is transported to the lower atmosphere, which results in
chromospheric heating. The mechanisms that transport energy to the lower
solar atmosphere during a flare are still not fully understood. 
Aims: We aim to estimate the temporal evolution of the radiative losses
in the chromosphere at the footpoints of a C-class flare, in order
to set observational constraints on the electron beam parameters of
a RADYN flare simulation. Methods: We estimated the radiative
losses from hydrogen, and singly ionized Ca and Mg using semiempirical
model atmospheres, which were inferred from a multiline inversion of
observed Stokes profiles obtained with the CRISP and CHROMIS instruments
on the Swedish 1-m Solar Telescope. The radiative losses were computed
taking into account the effect of partial redistribution and non-local
thermodynamic equilibrium. To estimate the integrated radiative losses
in the chromosphere, the net cooling rates were integrated between the
temperature minimum and the height where the temperature reaches 10
kK. We also compared our time series of radiative losses with those
from the RADYN flare simulations. Results: We obtained a high
spatial-resolution map of integrated radiative losses around the flare
peak time. The stratification of the net cooling rate suggests that the
Ca IR triplet lines are responsible for most of the radiative losses in
the flaring atmosphere. During the flare peak time, the contribution
from Ca II H and K and Mg II h and k lines are strong and comparable
to the Ca IR triplet (∼32 kW m−2). Since our flare is a
relatively weak event, the chromosphere is not heated above 11 kK, which
in turn yields a subdued Lyα contribution (∼7 kW m−2)
in the selected limits of the chromosphere. The temporal evolution
of total integrated radiative losses exhibits sharply rising losses
(0.4 kW m−2 s−1) and a relatively slow decay
(0.23 kW m−2 s−1). The maximum value of total
radiative losses is reached around the flare peak time, and can go up to
175 kW m−2 for a single pixel located at footpoint. After
a small parameter study, we find the best model-data consistency in
terms of the amplitude of radiative losses and the overall atmospheric
structure with a RADYN flare simulation in the injected energy flux
of 5 × 1010 erg s−1 cm−2.

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Title: Spatio-temporal analysis of chromospheric heating in a
plage region
Authors: Morosin, R.; de la Cruz Rodríguez, J.; Díaz Baso, C. J.;
Leenaarts, J.
2022A&A...664A...8M Altcode: 2022arXiv220301688M
Context. Our knowledge of the heating mechanisms that are at work
in the chromosphere of plage regions remains highly unconstrained
from observational studies. While many heating candidates have been
proposed in theoretical studies, the exact contribution from each of
them is still unknown. The problem is rather difficult because there
is no direct way of estimating the heating terms from chromospheric
observations. Aims: The purpose of our study is to estimate the
chromospheric heating terms from a multi-line high-spatial-resolution
plage dataset, characterize their spatio-temporal distribution
and set constraints on the heating processes that are at work in
the chromosphere. Methods: We used nonlocal thermodynamical
equilibrium inversions in order to infer a model of the photosphere
and chromosphere of a plage dataset acquired with the Swedish 1-m
Solar Telescope (SST). We used this model atmosphere to calculate
the chromospheric radiative losses from the main chromospheric cooler
from H I, Ca II, and Mg II atoms. In this study, we approximate the
chromospheric heating terms by the net radiative losses predicted
by the inverted model. In order to make the analysis of time-series
over a large field of view computationally tractable, we made use
of a neural network which is trained from the inverted models of
two non-consecutive time-steps. We have divided the chromosphere
in three regions (lower, middle, and upper) and analyzed how the
distribution of the radiative losses is correlated with the physical
parameters of the model. Results: In the lower chromosphere,
the contribution from the Ca II lines is dominant and predominantly
located in the surroundings of the photospheric footpoints. In the
upper chromosphere, the H I contribution is dominant. Radiative
losses in the upper chromosphere form a relatively homogeneous
patch that covers the entire plage region. The Mg II also peaks in
the upper chromosphere. Our time analysis shows that in all pixels,
the net radiative losses can be split in a periodic component with
an average amplitude of amp̅Q = 7.6 kW m−2
and a static (or very slowly evolving) component with a mean value of
−26.1 kW m−2. The period of the modulation present in
the net radiative losses matches that of the line-of-sight velocity
of the model. Conclusions: Our interpretation is that in the
lower chromosphere, the radiative losses are tracing the sharp lower
edge of the hot magnetic canopy that is formed above the photosphere,
where the electric current is expected to be large. Therefore, Ohmic
current dissipation could explain the observed distribution. In the
upper chromosphere, both the magnetic field and the distribution of net
radiative losses are room-filling and relatively smooth, whereas the
amplitude of the periodic component is largest. Our results suggest that
acoustic wave heating may be responsible for one-third of the energy
deposition in the upper chromosphere, whereas other heating mechanisms
must be responsible for the rest: turbulent Alfvén wave dissipation
or ambipolar diffusion could be among them. Given the smooth nature
of the magnetic field in the upper chromosphere, we are inclined to
rule out Ohmic dissipation of current sheets in the upper chromosphere.

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Title: An observationally-constrained model of strong magnetic
reconnection in the solar chromosphere: atmospheric stratification
and estimates of heating rates
Authors: Diaz Baso, Carlos Jose; De la Cruz Rodriguez, Jaime;
Leenaarts, Jorritt
2022cosp...44.2514D Altcode:
The evolution of the photospheric magnetic field plays a key role in
the energy transport into the chromosphere and the corona. In active
regions, the emergence of new magnetic flux leads to interactions with
the pre-existing magnetic field, releasing magnetic energy, heating
the solar atmosphere, and accelerating the solar plasma. We have
studied the heating caused by a strong reconnection event triggered by
the cancellation of the magnetic flux for two hours at high spatial
resolution. For this we used spectropolarimetric imaging data on Fe
I 6301A, Fe I 6302A, Ca II 8542A and Ca II K obtained with the CRISP
and CHROMIS instruments on the Swedish 1-m Solar Telescope. This data
was inverted using multi-atom, multi-line non-LTE inversions using
the STiC code. The inversion yielded a 3D model of the reconnection
event and surrounding atmosphere, including temperature, velocity,
microturbulence, magnetic file configuration, and the radiative loss
rate. The model atmosphere shows the emergence of magnetic loops with
a size of several arcsecs into a pre-existing predominantly unipolar
field. Where the reconnection region is expected to be, we see an
increase in the chromospheric temperature of roughly 3000 K as well
as bidirectional flows of the order of 10 km/s emanating from the
region. We also detected bright blobs of roughly 0.2 arcsec diameter
moving at a plane-of-the-sky velocity of order 100 km/s and a blueshift
of 100 km/s, which we interpret as plasmoids. The chromospheric
radiative losses at the reconnection site in our inferred model are as
high as 160 kW/m$^{2}$ (about a factor 8 higher than canonical values
for active regions) and the magnetic energy release estimated from
the flux cancellation (5. 10$^{29}$erg) is sufficient to sustain the
chromospheric losses for more than two hours (5. 10$^{27}$erg). These
results provide a quantitative constraint on theoretical models that aim
to simulate reconnection caused by flux emergence in the chromosphere.

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Title: The European Solar Telescope
Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.;
Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein,
C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz
Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados,
M.; the EST team
2022arXiv220710905Q Altcode:
The European Solar Telescope (EST) is a project aimed at studying
the magnetic connectivity of the solar atmosphere, from the deep
photosphere to the upper chromosphere. Its design combines the knowledge
and expertise gathered by the European solar physics community during
the construction and operation of state-of-the-art solar telescopes
operating in visible and near-infrared wavelengths: the Swedish 1m Solar
Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR,
the French Télescope Héliographique pour l'Étude du Magnétisme
et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope
(DOT). With its 4.2 m primary mirror and an open configuration, EST will
become the most powerful European ground-based facility to study the Sun
in the coming decades in the visible and near-infrared bands. EST uses
the most innovative technological advances: the first adaptive secondary
mirror ever used in a solar telescope, a complex multi-conjugate
adaptive optics with deformable mirrors that form part of the optical
design in a natural way, a polarimetrically compensated telescope design
that eliminates the complex temporal variation and wavelength dependence
of the telescope Mueller matrix, and an instrument suite containing
several (etalon-based) tunable imaging spectropolarimeters and several
integral field unit spectropolarimeters. This publication summarises
some fundamental science questions that can be addressed with the
telescope, together with a complete description of its major subsystems.

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Title: Quiet-Sun radiative losses: contribution to chromospheric
heating
Authors: Pastor Yabar, Adur; De la Cruz Rodriguez, Jaime
2022cosp...44.2517P Altcode:
Out-of-sunspots solar magnetism is known to host a plethora of
small-scale, highly dynamic, and fast evolving transient magnetic
events whose contribution to the energetic and mass balance of the solar
atmosphere remains to be clarified. A particularly intriguing property
of the solar atmosphere is that it is not in radiative equilibrium,
that is, the amount of energy escaping through radiation from higher
layers is larger than from lower layers. This is a clear indication that
there is a significant energy transport (other than radiation) needed to
explain at least the additional energy loss through radiation in upper
layers. Recently, it has been shown that this excess of energy loss
is not spatially homogeneous but its variability is extreme, namely,
though the average losses measured for the quiet-Sun match historical
values, the spatial analysis of these losses exhibit variations
in the orders of magnitude. One possible approach to shed light
into this problem is to analyze what kind of thermodynamic/magnetic
event (if any) is associated with these increased losses. To do so,
in this contribution we present the inversion of a quiet-Sun area
co-observed with photospheric and chromospheric polarimetry with CRISP
(FeI 6173{\AA} and CaII 8542{\AA}) and spectroscopy in CaII K with
CHROMIS and MgII h&amp;k lines with IRIS. In order to properly handle
the disparate optical systems associated with the usage of different
wavelengths and facilities, we have employed a novel inversion strategy
that handles coupled inversions in NLTE. In this upgrade, different
optical distortions such as the point-spread-function of the telescope
or different spatial sampling at different datasets are considered by
means of linear transformations, not only in the forward modeling but
also during the inversion step. In doing so, it is possible to achieve
a much more accurate inference of the physical parameters. Here we
will show the physical parameter inferences as well as the radiative
losses associated and their possible source.

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Title: Active region chromospheric magnetic fields. Observational
inference versus magnetohydrostatic modelling
Authors: Vissers, G. J. M.; Danilovic, S.; Zhu, X.; Leenaarts, J.;
Díaz Baso, C. J.; da Silva Santos, J. M.; de la Cruz Rodríguez,
J.; Wiegelmann, T.
2022A&A...662A..88V Altcode: 2021arXiv210902943V
Context. A proper estimate of the chromospheric magnetic fields is
thought to improve modelling of both active region and coronal mass
ejection evolution. However, because the chromospheric field is not
regularly obtained for sufficiently large fields of view, estimates
thereof are commonly obtained through data-driven models or field
extrapolations, based on photospheric boundary conditions alone and
involving pre-processing that may reduce details and dynamic range in
the magnetograms. Aims: We investigate the similarity between
the chromospheric magnetic field that is directly inferred from
observations and the field obtained from a magnetohydrostatic (MHS)
extrapolation based on a high-resolution photospheric magnetogram. Methods: Based on Swedish 1-m Solar Telescope Fe I 6173 Å and
Ca II 8542 Å observations of NOAA active region 12723, we employed
the spatially regularised weak-field approximation (WFA) to derive
the vector magnetic field in the chromosphere from Ca II, as well as
non-local thermodynamic equilibrium (non-LTE) inversions of Fe I and Ca
II to infer a model atmosphere for selected regions. Milne-Eddington
inversions of Fe I serve as photospheric boundary conditions for the
MHS model that delivers the three-dimensional field, gas pressure,
and density self-consistently. Results: For the line-of-sight
component, the MHS chromospheric field generally agrees with the
non-LTE inversions and WFA, but tends to be weaker by 16% on average
than these when larger in magnitude than 300 G. The observationally
inferred transverse component is systematically stronger, up to an
order of magnitude in magnetically weaker regions, but the qualitative
distribution with height is similar to the MHS results. For either
field component, the MHS chromospheric field lacks the fine structure
derived from the inversions. Furthermore, the MHS model does not
recover the magnetic imprint from a set of high fibrils connecting
the main polarities. Conclusions: The MHS extrapolation and
WFA provide a qualitatively similar chromospheric field, where the
azimuth of the former is better aligned with Ca II 8542 Å fibrils than
that of the WFA, especially outside strong-field concentrations. The
amount of structure as well as the transverse field strengths are,
however, underestimated by the MHS extrapolation. This underscores the
importance of considering a chromospheric magnetic field constraint in
data-driven modelling of active regions, particularly in the context
of space weather predictions.

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Title: Heating of the solar chromosphere through current dissipation
Authors: da Silva Santos, J. M.; Danilovic, S.; Leenaarts, J.; de
la Cruz Rodríguez, J.; Zhu, X.; White, S. M.; Vissers, G. J. M.;
Rempel, M.
2022A&A...661A..59D Altcode: 2022arXiv220203955D
Context. The solar chromosphere is heated to temperatures higher than
predicted by radiative equilibrium. This excess heating is greater
in active regions where the magnetic field is stronger. 
Aims: We aim to investigate the magnetic topology associated with an
area of enhanced millimeter (mm) brightness temperatures in a solar
active region mapped by the Atacama Large Millimeter/submillimeter
Array (ALMA) using spectropolarimetric co-observations with the 1-m
Swedish Solar Telescope (SST). Methods: We used Milne-Eddington
inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions,
and a magnetohydrostatic extrapolation to obtain constraints on the
three-dimensional (3D) stratification of temperature, magnetic field,
and radiative energy losses. We compared the observations to a snapshot
of a magnetohydrodynamics simulation and investigate the formation
of the thermal continuum at 3 mm using contribution functions. 
Results: We find enhanced heating rates in the upper chromosphere of up
to ∼5 kW m−2, where small-scale emerging loops interact
with the overlying magnetic canopy leading to current sheets as shown
by the magnetic field extrapolation. Our estimates are about a factor
of two higher than canonical values, but they are limited by the ALMA
spatial resolution (∼1.2″). Band 3 brightness temperatures reach
about ∼104 K in the region, and the transverse magnetic
field strength inferred from the non-LTE inversions is on the order
of ∼500 G in the chromosphere. Conclusions: We are able to
quantitatively reproduce many of the observed features including the
integrated radiative losses in our numerical simulation. We conclude
that the heating is caused by dissipation in current sheets. However,
the simulation shows a complex stratification in the flux emergence
region where distinct layers may contribute significantly to the
emission in the mm continuum. The movie is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243191/olm">https://www.aanda.org</A>

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Title: Physical properties of a fan-shaped jet backlit by an X9.3
flare
Authors: Pietrow, A. G. M.; Druett, M. K.; de la Cruz Rodriguez, J.;
Calvo, F.; Kiselman, D.
2022A&A...659A..58P Altcode: 2021arXiv211010541P
Context. Fan-shaped jets sometimes form above light bridges and are
believed to be driven by the reconnection of the vertical umbral
field with the more horizontal field above the light bridges. Because
these jets are not fully opaque in the wings of most chromospheric
lines, it is not possible to study their spectra without highly
complex considerations of radiative transfer in spectral lines from
the atmosphere behind the fan. Aims: We take advantage of a
unique set of observations of the Hα line along with the Ca II 8542
Å and Ca II K lines obtained with the CRISP and CHROMIS instrument
of the Swedish 1-m Solar Telescope to study the physical properties
of a fan-shaped jet that was backlit by an X9.3 flare. For what we
believe to be the first time, we report an observationally derived
estimate of the mass and density of material in a fan-shaped jet. Methods: The Hα flare ribbon emission profiles from behind the fan
are highly broadened and flattened, allowing us to investigate the fan
with a single slab via Beckers' cloud model, as if it were backlit
by a flat spectral profile of continuum emission. Using this model
we derived the opacity and velocity of the material in the jet. Using
inversions of Ca II 8542 Å emission via the STockholm inversion Code,
we were also able to estimate the temperature and to cross-check the
velocity of the material in the jet. Finally, we used the masses
and the plane-of-sky and line-of-sight velocities as functions of
time to investigate the downward supply of energy and momentum to the
photosphere in the collapse of this jet, and evaluated it as a potential
driver for a sunquake beneath. Results: We find that the physical
properties of the fan material are reasonably chromospheric in nature,
with a temperature of 7050 ± 250 K and a mean density of 2 ± 0.3 ×
10−11 g cm−3. Conclusions: The total
mass observed in Hα was found to be 3.9 ± 0.7 × 1013
g and the kinetic energy delivered to the base of the fan in its
collapse was nearly two orders of magnitude below typical sunquake
energies. We therefore rule out this jet as the sunquake driver,
but cannot completely rule out larger fan jets as potential drivers.

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Title: Bayesian Stokes inversion with normalizing flows
Authors: Díaz Baso, C. J.; Asensio Ramos, A.; de la Cruz Rodríguez,
J.
2022A&A...659A.165D Altcode: 2021arXiv210807089D
Stokes inversion techniques are very powerful methods for obtaining
information on the thermodynamic and magnetic properties of solar
and stellar atmospheres. In recent years, highly sophisticated
inversion codes have been developed that are now routinely applied
to spectro-polarimetric observations. Most of these inversion codes
are designed to find an optimum solution to the nonlinear inverse
problem. However, to obtain the location of potentially multimodal
cases (ambiguities), the degeneracies and the uncertainties of each
parameter inferred from the inversions algorithms - such as Markov chain
Monte Carlo (MCMC) - require evaluation of the likelihood of the model
thousand of times and are computationally costly. Variational methods
are a quick alternative to Monte Carlo methods, and approximate the
posterior distribution by a parametrized distribution. In this study,
we introduce a highly flexible variational inference method to perform
fast Bayesian inference, known as normalizing flows. Normalizing flows
are a set of invertible, differentiable, and parametric transformations
that convert a simple distribution into an approximation of any
other complex distribution. If the transformations are conditioned on
observations, the normalizing flows can be trained to return Bayesian
posterior probability estimates for any observation. We illustrate
the ability of the method using a simple Milne-Eddington model and
a complex non-local thermodynamic equilibrium (NLTE) inversion. The
method is extremely general and other more complex forward models
can be applied. The training procedure need only be performed once
for a given prior parameter space and the resulting network can then
generate samples describing the posterior distribution several orders
of magnitude faster than existing techniques.

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Title: ISP-SST/ISPy: ISPy release v0.2.0
Authors: Díaz Baso, Carlos José; Vissers, Gregal; Calvo, Flavio;
Pietrow, Alexander; Yadav, Rahul; De La Cruz Rodríguez, Jaime;
Zivadinovic, Lazar
2021zndo...5608441D Altcode: 2021zndo...5608441J
This new version includes all previous features (basic functionality
for SST data I/O and calibration, image alignment and inversion
result visualisations) and the following new ones: neuralDenoising,
potentialExtrapolation, field-dependent crosstalk, mapping to CEA
coordinates, neuralEstimation, cube visualisation and routines for
creating SST spectral line programs.

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Title: On the (Mis)Interpretation of the Scattering Polarization
Signatures in the Ca II 8542 Å Line through Spectral Line Inversions
Authors: Centeno, Rebecca; de la Cruz Rodríguez, Jaime; del Pino
Alemán, Tanausú
2021ApJ...918...15C Altcode: 2021arXiv210604478C
Scattering polarization tends to dominate the linear polarization
signals of the Ca II 8542 Å line in weakly magnetized areas (B ≲ 100
G), especially when the observing geometry is close to the limb. In
this paper, we evaluate the degree of applicability of existing
non-LTE spectral line inversion codes (which assume that the spectral
line polarization is due to the Zeeman effect only) at inferring the
magnetic field vector and, particularly, its transverse component. To
this end, we use the inversion code STiC to extract the strength and
orientation of the magnetic field from synthetic spectropolarimetric
data generated with the Hanle-RT code. The latter accounts for the
generation of polarization through scattering processes as well as
the joint actions of the Hanle and the Zeeman effects. We find that,
when the transverse component of the field is stronger than ~80 G,
the inversion code is able to retrieve accurate estimates of the
transverse field strength as well as its azimuth in the plane of the
sky. Below this threshold, the scattering polarization signatures
become the major contributors to the linear polarization signals and
often mislead the inversion code into severely over- or underestimating
the field strength. Since the line-of-sight component of the field is
derived from the circular polarization signal, which is not affected
by atomic alignment, the corresponding inferences are always good.

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Title: SSTRED: Data- and metadata-processing pipeline for CHROMIS
and CRISP
Authors: Löfdahl, Mats G.; Hillberg, Tomas; de la Cruz Rodríguez,
Jaime; Vissers, Gregal; Andriienko, Oleksii; Scharmer, Göran B.;
Haugan, Stein V. H.; Fredvik, Terje
2021A&A...653A..68L Altcode: 2018arXiv180403030L
Context. Data from ground-based, high-resolution solar telescopes
can only be used for science with calibrations and processing, which
requires detailed knowledge about the instrumentation. Space-based
solar telescopes provide science-ready data, which are easier to
work with for researchers whose expertise is in the interpretation of
data. Recently, data-processing pipelines for ground-based instruments
have been constructed. Aims: We aim to provide observers
with a user-friendly data pipeline for data from the Swedish 1-meter
Solar Telescope (SST) that delivers science-ready data together with
the metadata needed for proper interpretation and archiving. 
Methods: We briefly describe the CHROMospheric Imaging Spectrometer
(CHROMIS) instrument, including its (pre)filters, as well as recent
upgrades to the CRisp Imaging SpectroPolarimeter (CRISP) prefilters and
polarization optics. We summarize the processing steps from raw data
to science-ready data cubes in FITS files. We report calibrations
and compensations for data imperfections in detail. Misalignment
of Ca II data due to wavelength-dependent dispersion is identified,
characterized, and compensated for. We describe intensity calibrations
that remove or reduce the effects of filter transmission profiles
as well as solar elevation changes. We present REDUX, a new version
of the MOMFBD image restoration code, with multiple enhancements and
new features. It uses projective transforms for the registration of
multiple detectors. We describe how image restoration is used with
CRISP and CHROMIS data. The science-ready output is delivered in FITS
files, with metadata compliant with the SOLARNET recommendations. Data
cube coordinates are specified within the World Coordinate System
(WCS). Cavity errors are specified as distortions of the WCS wavelength
coordinate with an extension of existing WCS notation. We establish
notation for specifying the reference system for Stokes vectors with
reference to WCS coordinate directions. The CRIsp SPectral EXplorer
(CRISPEX) data-cube browser has been extended to accept SSTRED output
and to take advantage of the SOLARNET metadata. Results: SSTRED
is a mature data-processing pipeline for imaging instruments, developed
and used for the SST/CHROMIS imaging spectrometer and the SST/CRISP
spectropolarimeter. SSTRED delivers well-characterized, science-ready,
archival-quality FITS files with well-defined metadata. The SSTRED
code, as well as REDUX and CRISPEX, is freely available through git
repositories.

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Title: Line formation of He I D3 and He I 10 830 Å in
a small-scale reconnection event
Authors: Libbrecht, Tine; Bjørgen, Johan P.; Leenaarts, Jorrit;
de la Cruz Rodríguez, Jaime; Hansteen, Viggo; Joshi, Jayant
2021A&A...652A.146L Altcode: 2020arXiv201015946L
Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection
events that occur in the region of the upper photosphere to the
chromosphere. It has recently been discovered that these events can
have emission signatures in the He I D3 and He I 10 830 Å
lines, suggesting that their temperatures are higher than previously
expected. Aims: We aim to explain the line formation of He I
D3 and He I 10 830 Å in small-scale reconnection events. Methods: We used a simulated EB in a Bifrost-generated radiative
magnetohydrodynamics snapshot. The resulting He I D3 and He
I 10 830 Å line intensities were synthesized in 3D using the non-local
thermal equilibrium (non-LTE) Multi3D code. The presence of coronal
extreme UV (EUV) radiation was included self-consistently. We compared
the synthetic helium spectra with observed raster scans of EBs in He I
10 830 Å and He I D3 obtained at the Swedish Solar Telescope
with the TRI-Port Polarimetric Echelle-Littrow Spectrograph. 
Results: Emission in He I D3 and He I 10 830 Å is formed
in a thin shell around the EB at a height of ∼0.8 Mm, while the He I
D3 absorption is formed above the EB at ∼4 Mm. The height
at which the emission is formed corresponds to the lower boundary of the
EB, where the temperature increases rapidly from 6 × 103 K
to 106 K. The synthetic line profiles at a heliocentric angle
of μ = 0.27 are qualitatively similar to the observed profiles at the
same μ-angle in dynamics, broadening, and line shape: emission in the
wing and absorption in the line core. The opacity in He I D3
and He I 10 830 Å is generated through photoionization-recombination
driven by EUV radiation that is locally generated in the EB at
temperatures in the range of 2 × 104 − 2 × 106
K and electron densities between 1011 and 1013
cm−3. The synthetic emission signals are a result of
coupling to local conditions in a thin shell around the EB, with
temperatures between 7 × 103 and 104 K and
electron densities ranging from ∼1012 to 1013
cm−3. This shows that both strong non-LTE and thermal
processes play a role in the formation of He I D3 and
He I 10 830 Å in the synthetic EB/UV burst that we studied. 
Conclusions: In conclusion, the synthetic He I D3 and He I 10
830 Å emission signatures are an indicator of temperatures of at least
2 × 104 K; in this case, as high as ∼106 K.

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Title: Stratification of physical parameters in a C-class solar
flare using multiline observations
Authors: Yadav, R.; Díaz Baso, C. J.; de la Cruz Rodríguez, J.;
Calvo, F.; Morosin, R.
2021A&A...649A.106Y Altcode: 2020arXiv201102953Y
We present high-resolution and multiline observations of a C2-class
solar flare (SOL2019-05-06T08:47), which occurred in NOAA AR 12740
on May 6, 2019. The rise, peak, and decay phases of the flare
were recorded continuously and quasi-simultaneously in the Ca II
K line with the CHROMIS instrument and in the Ca II 8542 Å and Fe
I 6173 Å lines with the CRISP instrument at the Swedish 1 m Solar
Telescope. The observations in the chromospheric Ca II lines exhibit
intense brightening near the flare footpoints. At these locations,
a nonlocal thermodynamic equilibrium inversion code was employed to
infer the temperature, magnetic field, line-of-sight (LOS) velocity, and
microturbulent velocity stratification in the flaring atmosphere. The
temporal analysis of the inferred temperature at the flare footpoints
shows that the flaring atmosphere from log τ500 ∼ −2.5
to −3.5 is heated up to 7 kK, whereas from log τ500
∼ −3.5 to −5 the inferred temperature ranges between ∼7.5
kK and ∼11 kK. During the flare peak time, the LOS velocity
shows both upflows and downflows around the flare footpoints in the
upper chromosphere and lower chromosphere, respectively. Moreover,
the temporal analysis of the LOS magnetic field at the flare points
exhibits a maximum change of ∼600 G. After the flare, the LOS magnetic
field decreases to the non-flaring value, exhibiting no permanent or
step-wise change. The analysis of response functions to the temperature,
LOS magnetic field, and velocity shows that the Ca II lines exhibit
enhanced sensitivity to the deeper layers (i.e., log τ500
∼ −3) of the flaring atmosphere, whereas for the non-flaring
atmosphere they are mainly sensitive around log τ500 ∼
−4. We suggest that a fraction of the apparent increase in the LOS
magnetic field at the flare footpoints may be due to the increase in the
sensitivity of the Ca II 8542 Å line in the deeper layers, where the
field strength is relatively strong. The rest may be due to magnetic
field reconfiguration during the flare. In the photosphere, we do not
notice significant changes in the physical parameters during the flare
or non-flare times. Our observations illustrate that even a less intense
C-class flare can heat the deeper layers of the solar chromosphere,
mainly at the flare footpoints, without affecting the photosphere.

---------------------------------------------------------
Title: An observationally constrained model of strong magnetic
reconnection in the solar chromosphere. Atmospheric stratification
and estimates of heating rates
Authors: Díaz Baso, C. J.; de la Cruz Rodríguez, J.; Leenaarts, J.
2021A&A...647A.188D Altcode: 2020arXiv201206229D
Context. The evolution of the photospheric magnetic field plays
a key role in the energy transport into the chromosphere and the
corona. In active regions, newly emerging magnetic flux interacts
with the pre-existent magnetic field, which can lead to reconnection
events that convert magnetic energy into thermal energy. Aims:
We aim to study the heating caused by a strong reconnection event
that was triggered by magnetic flux cancelation. Methods: We
use imaging and spectropolarimetric data in the Fe I 6301&amp; 6302
Å, Ca II 8542 Å, and Ca II K spectral lines obtained with the CRISP
and CHROMIS instruments at the Swedish 1-m Solar Telescope. These data
were inverted with the STiC code by performing multi-atom, multi-line,
non-local thermodynamic equilibrium inversions. These inversions yielded
a three-dimensional model of the reconnection event and surrounding
atmosphere, including temperature, velocity, microturbulence, magnetic
field, and radiative loss rate. Results: The model atmosphere
shows the emergence of magnetic loops with a size of several arcseconds
into a pre-existing predominantly unipolar field. Where the reconnection
region is expected to be, we see an increase in the chromospheric
temperature of roughly 2000 K as well as bidirectional flows of the
order of 10 km s−1 emanating from there. We see bright
blobs of roughly 0.2 arcsec in diameter in the Ca II K, moving at a
plane-of-the-sky velocity of the order of 100 km s−1
and a blueshift of 100 km s−1, which we interpret as
ejected plasmoids from the same region. This scenario is consistent
with theoretical reconnection models, and therefore provides evidence
of a reconnection event taking place. The chromospheric radiative
losses at the reconnection site are as high as 160 kW m−2,
providing a quantitative constraint on theoretical models that aim to
simulate reconnection caused by flux emergence in the chromosphere.

---------------------------------------------------------
Title: Design and Performance Analysis of a Highly Efficient
Polychromatic Full Stokes Polarization Modulator for the CRISP
Imaging Spectrometer
Authors: de Wijn, A. G.; de la Cruz Rodríguez, J.; Scharmer, G. B.;
Sliepen, G.; Sütterlin, P.
2021AJ....161...89D Altcode: 2021arXiv210201231D
We present the design and performance of a polychromatic polarization
modulator for the CRisp Imaging SpectroPolarimeter (CRISP) Fabry-Perot
tunable narrow-band imaging spectropolarimer at the Swedish 1 m Solar
Telescope (SST). We discuss the design process in depth, compare
two possible modulator designs through a tolerance analysis, and
investigate thermal sensitivity of the selected design. The trade-offs
and procedures described in this paper are generally applicable in the
development of broadband polarization modulators. The modulator was
built and has been operational since 2015. Its measured performance
is close to optimal between 500 and 900 nm, and differences between
the design and as-built modulator are largely understood. We show some
example data, and briefly review scientific work that used data from
SST/CRISP and this modulator.

---------------------------------------------------------
Title: ALMA and IRIS Observations of the Solar
Chromosphere. II. Structure and Dynamics of Chromospheric Plages
Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora,
Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski,
Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.;
Sainz Dalda, Alberto
2021ApJ...906...83C Altcode: 2020arXiv201205970C
We propose and employ a novel empirical method for determining
chromospheric plage regions, which seems to better isolate a plage from
its surrounding regions than other methods commonly used. We caution
that isolating a plage from its immediate surroundings must be done
with care in order to successfully mitigate statistical biases that,
for instance, can impact quantitative comparisons between different
chromospheric observables. Using this methodology, our analysis suggests
that λ = 1.25 mm free-free emission in plage regions observed with
the Atacama Large Millimeter/submillimeter Array (ALMA)/Band6 may
not form in the low chromosphere as previously thought, but rather
in the upper chromospheric parts of dynamic plage features (such as
spicules and other bright structures), i.e., near geometric heights
of transition-region temperatures. We investigate the high degree of
similarity between chromospheric plage features observed in ALMA/Band6
(at 1.25 mm wavelengths) and the Interface Region Imaging Spectrograph
(IRIS)/Si IV at 1393 Å. We also show that IRIS/Mg II h and k are
not as well correlated with ALMA/Band6 as was previously thought,
and we discuss discrepancies with previous works. Lastly, we report
indications of chromospheric heating due to propagating shocks supported
by the ALMA/Band6 observations.

---------------------------------------------------------
Title: Non-LTE inversions of a confined X2.2 flare. I. The vector
magnetic field in the photosphere and chromosphere
Authors: Vissers, G. J. M.; Danilovic, S.; de la Cruz Rodríguez,
J.; Leenaarts, J.; Morosin, R.; Díaz Baso, C. J.; Reid, A.; Pomoell,
J.; Price, D. J.; Inoue, S.
2021A&A...645A...1V Altcode: 2020arXiv200901537V
Context. Obtaining an accurate measurement of magnetic field vector
in the solar atmosphere is essential for studying changes in field
topology during flares and reliably modelling space weather. 
Aims: We tackle this problem by applying various inversion methods to a
confined X2.2 flare that occurred in NOAA AR 12673 on 6 September 2017
and comparing the photospheric and chromospheric magnetic field vector
with the results of two numerical models of this event. Methods:
We obtained the photospheric magnetic field from Milne-Eddington
and (non-)local thermal equilibrium (non-LTE) inversions of Hinode
SOT/SP Fe I 6301.5 Å and 6302.5 Å. The chromospheric field was
obtained from a spatially regularised weak-field approximation (WFA)
and non-LTE inversions of Ca II 8542 Å observed with CRISP at the
Swedish 1 m Solar Telescope. We investigated the field strengths
and photosphere-to-chromosphere shear in the field vector. 
Results: The LTE- and non-LTE-inferred photospheric magnetic field
components are strongly correlated across several optical depths in
the atmosphere, with a tendency towards a stronger field and higher
temperatures in the non-LTE inversions. For the chromospheric field,
the non-LTE inversions correlate well with the spatially regularised
WFA, especially in terms of the line-of-sight field strength and field
vector orientation. The photosphere exhibits coherent strong-field
patches of over 4.5 kG, co-located with similar concentrations
exceeding 3 kG in the chromosphere. The obtained field strengths
are up to two to three times higher than in the numerical models,
while the photosphere-to-chromosphere shear close to the polarity
inversion line is more concentrated and structured. Conclusions:
In the photosphere, the assumption of LTE for Fe I line formation
does not yield significantly different magnetic field results in
comparison to the non-LTE case, while Milne-Eddington inversions
fail to reproduce the magnetic field vector orientation where Fe
I is in emission. In the chromosphere, the non-LTE-inferred field
is excellently approximated by the spatially regularised WFA. Our
inversions confirm the locations of flux rope footpoints that have
been predicted by numerical models. However, pre-processing and lower
spatial resolution lead to weaker and smoother field in the models than
what our data indicate. This highlights the need for higher spatial
resolution in the models to better constrain pre-eruptive flux ropes.

---------------------------------------------------------
Title: Downflowing umbral flashes as evidence of standing waves in
sunspot umbrae
Authors: Felipe, T.; Henriques, V. M. J.; de la Cruz Rodríguez, J.;
Socas-Navarro, H.
2021A&A...645L..12F Altcode: 2021arXiv210104188F
Context. Umbral flashes are sudden brightenings commonly visible
in the core of some chromospheric lines. Theoretical and numerical
modeling suggests that they are produced by the propagation of shock
waves. According to these models and early observations, umbral flashes
are associated with upflows. However, recent studies have reported
umbral flashes in downflowing atmospheres. Aims: We aim to
understand the origin of downflowing umbral flashes. We explore how
the existence of standing waves in the umbral chromosphere impacts the
generation of flashed profiles. Methods: We performed numerical
simulations of wave propagation in a sunspot umbra with the code
MANCHA. The Stokes profiles of the Ca II 8542 Å line were synthesized
with the NICOLE code. Results: For freely propagating waves,
the chromospheric temperature enhancements of the oscillations are
in phase with velocity upflows. In this case, the intensity core of
the Ca II 8542 Å atmosphere is heated during the upflowing stage of
the oscillation. However, a different scenario with a resonant cavity
produced by the sharp temperature gradient of the transition region
leads to chromospheric standing oscillations. In this situation,
temperature fluctuations are shifted backward and temperature
enhancements partially coincide with the downflowing stage of the
oscillation. In umbral flash events produced by standing oscillations,
the reversal of the emission feature is produced when the oscillation
is downflowing. The chromospheric temperature keeps increasing while
the atmosphere is changing from a downflow to an upflow. During the
appearance of flashed Ca II 8542 Å cores, the atmosphere is upflowing
most of the time, and only 38% of the flashed profiles are associated
with downflows. Conclusions: We find a scenario that remarkably
explains the recent empirical findings of downflowing umbral flashes
as a natural consequence of the presence of standing oscillations
above sunspot umbrae.

---------------------------------------------------------
Title: ALMA and IRIS Observations of the Solar Chromosphere. I. An
On-disk Type II Spicule
Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora,
Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski,
Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.;
Sainz Dalda, Alberto
2021ApJ...906...82C Altcode: 2020arXiv200512717C
We present observations of the solar chromosphere obtained
simultaneously with the Atacama Large Millimeter/submillimeter Array
(ALMA) and the Interface Region Imaging Spectrograph. The observatories
targeted a chromospheric plage region of which the spatial distribution
(split between strongly and weakly magnetized regions) allowed the
study of linear-like structures in isolation, free of contamination
from background emission. Using these observations in conjunction with
a radiative magnetohydrodynamic 2.5D model covering the upper convection
zone all the way to the corona that considers nonequilibrium ionization
effects, we report the detection of an on-disk chromospheric spicule
with ALMA and confirm its multithermal nature.

---------------------------------------------------------
Title: Open Questions and New Challenges in Chromospheric Solar
Physics
Authors: de la Cruz Rodriguez, J.
2020AGUFMSH004..01D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Probing chromospheric heating with millimeter interferometry
Authors: da Silva Santos, J. M.; de la Cruz Rodriguez, J.; White,
S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.; Danilovic, S.
2020AGUFMSH0010001D Altcode:
Observations at visible and ultraviolet wavelengths have shown that
solar active regions host different kinds of small-scale, transient,
bright structures that are believed to be heating events resulting
from the release of magnetic energy in the low atmosphere of the Sun,
especially at the early stages of flux emergence. It is of great
scientific interest to be able to accurately infer temperatures and
formation heights of the most localized events, which are still
matter of debate, in the hope of learning about the evolution of
active regions where occasionally more energetic phenomena lead to
much larger outbursts that propagate across the Solar System. The
millimeter (mm) continuum is a new complementary diagnostic for
chromospheric heating that is now available thanks to the Atacama
Large Millimeter/submillimeter Array (ALMA). We report on the
first ALMA 3 mm observations of small-scale heating events in a
solar active region. In contrast with the low-amplitude brightness
temperature variations in the quiet-Sun, the interferometric maps show
that the active region consists of long, warm, fibril-like structures
that connect magnetic concentrations of opposite polarity and often
flare up along with compact, flickering mm-bursts -- reminiscent of
ultraviolet bursts -- with brightness temperatures of up to 14000 K at
1.2" scales. These events also show simultaneous EUV emission observed
by the Solar Dynamics Observatory (SDO). We find a weak correlation
between the photospheric bright patches and the 3 mm continuum
brightness and, in particular, we do not detect any mm counterpart of
Ellerman bombs which confirms that they are photospheric phenomena. Our observations and modelling highlight the diagnostic capabilities
of ALMA for local heating in solar active regions and emphasize the
need for coordinated observations with IRIS and DKIST in the future.

---------------------------------------------------------
Title: ALMA and IRIS Observations Highlighting the Dynamics and
Structure of Chromospheric Plage
Authors: Chintzoglou, G.; De Pontieu, B.; Martinez-Sykora, J.;
Hansteen, V. H.; de la Cruz Rodriguez, J.; Szydlarski, M.; Jafarzadeh,
S.; Wedemeyer, S.; Bastian, T.; Sainz Dalda, A.
2020AGUFMSH0010009C Altcode:
We present observations of the solar chromosphere obtained
simultaneously with the Atacama Large Millimeter/submillimeter Array
(ALMA) and the Interface Region Imaging Spectrograph (IRIS). The
observatories targeted a chromospheric plage region of which the spatial
distribution (split between strongly and weakly magnetized regions)
allowed the study of linear-like structures in isolation, free of
contamination from background emission. Using these observations
in conjunction with a radiative magnetohydrodynamic 2.5D model
covering the upper convection zone all the way to the corona
that considers non-equilibrium ionization effects, we report the
detection of an on-disk chromospheric spicule with ALMA and confirm
its multithermal nature. In addition, we discuss the strikingly high
degree of similarity between chromospheric plage features observed
in ALMA/Band6 and IRIS/\ion{Si}{4} (also reproduced in our model)
suggesting that ALMA/Band6 does not observe in the low chromosphere as
previously thought but rather observes the upper chromospheric parts
of structures such as spicules and other bright structures above plage
at geometric heights near transition region temperatures. We also show
that IRIS/\ion{Mg}{2} is not as well correlated with ALMA/Band6 as was
previously thought. For these comparisons, we propose and employ a novel
empirical method for the determination of plage regions, which seems
to better isolate plage from its surrounding regions as compared to
other methods commonly used. We caution that isolating plage from its
immediate surroundings must be done with care to mitigate statistical
bias in quantitative comparisons between different chromospheric
observables. Lastly, we report indications for chromospheric heating
due to traveling shocks supported by the ALMA/Band6 observations.

---------------------------------------------------------
Title: Inference of the chromospheric magnetic field configuration
of solar plage using the Ca II 8542 Å line
Authors: Pietrow, A. G. M.; Kiselman, D.; de la Cruz Rodríguez, J.;
Díaz Baso, C. J.; Pastor Yabar, A.; Yadav, R.
2020A&A...644A..43P Altcode: 2020arXiv200614486P
Context. It has so far proven impossible to reproduce all aspects of
the solar plage chromosphere in quasi-realistic numerical models. The
magnetic field configuration in the lower atmosphere is one of the
few free parameters in such simulations. The literature only offers
proxy-based estimates of the field strength, as it is difficult to
obtain observational constraints in this region. Sufficiently sensitive
spectro-polarimetric measurements require a high signal-to-noise
ratio, spectral resolution, and cadence, which are at the limit
of current capabilities. Aims: We use critically sampled
spectro-polarimetric observations of the Ca II 8542 Å line obtained
with the CRISP instrument of the Swedish 1-m Solar Telescope to study
the strength and inclination of the chromospheric magnetic field of
a plage region. This will provide direct physics-based estimates of
these values, which could aid modelers to put constraints on plage
models. Methods: We increased the signal-to-noise ratio of the
data by applying several methods including deep learning and PCA. We
estimated the noise level to be 1 × 10-3 Ic. We
then used STiC, a non-local thermodynamic equilibrium inversion
code to infer the atmospheric structure and magnetic field pixel by
pixel. Results: We are able to infer the magnetic field strength
and inclination for a plage region and for fibrils in the surrounding
canopy. In the plage we report an absolute field strength of |B| = 440
± 90 G, with an inclination of 10° ±16° with respect to the local
vertical. This value for |B| is roughly double of what was reported
previously, while the inclination matches previous studies done in
the photosphere. In the fibrillar region we found |B| = 300 ± 50 G,
with an inclination of 50° ±13°.

---------------------------------------------------------
Title: ALMA observations of transient heating in a solar active region
Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; White,
S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.
2020A&A...643A..41D Altcode: 2020arXiv200614564D
 Aims: We aim to investigate the temperature enhancements and
formation heights of solar active-region brightenings such as Ellerman
bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region
fibrils (FAFs) using interferometric observations in the millimeter
(mm) continuum provided by the Atacama Large Millimeter/submillimeter
Array (ALMA). Methods: We examined 3 mm signatures of heating
events identified in Solar Dynamics Observatory observations of an
active region and compared the results with synthetic spectra from a 3D
radiative magnetohydrodynamic simulation. We estimated the contribution
from the corona to the mm brightness using differential emission measure
analysis. Results: We report the null detection of EBs in the 3
mm continuum at ∼1.2″ spatial resolution, which is evidence that
they are sub-canopy events that do not significantly contribute to
heating the upper chromosphere. In contrast, we find the active region
to be populated with multiple compact, bright, flickering mm-bursts -
reminiscent of UVBs. The high brightness temperatures of up to ∼14 200
K in some events have a contribution (up to ∼7%) from the corona. We
also detect FAF-like events in the 3 mm continuum. These events show
rapid motions of &gt; 10 kK plasma launched with high plane-of-sky
velocities (37 - 340 km s-1) from bright kernels. The mm FAFs
are the brightest class of warm canopy fibrils that connect magnetic
regions of opposite polarities. The simulation confirms that ALMA
should be able to detect the mm counterparts of UVBs and small flares
and thus provide a complementary diagnostic for localized heating in the
solar chromosphere. Movie associated to Fig. 5 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038755/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Stratification of canopy magnetic fields in a plage
region. Constraints from a spatially-regularized weak-field
approximation method
Authors: Morosin, Roberta; de la Cruz Rodríguez, Jaime; Vissers,
Gregal J. M.; Yadav, Rahul
2020A&A...642A.210M Altcode: 2020arXiv200614487M
Context. 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; the details of which have not been well established with
observational studies for many chromospheric scenarios. High-resolution
studies of chromospheric magnetic fields in plage are very scarce
or non existent in general. Aims: Our aim is to study the
stratification of the magnetic field vector in plage regions. Previous
studies predict the presence of a magnetic canopy in the chromosphere
that has not yet been studied with full-Stokes observations. We
use high-spatial resolution full-Stokes observations acquired with
the CRisp Imaging Spectro-Polarimeter (CRISP) at the Swedish 1-m
Solar Telescope in the Mg I 5173 Å, Na I 5896 Å and Ca II 8542
Å lines. Methods: 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. Results: 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 (z ≈ 760 km) is 658 G. At z = 1160 km,
we estimate ⟨B∥⟩ ≈ 417 G. Conclusions:
In this study we propose a modification to the WFA that improves
its applicability to data with a worse signal-to-noise ratio. We
have used this technique to study the magnetic properties of the hot
chromospheric canopy that is observed in plage regions. The methods
described in this paper provide a quick and reliable way of studying
multi layer magnetic field observations without the many difficulties
inherent to other inversion methods.

---------------------------------------------------------
Title: Signatures of ubiquitous magnetic reconnection in the lower
solar atmosphere
Authors: Joshi, Jayant; Rouppe van der Voort, Luc H. M.; de la Cruz
Rodríguez, Jaime
2020A&A...641L...5J Altcode: 2020arXiv200614975J
Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in
the quiet Sun, also known as quiet Sun Ellerman bombs (QSEBs), are a
signature of the fundamental process of magnetic reconnection at the
smallest observable scale in the lower solar atmosphere. We analyze
high spatial resolution observations (0.″1)
obtained with the Swedish 1-m Solar Telescope to explore signatures of
QSEBs in the Hβ line. We find that QSEBs are ubiquitous and uniformly
distributed throughout the quiet Sun, predominantly occurring in
intergranular lanes. We find up to 120 QSEBs in the field of view for a
single moment in time; this is more than an order of magnitude higher
than the number of QSEBs found in earlier Hα observations. This
suggests that about half a million QSEBs could be present in the
lower solar atmosphere at any given time. The QSEB brightenings
found in the Hβ line wings also persist in the line core with a
temporal delay and spatial offset toward the nearest solar limb. Our
results suggest that QSEBs emanate through magnetic reconnection along
vertically extended current sheets in the lower solar atmosphere. The
apparent omnipresence of small-scale magnetic reconnection may play
an important role in the energy balance of the solar chromosphere. Movies associated to Figs. 1-3, B1, and B2 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038769/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: High-resolution observations of the solar photosphere,
chromosphere, and transition region. A database of coordinated IRIS
and SST observations
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson,
M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.;
Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques,
V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.;
Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.;
Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.;
Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski,
M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P.
2020A&A...641A.146R Altcode: 2020arXiv200514175R
NASA's Interface Region Imaging Spectrograph (IRIS) provides
high-resolution observations of the solar atmosphere through ultraviolet
spectroscopy and imaging. Since the launch of IRIS in June 2013, we
have conducted systematic observation campaigns in coordination with
the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides
complementary high-resolution observations of the photosphere and
chromosphere. The SST observations include spectropolarimetric imaging
in photospheric Fe I lines and spectrally resolved imaging in the
chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present
a database of co-aligned IRIS and SST datasets that is open for
analysis to the scientific community. The database covers a variety
of targets including active regions, sunspots, plages, the quiet Sun,
and coronal holes.

---------------------------------------------------------
Title: Physical properties of bright Ca II K fibrils in the solar
chromosphere
Authors: Kianfar, Sepideh; Leenaarts, Jorrit; Danilovic, Sanja;
de la Cruz Rodríguez, Jaime; Díaz Baso, Carlos José
2020A&A...637A...1K Altcode: 2020arXiv200311302K
Context. Broad-band images of the solar chromosphere in the Ca
II H&amp;K line cores around active regions are covered with fine
bright elongated structures called bright fibrils. The mechanisms
that form these structures and cause them to appear bright are still
unknown. Aims: We aim to investigate the physical properties,
such as temperature, line-of-sight velocity, and microturbulence,
in the atmosphere that produces bright fibrils and to compare those
to the properties of their surrounding atmosphere. Methods:
We used simultaneous observations of a plage region in Fe I 6301-2
Å, Ca II 8542 Å, Ca II K, and Hα acquired by the CRISP and CHROMIS
instruments on the Swedish 1 m Solar Telescope. We manually selected
a sample of 282 Ca II K bright fibrils. We compared the appearance
of the fibrils in our sample to the Ca II 8542 Å and Hα data. We
performed non-local thermodynamic equilibrium inversions using the
inversion code STiC on the Fe I 6301-2 Å, Ca II 8542 Å, and Ca II
K lines to infer the physical properties of the atmosphere. 
Results: The line profiles in bright fibrils have a higher intensity in
their K2 peaks compared to profiles formed in the surrounding
atmosphere. The inversion results show that the atmosphere in fibrils is
on average -100 K hotter at an optical depth log(τ500 nm)
= -4.3 compared to their surroundings. The line-of-sight velocity
at chromospheric heights in the fibrils does not show any preference
towards upflows or downflows. The microturbulence in the fibrils is on
average 0.5 km s-1 higher compared to their surroundings. Our
results suggest that the fibrils have a limited extent in height, and
they should be viewed as hot threads pervading the chromosphere. Movies associated to Figs. 9, 11, and 15 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202037572/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: The Sun at millimeter wavelengths. I. Introduction to ALMA
Band 3 observations
Authors: Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin;
Eklund, Henrik; Guevara Gomez, Juan Camilo; Bastian, Tim; Fleck,
Bernhard; de la Cruz Rodriguez, Jaime; Rodger, Andrew; Carlsson, Mats
2020A&A...635A..71W Altcode: 2020arXiv200102185W
Context. The Atacama Large Millimeter/submillimeter Array (ALMA) started
regular observations of the Sun in 2016, first offering receiver Band
3 at wavelengths near 3 mm (100 GHz) and Band 6 at wavelengths around
1.25 mm (239 GHz). Aims: Here we present an initial study
of one of the first ALMA Band 3 observations of the Sun. Our aim is
to characterise the diagnostic potential of brightness temperatures
measured with ALMA on the Sun. Methods: The observation covers
a duration of 48 min at a cadence of 2 s targeting a quiet Sun region
at disc-centre. Corresponding time series of brightness temperature
maps are constructed with the first version of the Solar ALMA Pipeline
and compared to simultaneous observations with the Solar Dynamics
Observatory (SDO). Results: The angular resolution of the
observations is set by the synthesised beam, an elliptical Gaussian
that is approximately 1.4″ × 2.1″ in size. The ALMA maps exhibit
network patches, internetwork regions, and elongated thin features
that are connected to large-scale magnetic loops, as confirmed by a
comparison with SDO maps. The ALMA Band 3 maps correlate best with
the SDO/AIA 171 Å, 131 Å, and 304 Å channels in that they exhibit
network features and, although very weak in the ALMA maps, imprints
of large-scale loops. A group of compact magnetic loops is very
clearly visible in ALMA Band 3. The brightness temperatures in the
loop tops reach values of about 8000-9000 K and in extreme moments
up to 10 000 K. Conclusions: ALMA Band 3 interferometric
observations from early observing cycles already reveal temperature
differences in the solar chromosphere. The weak imprint of magnetic
loops and the correlation with the 171, 131, and 304 SDO channels
suggests, however, that the radiation mapped in ALMA Band 3 might
have contributions from a wider range of atmospheric heights than
previously assumed, but the exact formation height of Band 3 needs to
be investigated in more detail. The absolute brightness temperature
scale as set by total power measurements remains less certain and
must be improved in the future. Despite these complications and the
limited angular resolution, ALMA Band 3 observations have a large
potential for quantitative studies of the small-scale structure and
dynamics of the solar chromosphere. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201937122/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: The Formation Height of Millimeter-wavelength Emission in
the Solar Chromosphere
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; de la Cruz
Rodriguez, Jaime; Chintzoglou, Georgios
2020ApJ...891L...8M Altcode: 2020arXiv200110645M
In the past few years, the ALMA radio telescope has become available
for solar observations. ALMA diagnostics of the solar atmosphere are of
high interest because of the theoretically expected linear relationship
between the brightness temperature at millimeter wavelengths and
the local gas temperature in the solar atmosphere. Key for the
interpretation of solar ALMA observations is understanding where in
the solar atmosphere the ALMA emission originates. Recent theoretical
studies have suggested that ALMA bands at 1.2 (band 6) and 3 mm
(band 3) form in the middle and upper chromosphere at significantly
different heights. We study the formation of ALMA diagnostics using
a 2.5D radiative MHD model that includes the effects of ion-neutral
interactions (ambipolar diffusion) and nonequilibrium ionization
of hydrogen and helium. Our results suggest that in active regions
and network regions, observations at both wavelengths most often
originate from similar heights in the upper chromosphere, contrary to
previous results. Nonequilibrium ionization increases the opacity in the
chromosphere so that ALMA mostly observes spicules and fibrils along the
canopy fields. We combine these modeling results with observations from
IRIS, SDO, and ALMA to suggest a new interpretation for the recently
reported "dark chromospheric holes," regions of very low temperatures
in the chromosphere.

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Title: The multi-thermal chromosphere. Inversions of ALMA and
IRIS data
Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; Leenaarts,
J.; Chintzoglou, G.; De Pontieu, B.; Wedemeyer, S.; Szydlarski, M.
2020A&A...634A..56D Altcode: 2019arXiv191209886D
Context. Numerical simulations of the solar chromosphere predict a
diverse thermal structure with both hot and cool regions. Observations
of plage regions in particular typically feature broader and brighter
chromospheric lines, which suggests that they are formed in hotter
and denser conditions than in the quiet Sun, but also implies a
nonthermal component whose source is unclear. Aims: We revisit
the problem of the stratification of temperature and microturbulence
in plage and the quiet Sun, now adding millimeter (mm) continuum
observations provided by the Atacama Large Millimiter Array (ALMA) to
inversions of near-ultraviolet Interface Region Imaging Spectrograph
(IRIS) spectra as a powerful new diagnostic to disentangle the
two parameters. We fit cool chromospheric holes and track the fast
evolution of compact mm brightenings in the plage region. 
Methods: We use the STiC nonlocal thermodynamic equilibrium (NLTE)
inversion code to simultaneously fit real ultraviolet and mm spectra
in order to infer the thermodynamic parameters of the plasma. 
Results: We confirm the anticipated constraining potential of ALMA
in NLTE inversions of the solar chromosphere. We find significant
differences between the inversion results of IRIS data alone compared to
the results of a combination with the mm data: the IRIS+ALMA inversions
have increased contrast and temperature range, and tend to favor lower
values of microturbulence (∼3-6 km s-1 in plage compared
to ∼4-7 km s-1 from IRIS alone) in the chromosphere. The
average brightness temperature of the plage region at 1.25 mm is 8500
K, but the ALMA maps also show much cooler (∼3000 K) and hotter
(∼11 000 K) evolving features partially seen in other diagnostics. To
explain the former, the inversions require the existence of localized
low-temperature regions in the chromosphere where molecules such as CO
could form. The hot features could sustain such high temperatures due to
non-equilibrium hydrogen ionization effects in a shocked chromosphere
- a scenario that is supported by low-frequency shock wave patterns
found in the Mg II lines probed by IRIS.

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Title: Three-dimensional magnetic field structure of a flux-emerging
region in the solar atmosphere
Authors: Yadav, Rahul; de la Cruz Rodríguez, Jaime; Díaz Baso,
Carlos José; Prasad, Avijeet; Libbrecht, Tine; Robustini, Carolina;
Asensio Ramos, Andrés
2019A&A...632A.112Y Altcode: 2019arXiv191013279Y
We analyze high-resolution spectropolarimetric observations of a
flux-emerging region (FER) in order to understand its magnetic and
kinematic structure. Our spectropolarimetric observations in the He
I 10830 Å spectral region of a FER were recorded with GRIS at the
1.5 m aperture GREGOR telescope. A Milne-Eddington-based inversion
code was employed to extract the photospheric information of the Si I
spectral line, whereas the He I triplet line was analyzed with the Hazel
inversion code, which takes into account the joint action of the Hanle
and the Zeeman effects. The spectropolarimetric analysis of the Si I
line reveals a complex magnetic structure near the vicinity of the FER,
where a weak (350-600 G) and horizontal magnetic field was observed. In
contrast to the photosphere, the analysis of the He I triplet presents
a smooth variation of the magnetic field vector (ranging from 100 to
400 G) and velocities across the FER. Moreover, we find supersonic
downflows of ∼40 km s-1 appearing near the foot points
of loops connecting two pores of opposite polarity, whereas strong
upflows of 22 km s-1 appear near the apex of the loops. At
the location of supersonic downflows in the chromosphere, we observed
downflows of 3 km s-1 in the photosphere. Furthermore,
nonforce-free field extrapolations were performed separately at
two layers in order to understand the magnetic field topology of
the FER. We determine, using extrapolations from the photosphere and
the observed chromospheric magnetic field, that the average formation
height of the He I triplet line is ∼2 Mm from the solar surface. The
reconstructed loops using photospheric extrapolations along an arch
filament system have a maximum height of ∼10.5 Mm from the solar
surface with a foot-point separation of ∼19 Mm, whereas the loops
reconstructed using chromospheric extrapolations reach around ∼8.4
Mm above the solar surface with a foot-point separation of ∼16 Mm at
the chromospheric height. The magnetic topology in the FER suggests
the presence of small-scale loops beneath the large loops. Under
suitable conditions, due to magnetic reconnection, these loops can
trigger various heating events in the vicinity of the FER.

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Title: Three-dimensional modeling of chromospheric spectral lines
in a simulated active region
Authors: Bjørgen, Johan P.; Leenaarts, Jorrit; Rempel, Matthias;
Cheung, Mark C. M.; Danilovic, Sanja; de la Cruz Rodríguez, Jaime;
Sukhorukov, Andrii V.
2019A&A...631A..33B Altcode: 2019arXiv190601098B
Context. Because of the complex physics that governs the formation of
chromospheric lines, interpretation of solar chromospheric observations
is difficult. The origin and characteristics of many chromospheric
features are, because of this, unresolved. Aims: We focus on
studying two prominent features: long fibrils and flare ribbons. To
model these features, we use a 3D magnetohydrodynamic simulation of
an active region, which self-consistently reproduces both of these
features. Methods: We modeled the Hα, Mg II k, Ca II K,
and Ca II 8542 Å lines using the 3D non-LTE radiative transfer
code Multi3D. To obtain non-LTE electron densities, we solved the
statistical equilibrium equations for hydrogen simultaneously with the
charge conservation equation. We treated the Ca II K and Mg II k lines
with partially coherent scattering. Results: This simulation
reproduces long fibrils that span between the opposite-polarity
sunspots and go up to 4 Mm in height. They can be traced in all lines
owing to density corrugation. In contrast to previous studies, Hα,
Mg II h&amp;k, and Ca II H&amp;K are formed at similar height in this
model. Although some of the high fibrils are also visible in the Ca II
8542 Å line, this line tends to sample loops and shocks lower in the
chromosphere. Magnetic field lines are aligned with the Hα fibrils,
but the latter holds to a lesser extent for the Ca II 8542 Å line. The
simulation shows structures in the Hα line core that look like flare
ribbons. The emission in the ribbons is caused by a dense chromosphere
and a transition region at high column mass. The ribbons are visible in
all chromospheric lines, but least prominent in Ca II 8542 Å line. In
some pixels, broad asymmetric profiles with a single emission peak
are produced similar to the profiles observed in flare ribbons. They
are caused by a deep onset of the chromospheric temperature rise
and large velocity gradients. Conclusions: The simulation
produces long fibrils similar to what is seen in observations. It
also produces structures similar to flare ribbons despite the lack
of nonthermal electrons in the simulation. The latter suggests that
thermal conduction might be a significant agent in transporting flare
energy to the chromosphere in addition to nonthermal electrons.

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Title: A method for global inversion of multi-resolution solar data
Authors: de la Cruz Rodríguez, J.
2019A&A...631A.153D Altcode: 2019arXiv190902604D
Understanding the complex dynamics and structure of the upper solar
atmosphere strongly benefits from the use of a combination of several
diagnostics. Frequently, such diverse diagnostics can only be obtained
from telescopes and/or instrumentation operating at widely different
spatial resolution. To optimize the utilization of such data, we propose
a new method for the global inversion of data acquired at different
spatial resolution. The method has its roots in the Levenberg-Marquardt
algorithm but involves the use of linear operators to transform and
degrade the synthetic spectra of a highly resolved guess model to
account for the effects of spatial resolution, data sampling, alignment,
and image rotation of each of the datasets. We have carried out a
list of numerical experiments to show that our method allows for the
extraction of spatial information from two simulated datasets that have
gone through two different telescope apertures and that are sampled in
different spatial grids. Our results show that each dataset contributes
in the inversion by constraining information at the spatial scales
that are present in each of the datasets, and no negative effects are
derived from the combination of multiple resolution data. This method
is especially relevant for chromospheric studies that attempt to combine
datasets acquired with different telescopes and/or datasets acquired at
different wavelengths. The techniques described in the present study
will also help to address the ever increasing resolution gap between
space-borne missions and forthcoming ground-based facilities.

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Title: Solar image denoising with convolutional neural networks
Authors: Díaz Baso, C. J.; de la Cruz Rodríguez, J.; Danilovic, S.
2019A&A...629A..99D Altcode: 2019arXiv190802815D
The topology and dynamics of the solar chromosphere are greatly
affected by the presence of magnetic fields. The magnetic field
can be inferred by analyzing polarimetric observations of spectral
lines. Polarimetric signals induced by chromospheric magnetic fields
are, however, particularly weak, and in most cases very close to
the detection limit of current instrumentation. Because of this,
there are only few observational studies that have successfully
reconstructed the three components of the magnetic field vector in the
chromosphere. Traditionally, the signal-to-noise ratio of observations
has been improved by performing time-averages or spatial averages, but
in both cases, some information is lost. More advanced techniques, like
principal-component analysis, have also been employed to take advantage
of the sparsity of the observations in the spectral direction. In the
present study, we use the spatial coherence of the observations to
reduce the noise using deep-learning techniques. We designed a neural
network that is capable of recovering weak signals under a complex
noise corruption (including instrumental artifacts and non-linear
post-processing). The training of the network is carried out without
a priori knowledge of the clean signals, or an explicit statistical
characterization of the noise or other corruption. We only use the same
observations as our generative model. The performance of this method
is demonstrated on both synthetic experiments and real data. We show
examples of the improvement in typical signals obtained in current
telescopes such as the Swedish 1 m Solar Telescope. The presented
method can recover weak signals equally well no matter what spectral
line or spectral sampling is used. It is especially suitable for cases
when the wavelength sampling is scarce.

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Title: Chromospheric polarimetry through multiline observations of
the 850 nm spectral region III: Chromospheric jets driven by twisted
magnetic fields
Authors: Quintero Noda, C.; Iijima, H.; Katsukawa, Y.; Shimizu,
T.; Carlsson, M.; de la Cruz Rodríguez, J.; Ruiz Cobo, B.; Orozco
Suárez, D.; Oba, T.; Anan, T.; Kubo, M.; Kawabata, Y.; Ichimoto,
K.; Suematsu, Y.
2019MNRAS.486.4203Q Altcode: 2019MNRAS.tmp.1081N; 2019arXiv190409151Q
We investigate the diagnostic potential of the spectral lines at 850
nm for understanding the magnetism of the lower atmosphere. For that
purpose, we use a newly developed 3D simulation of a chromospheric
jet to check the sensitivity of the spectral lines to this phenomenon
as well as our ability to infer the atmospheric information through
spectropolarimetric inversions of noisy synthetic data. We start
comparing the benefits of inverting the entire spectrum at 850 nm versus
only the Ca II 8542 Å spectral line. We found a better match of the
input atmosphere for the former case, mainly at lower heights. However,
the results at higher layers were not accurate. After several tests,
we determined that we need to weight more the chromospheric lines
than the photospheric ones in the computation of the goodness of the
fit. The new inversion configuration allows us to obtain better fits and
consequently more accurate physical parameters. Therefore, to extract
the most from multiline inversions, a proper set of weights needs to
be estimated. Besides that, we conclude again that the lines at 850
nm, or a similar arrangement with Ca II 8542 Å plus Zeeman-sensitive
photospheric lines, pose the best-observing configuration for examining
the thermal and magnetic properties of the lower solar atmosphere.

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Title: Dissecting bombs and bursts: non-LTE inversions of
low-atmosphere reconnection in SST and IRIS observations
Authors: Vissers, G. J. M.; de la Cruz Rodríguez, J.; Libbrecht,
T.; Rouppe van der Voort, L. H. M.; Scharmer, G. B.; Carlsson, M.
2019A&A...627A.101V Altcode: 2019arXiv190502035V
Ellerman bombs and UV bursts are transient brightenings that are
ubiquitously observed in the lower atmospheres of active and emerging
flux regions. As they are believed to pinpoint sites of magnetic
reconnection in reconfiguring fields, understanding their occurrence
and detailed evolution may provide useful insight into the overall
evolution of active regions. Here we present results from inversions
of SST/CRISP and CHROMIS, as well as IRIS data of such transient
events. Combining information from the Mg II h &amp; k, Si IV, and Ca
II 8542 Å and Ca II H &amp; K lines, we aim to characterise their
temperature and velocity stratification, as well as their magnetic
field configuration. We find average temperature enhancements of
a few thousand kelvin, close to the classical temperature minimum
and similar to previous studies, but localised peak temperatures
of up to 10 000-15 000 K from Ca II inversions. Including Mg II
appears to generally dampen these temperature enhancements to below
8000 K, while Si IV requires temperatures in excess of 10 000 K at
low heights, but may also be reproduced with secondary temperature
enhancements of 35 000-60 000 K higher up. However, reproducing Si
IV comes at the expense of overestimating the Mg II emission. The
line-of-sight velocity maps show clear bi-directional jet signatures
for some events and strong correlation with substructure in the
intensity images in general. Absolute line-of-sight velocities range
between 5 and 20 km s-1 on average, with slightly larger
velocities towards, rather than away from, the observer. The inverted
magnetic field parameters show an enhancement of the horizontal
field co-located with the brightenings at heights similar to that of
the temperature increase. We are thus able to largely reproduce the
observational properties of Ellerman bombs with the UV burst signature
(e.g. intensities, profile asymmetries, morphology, and bi-directional
jet signatures), with temperature stratifications peaking close
to the classical temperature minimum. Correctly modelling the Si
IV emission in agreement with all other diagnostics is however an
outstanding issue and remains paramount in explaining its apparent
coincidence with Hα emission. Fine-tuning the approach (accounting for
resolution differences, fitting localised temperature enhancements,
and/or performing spatially coupled inversions) is likely necessary
in order to obtain better agreement between all considered diagnostics.

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Title: Is the sky the limit?. Performance of the revamped Swedish
1-m Solar Telescope and its blue- and red-beam reimaging systems
Authors: Scharmer, G. B.; Löfdahl, M. G.; Sliepen, G.; de la Cruz
Rodríguez, J.
2019A&A...626A..55S Altcode: 2019arXiv190505588S
We discuss the use of measurements of the solar granulation contrast
as a measure of optical quality. We demonstrate that for data recorded
with a telescope that uses adaptive optics and/or post-processing to
compensate for many low- and high-order aberrations, the RMS granulation
contrast is directly proportional to the Strehl ratio calculated
from the residual (small-scale) wavefront error (static and/or from
seeing). We demonstrate that the wings of the high-order compensated
point spread function for the Swedish 1-m Solar Telescope (SST) are
likely to extend to a radius of not more than about 2″, which is
consistent with earlier conclusions drawn from stray-light compensation
of sunspot images. We report on simultaneous measurements of seeing
and solar granulation contrast averaged over 2 s time intervals at
several wavelengths from 525 nm to 853.6 nm on the red-beam (CRISP
beam) and wavelengths from 395 nm to 484 nm on the blue-beam (CHROMIS
beam). These data were recorded with the SST, which has been revamped
with an 85-electrode adaptive mirror and a new tip-tilt mirror, both of
which were polished to exceptionally high optical quality. Compared
to similar data obtained with the previous 37-electrode adaptive
mirror in 2009 and 2011, there is a significant improvement in image
contrast. The highest 2 s average image contrasts measured in April
2015 through 0.3-0.9 nm interference filters at 525 nm, 557 nm, 630
nm, and 853.5 nm with compensation only for the diffraction limited
point spread function of SST are 11.8%, 11.8%, 10.2%, and 7.2%,
respectively. Similarly, the highest 2 s contrasts measured at 395 nm,
400 nm, and 484 nm in May 2016 through 0.37-1.3 nm filters are 16%,
16%, and 12.5%, respectively. The granulation contrast observed with
SST compares favorably to measured values with SOT on Hinode and with
Sunrise as well as major ground-based solar telescopes. Simultaneously
with the above wideband red-beam data, we also recorded narrowband
continuum images with the CRISP imaging spectropolarimeter. We find
that contrasts measured with CRISP are entirely consistent with the
corresponding wideband contrasts, demonstrating that any additional
image degradation by the CRISP etalons and telecentric optical system
is marginal or even insignificant. Finally, we discuss the origin of
the 48 nm RMS wavefront error needed to bring consistency between the
measured granulation contrast and that obtained from 3D simulations
of convection.

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Title: The Dark Side of Penumbral Microjets: Observations in Hα
Authors: Buehler, D.; Esteban Pozuelo, S.; de la Cruz Rodriguez, J.;
Scharmer, G. B.
2019ApJ...876...47B Altcode: 2019arXiv190501245B
We present data of 10 penumbral microjets (PMJs) observed in a Hα,
Ca II 8542 Å, and Fe I 6302 Å line pair with the Swedish 1 m Solar
Telescope (SST) with CRISP and Ca II K with SST/CHROMIS in active region
NOAA 12599 on 2016 October 12 at μ = 0.68. All four Stokes parameters
of the Ca II 8542 Å and Fe I 6302 Å lines were observed and a series
of test pixels were inverted using the Stockholm inversion code. Our
analysis revealed for the first time that PMJs are visible in Hα,
where they appear as dark features with average line-of-sight (LOS)
upflows of 1.1 ± 0.6 km s-1, matching the LOS velocities
from the inversions. Based on the Hα observations we extend the
previous average length and lifetime of PMJs to 2815 ± 530 km and 163
± 25 s, respectively. The plane-of-sky (POS) velocities of our PMJs
of up to 17 km s-1 tend to give increased velocities with
distance traveled. Furthermore, two of our PMJs with significant Stokes
V signal indicate that the PMJs possess an increased LOS magnetic field
of up to 100 G compared to the local pre-/post- PMJ magnetic field,
which propagates as quickly as the PMJs’ POS velocities. Finally,
we present evidence that PMJs display an on average 1 minute gradual
precursory brightening that only manifests itself in the cores of the
Ca II lines. We conclude that PMJs are not ordinary jets but likely
are manifestations of heat fronts that propagate at the local Alfvén
velocity.

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Title: Recovering Thermodynamics from Spectral Profiles observed by
IRIS: A Machine and Deep Learning Approach
Authors: Sainz Dalda, Alberto; de la Cruz Rodríguez, Jaime; De
Pontieu, Bart; Gošić, Milan
2019ApJ...875L..18S Altcode: 2019arXiv190408390S
Inversion codes allow the reconstruction of a model atmosphere from
observations. With the inclusion of optically thick lines that form in
the solar chromosphere, such modeling is computationally very expensive
because a non-LTE evaluation of the radiation field is required. In this
study, we combine the results provided by these traditional methods
with machine and deep learning techniques to obtain similar-quality
results in an easy-to-use, much faster way. We have applied these
new methods to Mg II h and k lines observed by the Interface Region
Imaging Spectrograph (IRIS). As a result, we are able to reconstruct the
thermodynamic state (temperature, line-of-sight velocity, nonthermal
velocities, electron density, etc.) in the chromosphere and upper
photosphere of an area equivalent to an active region in a few CPU
minutes, speeding up the process by a factor of 105 -
106. The open-source code accompanying this Letter will
allow the community to use IRIS observations to open a new window to
a host of solar phenomena.

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Title: Diagnostic potential of the Ca II 8542 Å line for solar
filaments
Authors: Díaz Baso, C. J.; Martínez González, M. J.; Asensio Ramos,
A.; de la Cruz Rodríguez, J.
2019A&A...623A.178D Altcode: 2019arXiv190206574D
 Aims: In this study we explore the diagnostic potential of
the chromospheric Ca II line at 8542 Å for studying the magnetic and
dynamic properties of solar filaments. We have acquired high spatial
resolution spectropolarimetric observations in the Ca II 8542 Å line
using the CRISP instrument at the Swedish 1 m Solar Telescope. 
Methods: We used the NICOLE inversion code to infer physical properties
from observations of a solar filament. We discuss the validity of
the results due to the assumption of hydrostatic equilibrium. We have
used observations from other telescopes such as CHROTEL and SDO, in
order to study large scale dynamics and the long term evolution of the
filament. Results: We show that the Ca II 8542 Å line encodes
information of the temperature, line-of-sight velocity and magnetic
field vector from the region where the filament is located. The current
noise levels only allow us to estimate an upper limit of 260 G for the
total magnetic field of the filament. Our study also reveals that if we
consider information from the aforementioned spectral line alone, the
geometric height, the temperature and the density could be degenerated
parameters outside the hydrostatic equilibrium approach.

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Title: STiC: A multiatom non-LTE PRD inversion code for full-Stokes
solar observations
Authors: de la Cruz Rodríguez, J.; Leenaarts, J.; Danilovic, S.;
Uitenbroek, H.
2019A&A...623A..74D Altcode: 2018arXiv181008441D
The inference of the underlying state of the plasma in the solar
chromosphere remains extremely challenging because of the nonlocal
character of the observed radiation and plasma conditions in this
layer. Inversion methods allow us to derive a model atmosphere that
can reproduce the observed spectra by undertaking several physical
assumptions. The most advanced approaches involve a depth-stratified
model atmosphere described by temperature, line-of-sight velocity,
turbulent velocity, the three components of the magntic field vector,
and gas and electron pressure. The parameters of the radiative transfer
equation are computed from a solid ground of physical principles. In
order to apply these techniques to spectral lines that sample the
chromosphere, nonlocal thermodynamical equilibrium effects must be
included in the calculations. We developed a new inversion code STiC
(STockholm inversion Code) to study spectral lines that sample the
upper chromosphere. The code is based on the RH forward synthesis code,
which we modified to make the inversions faster and more stable. For
the first time, STiC facilitates the processing of lines from multiple
atoms in non-LTE, also including partial redistribution effects (PRD)
in angle and frequency of scattered photons. Furthermore, we include
a regularization strategy that allows for model atmospheres with a
complex depth stratification, without introducing artifacts in the
reconstructed physical parameters, which are usually manifested in
the form of oscillatory behavior. This approach takes steps toward
a node-less inversion, in which the value of the physical parameters
at each grid point can be considered a free parameter. In this paper
we discuss the implementation of the aforementioned techniques, the
description of the model atmosphere, and the optimizations that we
applied to the code. We carry out some numerical experiments to show
the performance of the code and the regularization techniques that we
implemented. We made STiC publicly available to the community.

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Title: Chromospheric observations and magnetic configuration of a
supergranular structure
Authors: Robustini, Carolina; Esteban Pozuelo, Sara; Leenaarts,
Jorrit; de la Cruz Rodríguez, Jaime
2019A&A...621A...1R Altcode: 2018A&A...621A...1R; 2018arXiv181010762R
Context. Unipolar magnetic regions are often associated with
supergranular cells. The chromosphere above these regions is regulated
by the magnetic field, but the field structure is poorly known. In
unipolar regions, the fibrillar arrangement does not always coincide
with magnetic field lines, and polarimetric observations are needed
to establish the chromospheric magnetic topology. Aims: In an
active region close to the limb, we observed a unipolar annular network
of supergranular size. This supergranular structure harbours a radial
distribution of the fibrils converging towards its centre. We aim to
improve the description of this structure by determining the magnetic
field configuration and the line-of-sight velocity distribution in both
the photosphere and the chromosphere. Methods: We observed the
supergranular structure at different heights by taking data in the Fe I
6301-6302 Å, Hα, Ca II 8542 Å, and the Ca II H&amp;K spectral lines
with the CRisp Imaging SpectroPolarimeter (CRISP) and CHROMospheric
Imaging Spectrometer (CHROMIS) at the Swedish 1-m Solar Telescope. We
performed Milne-Eddington inversions of the spectropolarimetric data
of Fe I 6301-6302 Å and applied the weak field approximation to Ca
II 8542 Å data to retrieve the magnetic field in the photosphere
and chromosphere. We used photospheric magnetograms of CRISP, Hinode
Solar Optical Telescope spectropolarimeter, and Helioseismic and
Magnetic Imager to calculate the magnetic flux. We investigated the
velocity distribution using the line-of-sight velocities computed
from the Milne-Eddington inversion and from the Doppler shift of the
K3 feature in the Ca II K spectral line. To describe the
typical spectral profiles characterising the chromosphere above the
inner region of the supergranular structure, we performed a K-mean
clustering of the spectra in Ca II K. Results: The photospheric
magnetic flux shows that the supergranular boundary has an excess
of positive polarity and the whole structure is not balanced. The
magnetic field vector at chromospheric heights, retrieved by the
weak field approximation, indicates that the field lines within the
supergranular cell tend to point inwards, and might form a canopy
above the unipolar region. In the centre of the supergranular cell
hosting the unipolar region, we observe a persistent chromospheric
brightening coinciding with a strong gradient in the line-of-sight
velocity. The movie associated to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201833246/olm">https://www.aanda.org</A>

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Title: Chromospheric condensations and magnetic field in a C3.6-class
flare studied via He I D3 spectro-polarimetry
Authors: Libbrecht, Tine; de la Cruz Rodríguez, Jaime; Danilovic,
Sanja; Leenaarts, Jorrit; Pazira, Hiva
2019A&A...621A..35L Altcode: 2018arXiv180606880L
Context. Magnetic reconnection during flares takes place in the
corona, but a substantial part of flare energy is deposited in
the chromosphere. However, high-resolution spectro-polarimetric
chromospheric observations of flares are very rare. The most used
observables are Ca II 8542 Å and He I 10830 Å. Aims:
We aim to study the chromosphere during a C3.6 class flare via
spectro-polarimetric observations of the He I D3 line. Methods: We present the first SST/CRISP spectro-polarimetric
observations of He I D3. We analyzed the data using the
inversion code HAZEL, and estimate the line-of-sight velocity and
the magnetic field vector. Results: Strong He I D3
emission at the flare footpoints, as well as strong He I D3
absorption profiles tracing the flaring loops are observed during the
flare. The He I D3 traveling emission kernels at the flare
footpoints exhibit strong chromospheric condensations of up to ∼60
km s-1 at their leading edge. Our observations suggest that
such condensations result in shocking the deep chromosphere, causing
broad and modestly blueshifted He I D3 profiles indicating
subsequent upflows. A strong and rather vertical magnetic field of up
to ∼2500 G is measured in the flare footpoints, confirming that the He
I D3 line is likely formed in the deep chromosphere at those
locations. We provide chromospheric line-of-sight velocity and magnetic
field maps obtained via He I D3 inversions. We propose a
fan-spine configuration as the flare magnetic field topology. 
Conclusions: The He I D3 line is an excellent diagnostic to
study the chromosphere during flares. The impact of strong condensations
on the deep chromosphere has been observed. Detailed maps of the flare
dynamics and the magnetic field are obtained.

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Title: Observationally Based Models of Penumbral Microjets
Authors: Esteban Pozuelo, S.; de la Cruz Rodríguez, J.; Drews, A.;
Rouppe van der Voort, L.; Scharmer, G. B.; Carlsson, M.
2019ApJ...870...88E Altcode: 2018arXiv181107881E
We study the polarization signals and physical parameters of penumbral
microjets (PMJs) by using high spatial resolution data taken in the
Fe I 630 nm pair, Ca II 854.2 nm, and Ca II K lines with the CRISP
and CHROMIS instruments at the Swedish 1 m Solar Telescope. We
infer their physical parameters, such as physical observables in
the photosphere and chromospheric velocity diagnostics, by different
methods, including inversions of the observed Stokes profiles with
the STiC code. PMJs harbor overall brighter Ca II K line profiles
and conspicuous polarization signals in Ca II 854.2 nm, specifically
in circular polarization that often shows multiple lobes mainly due
to the shape of Stokes I. They usually overlap photospheric regions
with a sheared magnetic field configuration, suggesting that magnetic
reconnections could play an important role in the origin of PMJs. The
discrepancy between their low LOS velocities and the high apparent
speeds reported on earlier, as well as the existence of different
vertical velocity gradients in the chromosphere, indicate that PMJs
might not be entirely related to mass motions. Instead, PMJs could
be due to perturbation fronts induced by magnetic reconnections
occurring in the deep photosphere that propagate through the
chromosphere. This reconnection may be associated with current heating
that produces temperature enhancements from the temperature minimum
region. Furthermore, enhanced collisions with electrons could also
increase the coupling to the local conditions at higher layers during
the PMJ phase, giving a possible explanation for the enhanced emission
in the overall Ca II K profiles emerging from these transients.

---------------------------------------------------------
Title: Temperature constraints from inversions of synthetic solar
optical, UV, and radio spectra
Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.;
Leenaarts, J.
2018A&A...620A.124D Altcode: 2018arXiv180606682D
Context. High-resolution observations of the solar chromosphere
at millimeter wavelengths are now possible with the Atacama Large
Millimeter Array (ALMA), bringing with them the promise of tackling
many open problems in solar physics. Observations from other ground
and space-based telescopes will greatly benefit from coordinated
endeavors with ALMA, yet the diagnostic potential of combined optical,
ultraviolet and mm observations has remained mostly unassessed. 
Aims: In this paper we investigate whether mm-wavelengths could aid
current inversion schemes to retrieve a more accurate representation
of the temperature structure of the solar atmosphere. Methods:
We performed several non-LTE inversion experiments of the emergent
spectra from a snapshot of 3D radiation-MHD simulation. We included
common line diagnostics such as Ca II H, K, 8542 Å and Mg II h and k,
taking into account partial frequency redistribution effects, along
with the continuum around 1.2 mm and 3 mm. Results: We find
that including the mm-continuum in inversions allows a more accurate
inference of temperature as function of optical depth. The addition
of ALMA bands to other diagnostics should improve the accuracy of the
inferred chromospheric temperatures between log τ ∼ [-6, -4.5]
where the Ca II and Mg II lines are weakly coupled to the local
conditions. However, we find that simultaneous multiatom, non-LTE
inversions of optical and UV lines present equally strong constraints
in the lower chromosphere and thus are not greatly improved by the
1.2 mm band. Nonetheless, the 3 mm band is still needed to better
constrain the mid-upper chromosphere.

---------------------------------------------------------
Title: Real-time, multiframe, blind deconvolution of solar images
Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.; Pastor Yabar, A.
2018A&A...620A..73A Altcode: 2018arXiv180607150A
The quality of images of the Sun obtained from the ground are
severely limited by the perturbing effect of the Earth's turbulent
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 make use of 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.

---------------------------------------------------------
Title: Magnetic field variations associated with umbral flashes and
penumbral waves
Authors: Joshi, Jayant; de la Cruz Rodríguez, Jaime
2018A&A...619A..63J Altcode: 2018arXiv180301737J
Context. Umbral flashes (UF) and running penumbral waves (RPWs)
in sunspot chromospheres leave a dramatic imprint in the intensity
profile of the Ca II 8542 Å line. Recent studies have focussed on also
explaining the observed polarization profiles, which show even more
dramatic variations during the passage of these shock fronts. While most
of these variations can be explained with an almost constant magnetic
field as a function of time, several studies have reported changes in
the inferred magnetic field strength during UF phases. These changes
could be explained by opacity effects or by intrinsic changes in the
magnetic field strength. Aims: In this study we investigate the
origin of these periodic variations of the magnetic field strength by
analyzing a time-series of high-temporal-cadence observations acquired
in the Ca II 8542 Å line with the CRISP instrument at the Swedish 1-m
Solar Telescope. In particular, we analyze how the inferred geometrical
height scale changes between quiescent and UF phases, and whether those
changes are enough to explain the observed changes in the magnetic
field, B. Methods: We have performed non local thermodynamical
equilibrium (non-LTE) data inversions with the NICOLE code of a
time-series of very high spatio-temporal-resolution observations in
the Ca II 8542 Å, Fe I 6301.5, and Fe I 6302.5 Å lines. We analyze
in detail the variations of the different physical parameters of the
model as a function of time. Results: Our results indicate
that the Ca II 8542 Å line in sunspots is greatly sensitive to
magnetic fields at log τ500 = -5 (hereafter log τ = -5)
during UFs and quiescence. However this optical depth value does not
correspond to the same geometrical height during the two phases. Our
results indicate that during UFs and RPWs the log τ = -5 is located
at a higher geometrical height than during quiescence. Additionally,
the inferred magnetic field values are higher in UFs (up to ∼270 G)
and in RPWs (∼100 G). Conclusions: Our results suggest that
opacity changes caused by UFs and RPWs cannot explain the observed
temporal variations in the magnetic field, as the line seems to form
at higher geometrical heights where the field is expected to be lower.

---------------------------------------------------------
Title: STiC: Stockholm inversion code
Authors: de la Cruz Rodríguez, J.; Leenaarts, J.; Danilovic, S.;
Uitenbroek, H.
2018ascl.soft10014D Altcode:
STiC is a MPI-parallel non-LTE inversion code for observed full-Stokes
observations. The code processes lines from multiple atoms in non-LTE,
including partial redistribution effects of scattered photons in
angle and frequency of scattered photons (PRD), and can be used with
model atmospheres that have a complex depth stratification without
introducing artifacts.

---------------------------------------------------------
Title: Modeling of the Hydrogen Lyman Lines in Solar Flares
Authors: Brown, Stephen A.; Fletcher, Lyndsay; Kerr, Graham S.;
Labrosse, Nicolas; Kowalski, Adam F.; De La Cruz Rodríguez, Jaime
2018ApJ...862...59B Altcode: 2018arXiv180703373B
The hydrogen Lyman lines (91.2 nm &lt; λ &lt; 121.6 nm) are significant
contributors to the radiative losses of the solar chromosphere, and
they are enhanced during flares. We have shown previously that the Lyman
lines observed by the Extreme Ultraviolet Variability instrument onboard
the Solar Dynamics Observatory exhibit Doppler motions equivalent
to speeds on the order of 30 km s-1. However, contrary to
expectations, both redshifts and blueshifts were present and no dominant
flow direction was observed. To understand the formation of the Lyman
lines, particularly their Doppler motions, we have used the radiative
hydrodynamic code, RADYN, along with the radiative transfer code, RH,
to simulate the evolution of the flaring chromosphere and the response
of the Lyman lines during solar flares. We find that upflows in the
simulated atmospheres lead to blueshifts in the line cores, which
exhibit central reversals. We then model the effects of the instrument
on the profiles, using the Extreme Ultraviolet Variability Experiment
(EVE) instrument's properties. What may be interpreted as downflows
(redshifted emission) in the lines, after they have been convolved
with the instrumental line profile, may not necessarily correspond to
actual downflows. Dynamic features in the atmosphere can introduce
complex features in the line profiles that will not be detected by
instruments with the spectral resolution of EVE, but which leave more
of a signature at the resolution of the Spectral Investigation of the
Coronal Environment instrument onboard the Solar Orbiter.

---------------------------------------------------------
Title: Spectropolarimetric Inversions of the Ca II 8542 Å Line in
an M-class Solar Flare
Authors: Kuridze, D.; Henriques, V. M. J.; Mathioudakis, M.; Rouppe
van der Voort, L.; de la Cruz Rodríguez, J.; Carlsson, M.
2018ApJ...860...10K Altcode: 2018arXiv180500487K
We study the M1.9-class solar flare SOL2015-09-27T10:40 UT using
high-resolution full Stokes imaging spectropolarimetry of the Ca II
8542 Å line obtained with the CRISP imaging spectropolarimeter at the
Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the
non-LTE code NICOLE are used to construct semiempirical models of the
flaring atmosphere to investigate the structure and evolution of the
flare temperature and magnetic field. A comparison of the temperature
stratification in flaring and nonflaring areas reveals strong heating
of the flare ribbon during the flare peak. The polarization signals
of the ribbon in the chromosphere during the flare maximum become
stronger when compared to its surroundings and to pre- and post-flare
profiles. Furthermore, a comparison of the response functions to
perturbations in the line-of-sight magnetic field and temperature in
flaring and nonflaring atmospheres shows that during the flare, the
Ca II 8542 Å line is more sensitive to the lower atmosphere where the
magnetic field is expected to be stronger. The chromospheric magnetic
field was also determined with the weak-field approximation, which
led to results similar to those obtained with the NICOLE inversions.

---------------------------------------------------------
Title: Chromospheric Heating due to Cancellation of Quiet Sun
Internetwork Fields
Authors: Gošić, M.; de la Cruz Rodríguez, J.; De Pontieu, B.; Bellot
Rubio, L. R.; Carlsson, M.; Esteban Pozuelo, S.; Ortiz, A.; Polito, V.
2018ApJ...857...48G Altcode: 2018arXiv180207392G
The heating of the solar chromosphere remains one of the most
important questions in solar physics. Our current understanding is that
small-scale internetwork (IN) magnetic fields play an important role
as a heating agent. Indeed, cancellations of IN magnetic elements in
the photosphere can produce transient brightenings in the chromosphere
and transition region. These bright structures might be the signature
of energy release and plasma heating, probably driven by the magnetic
reconnection of IN field lines. Although single events are not expected
to release large amounts of energy, their global contribution to the
chromosphere may be significant due to their ubiquitous presence
in quiet Sun regions. In this paper, we study cancellations of IN
elements and analyze their impact on the energetics and dynamics of
the quiet Sun atmosphere. We use high-resolution, multiwavelength,
coordinated observations obtained with the Interface Region Imaging
Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify
cancellations of IN magnetic flux patches and follow their evolution. We
find that, on average, these events live for ∼3 minutes in the
photosphere and ∼12 minutes in the chromosphere and/or transition
region. Employing multi-line inversions of the Mg II h and k lines,
we show that cancellations produce clear signatures of heating in the
upper atmospheric layers. However, at the resolution and sensitivity
accessible to the SST, their number density still seems to be one
order of magnitude too low to explain the global chromospheric heating.

---------------------------------------------------------
Title: Chromospheric heating during flux emergence in the solar
atmosphere
Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Danilovic,
Sanja; Scharmer, Göran; Carlsson, Mats
2018A&A...612A..28L Altcode: 2017arXiv171200474L
Context. The radiative losses in the solar chromosphere vary from
4 kW m-2 in the quiet Sun, to 20 kW m-2 in
active regions. The mechanisms that transport non-thermal energy to
and deposit it in the chromosphere are still not understood. Aim. We
aim to investigate the atmospheric structure and heating of the solar
chromosphere in an emerging flux region. Methods: We have used
observations taken with the CHROMIS and CRISP instruments on the
Swedish 1-m Solar Telescope in the Ca II K , Ca II 854.2 nm, Hα,
and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line
profiles and in addition perform multi-line, multi-species, non-local
thermodynamic equilibrium (non-LTE) inversions to estimate the spatial
and temporal variation of the chromospheric structure. Results:
We investigate which spectral features of Ca II K contribute to the
frequency-integrated Ca II K brightness, which we use as a tracer
of chromospheric radiative losses. The majority of the radiative
losses are not associated with localised high-Ca II K-brightness
events, but instead with a more gentle, spatially extended, and
persistent heating. The frequency-integrated Ca II K brightness
correlates strongly with the total linear polarization in the Ca II
854.2 nm, while the Ca II K profile shapes indicate that the bulk
of the radiative losses occur in the lower chromosphere. Non-LTE
inversions indicate a transition from heating concentrated around
photospheric magnetic elements below log τ500 = -3 to a more
space-filling and time-persistent heating above log τ500
= -4. The inferred gas temperature at log τ500 = -3.8
correlates strongly with the total linear polarization in the Ca
II 854.2 nm line, suggesting that that the heating rate correlates
with the strength of the horizontal magnetic field in the low
chromosphere. Movies attached to Figs. 1 and 4 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201732027/olm">https://www.aanda.org/</A>

---------------------------------------------------------
Title: Three-dimensional modeling of the Ca II H and K lines in the
solar atmosphere
Authors: Bjørgen, Johan P.; Sukhorukov, Andrii V.; Leenaarts, Jorrit;
Carlsson, Mats; de la Cruz Rodríguez, Jaime; Scharmer, Göran B.;
Hansteen, Viggo H.
2018A&A...611A..62B Altcode: 2017arXiv171201045B
Context. CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar
Telescope (SST), can observe the chromosphere in the H and K lines of
Ca II at high spatial and spectral resolution. Accurate modeling as
well as an understanding of the formation of these lines are needed to
interpret the SST/CHROMIS observations. Such modeling is computationally
challenging because these lines are influenced by strong departures from
local thermodynamic equilibrium, three-dimensional radiative transfer,
and partially coherent resonance scattering of photons. Aim. We aim to
model the Ca II H and K lines in 3D model atmospheres to understand
their formation and to investigate their diagnostic potential for
probing the chromosphere. Methods: We model the synthetic
spectrum of Ca II using the radiative transfer code Multi3D in three
different radiation-magnetohydrodynamic model atmospheres computed with
the Bifrost code. We classify synthetic intensity profiles according
to their shapes and study how their features are related to the
physical properties in the model atmospheres. We investigate whether
the synthetic data reproduce the observed spatially-averaged line
shapes, center-to-limb variation and compare this data with SST/CHROMIS
images. Results: The spatially-averaged synthetic line profiles
show too low central emission peaks, and too small separation between
the peaks. The trends of the observed center-to-limb variation of
the profiles properties are reproduced by the models. The Ca II H and
K line profiles provide a temperature diagnostic of the temperature
minimum and the temperature at the formation height of the emission
peaks. The Doppler shift of the central depression is an excellent
probe of the velocity in the upper chromosphere.

---------------------------------------------------------
Title: The chromosphere above a δ-sunspot in the presence of
fan-shaped jets
Authors: Robustini, Carolina; Leenaarts, Jorrit; de la Cruz Rodríguez,
Jaime
2018A&A...609A..14R Altcode: 2017A&A...609A..14R; 2017arXiv170903864R
Context. Delta-sunspots are known to be favourable locations for
fast and energetic events like flares and coronal mass ejections. The
photosphere of this sunspot type has been thoroughly investigated in
the past three decades. The atmospheric conditions in the chromosphere
are not as well known, however. Aims: This study is focused on
the chromosphere of a δ-sunspot that harbours a series of fan-shaped
jets in its penumbra. The aim of this study is to establish the
magnetic field topology and the temperature distribution in the
presence of jets in the photosphere and the chromosphere. 
Methods: We use data from the Swedish 1m Solar Telescope (SST) and
the Solar Dynamics Observatory. We invert the spectropolarimetric
Fe I 6302 Å and Ca II 8542 Å data from the SST using the non-LTE
inversion code NICOLE to estimate the magnetic field configuration,
temperature, and velocity structure in the chromosphere. 
Results: A loop-like magnetic structure is observed to emerge
in the penumbra of the sunspot. The jets are launched from this
structure. Magnetic reconnection between this emerging field and the
pre-existing vertical field is suggested by hot plasma patches on the
interface between the two fields. The height at which the reconnection
takes place is located between log τ500 = -2 and log
τ500 = -3. The magnetic field vector and the atmospheric
temperature maps show a stationary configuration during the whole
observation. Movies associated to Figs. 3-5 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201731504/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Chromospheric Heating Driven by Cancellations of Internetwork
Magnetic Flux
Authors: Gosic, M.; de la Cruz Rodriguez, J.; De Pontieu, B.; Bellot
Rubio, L.; Esteban Pozuelo, S.; Ortiz-Carbonell, A. N.
2017AGUFMSH41C..02G Altcode:
The heating of the solar chromosphere remains to be one of the
most important questions in solar physics. It is believed that this
phenomenon may significantly be supported by small-scale internetwork
(IN) magnetic fields. Indeed, cancellations of IN magnetic flux can
generate transient brightenings in the chromosphere and transition
region. These bright structures might be the signature of energy release
and plasma heating, probably driven by magnetic reconnection of IN field
lines. Using high resolution, multiwavelength, coordinated observations
recorded with the Interface Region Imaging Spectrograph (IRIS) and
the Swedish 1-m Solar Telescope (SST), we analyzed cancellations of IN
flux and their impact on the energetics and dynamics of the quiet Sun
atmosphere. From their temporal and spatial evolution, we determine that
these events can heat locally the upper atmospheric layers. However,
employing multi-line inversions of the Mg II h &amp; k lines, we show
that cancellations, although occurring ubiquitously over IN regions,
are not capable of sustaining the total radiative losses of the quiet
Sun chromosphere.

---------------------------------------------------------
Title: Intermittent Reconnection and Plasmoids in UV Bursts in the
Low Solar Atmosphere
Authors: Rouppe van der Voort, L.; De Pontieu, B.; Scharmer, G. B.;
de la Cruz Rodríguez, J.; Martínez-Sykora, J.; Nóbrega-Siverio,
D.; Guo, L. J.; Jafarzadeh, S.; Pereira, T. M. D.; Hansteen, V. H.;
Carlsson, M.; Vissers, G.
2017ApJ...851L...6R Altcode: 2017arXiv171104581R
Magnetic reconnection is thought to drive a wide variety of dynamic
phenomena in the solar atmosphere. Yet, the detailed physical mechanisms
driving reconnection are difficult to discern in the remote sensing
observations that are used to study the solar atmosphere. In this
Letter, we exploit the high-resolution instruments Interface Region
Imaging Spectrograph and the new CHROMIS Fabry-Pérot instrument at
the Swedish 1-m Solar Telescope (SST) to identify the intermittency
of magnetic reconnection and its association with the formation of
plasmoids in so-called UV bursts in the low solar atmosphere. The Si IV
1403 Å UV burst spectra from the transition region show evidence of
highly broadened line profiles with often non-Gaussian and triangular
shapes, in addition to signatures of bidirectional flows. Such profiles
had previously been linked, in idealized numerical simulations, to
magnetic reconnection driven by the plasmoid instability. Simultaneous
CHROMIS images in the chromospheric Ca II K 3934 Å line now provide
compelling evidence for the presence of plasmoids by revealing highly
dynamic and rapidly moving brightenings that are smaller than 0.″2 and
that evolve on timescales of the order of seconds. Our interpretation
of the observations is supported by detailed comparisons with synthetic
observables from advanced numerical simulations of magnetic reconnection
and associated plasmoids in the chromosphere. Our results highlight
how subarcsecond imaging spectroscopy sensitive to a wide range of
temperatures combined with advanced numerical simulations that are
realistic enough to compare with observations can directly reveal the
small-scale physical processes that drive the wide range of phenomena
in the solar atmosphere.

---------------------------------------------------------
Title: Chromospheric polarimetry through multiline observations of
the 850-nm spectral region - II. A magnetic flux tube scenario
Authors: Quintero Noda, C.; Kato, Y.; Katsukawa, Y.; Oba, T.; de la
Cruz Rodríguez, J.; Carlsson, M.; Shimizu, T.; Orozco Suárez, D.;
Ruiz Cobo, B.; Kubo, M.; Anan, T.; Ichimoto, K.; Suematsu, Y.
2017MNRAS.472..727Q Altcode: 2017arXiv170801333Q
In this publication, we continue the work started in Quintero Noda et
al., examining this time a numerical simulation of a magnetic flux
tube concentration. Our goal is to study if the physical phenomena
that take place in it, in particular, the magnetic pumping, leaves
a specific imprint on the examined spectral lines. We find that the
profiles from the interior of the flux tube are periodically doppler
shifted following an oscillation pattern that is also reflected in
the amplitude of the circular polarization signals. In addition, we
analyse the properties of the Stokes profiles at the edges of the flux
tube discovering the presence of linear polarization signals for the Ca
II lines, although they are weak with an amplitude around 0.5 per cent
of the continuum intensity. Finally, we compute the response functions
to perturbations in the longitudinal field, and we estimate the field
strength using the weak-field approximation. Our results indicate
that the height of formation of the spectral lines changes during the
magnetic pumping process, which makes the interpretation of the inferred
magnetic field strength and its evolution more difficult. These results
complement those from previous works, demonstrating the capabilities and
limitations of the 850-nm spectrum for chromospheric Zeeman polarimetry
in a very dynamic and complex atmosphere.

---------------------------------------------------------
Title: Radiative Diagnostics in the Solar Photosphere and Chromosphere
Authors: de la Cruz Rodríguez, J.; van Noort, M.
2017SSRv..210..109D Altcode: 2016arXiv160908324D; 2016SSRv..tmp...73D
Magnetic fields on the surface of the Sun and stars in general imprint
or modify the polarization state of the electromagnetic radiation that
is leaving from the star. The inference of solar/stellar magnetic fields
is performed by detecting, studying and modeling polarized light from
the target star. In this review we present an overview of techniques
that are used to study the atmosphere of the Sun, and particularly those
that allow to infer magnetic fields. We have combined a small selection
of theory on polarized radiative transfer, inversion techniques and
we discuss a number of results from chromospheric inversions.

---------------------------------------------------------
Title: CRISPRED: CRISP imaging spectropolarimeter data reduction
pipeline
Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.;
Hillberg, T.; Rouppe van der Voort, L.
2017ascl.soft08003D Altcode:
CRISPRED reduces data from the CRISP imaging spectropolarimeter at
the Swedish 1 m Solar Telescope (SST). It performs fitting routines,
corrects optical aberrations from atmospheric turbulence as well as
from the optics, and compensates for inter-camera misalignments,
field-dependent and time-varying instrumental polarization, and
spatial variation in the detector gain and in the zero level offset
(bias). It has an object-oriented IDL structure with computationally
demanding routines performed in C subprograms called as dynamically
loadable modules (DLMs).

---------------------------------------------------------
Title: A Hot Downflowing Model Atmosphere for Umbral Flashes and
the Physical Properties of Their Dark Fibrils
Authors: Henriques, V. M. J.; Mathioudakis, M.; Socas-Navarro, H.;
de la Cruz Rodríguez, J.
2017ApJ...845..102H Altcode: 2017arXiv170605311H
We perform non-LTE 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 λ8542 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.

---------------------------------------------------------
Title: Chromospheric heating due to internetwork magnetic flux
cancellations
Authors: Gosic, Milan; de la Cruz Rodriguez, Jaime; De Pontieu, Bart;
Bellot Rubio, Luis; Ortiz, Ada; Esteban Pozuelo, Sara
2017SPD....4810404G Altcode:
The heating of the solar chromosphere is one of the most intriguing
unanswered problems in solar physics. It is believed that this
phenomenon may significantly be supported by small-scale internetwork
(IN) magnetic fields. Indeed, cancellations of IN magnetic flux
patches might be an efficient way to transport flux and energy from
the photosphere to the chromosphere. Because of this, it is essential
to determine where they occur, the rates at which they proceed, and
understand their influence on the chromosphere. Here we study the
spatial and temporal evolution of IN cancelling patches using high
resolution, multiwavelength, coordinated observations obtained with
the Interface Region Imaging Spectrograph (IRIS) and the Swedish
1-m Solar Telescope (SST). Employing multi-line inversions of the
Mg II h&amp;k lines we show that cancelling events, while occurring
ubiquitously over IN regions, produce clear signatures of heating in
the upper atmospheric layers. Using the RADYN code we determine the
energy released due to cancellations of IN elements and discuss about
their impact on the dynamics and energetics of the solar chromosphere.

---------------------------------------------------------
Title: Inference of the chromospheric magnetic field orientation in
the Ca II 8542 Å line fibrils
Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.; Martínez
González, M. J.; Socas-Navarro, H.
2017A&A...599A.133A Altcode: 2016arXiv161206088A
Context. Solar chromospheric fibrils, as observed in the core of
strong chromospheric spectral lines, extend from photospheric field
concentrations suggesting that they trace magnetic field lines. These
images have been historically used as proxies of magnetic fields
for many purposes. Aims: Use statistical analysis to test
whether the association between fibrils and magnetic field lines is
justified. Methods: We use a Bayesian hierarchical model to
analyze several tens of thousands of pixels in spectro-polarimetric
chromospheric images of penumbrae and chromospheric fibrils. We
compare the alignment between the field azimuth inferred from the
linear polarization signals through the transverse Zeeman effect and
the direction of the fibrils in the image. Results: We conclude
that, in the analyzed fields of view, fibrils are often well aligned
with the magnetic field azimuth. Despite this alignment, the analysis
also shows that there is a non-negligible dispersion. In penumbral
filaments, we find a dispersion with a standard deviation of 16°,
while this dispersion goes up to 34° in less magnetized regions.

---------------------------------------------------------
Title: Chromospheric polarimetry through multiline observations of
the 850-nm spectral region
Authors: Quintero Noda, C.; Shimizu, T.; Katsukawa, Y.; de la Cruz
Rodríguez, J.; Carlsson, M.; Anan, T.; Oba, T.; Ichimoto, K.;
Suematsu, Y.
2017MNRAS.464.4534Q Altcode: 2016arXiv161006651Q
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. focused on the infrared Ca II 8542 Å line and we
concluded that it 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 trying to
improve the results produced by this line observing additional spectral
lines. In that regard, we examined the neighbourhood solar spectrum
looking for spectral lines which could increase the sensitivity to
the atmospheric parameters. Interestingly, we discovered several
photospheric lines which greatly improve the photospheric sensitivity
to the magnetic field vector. Moreover, they are located close to a
second chromospheric line which also belongs to the Ca II infrared
triplet, I.e. the Ca II 8498 Å 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 Å line not only increase its
sensitivity to the atmospheric parameters at all layers, but also they
constitute an excellent spectral window for chromospheric polarimetry.

---------------------------------------------------------
Title: Observations of Ellerman bomb emission features in He I
D3 and He I 10 830 Å
Authors: Libbrecht, Tine; Joshi, Jayant; de la Cruz Rodríguez, Jaime;
Leenaarts, Jorrit; Ramos, Andrés Asensio
2017A&A...598A..33L Altcode: 2016arXiv161001321L
Context. Ellerman bombs (EBs) are short-lived emission features,
characterised by extended wing emission in hydrogen Balmer lines. Until
now, no distinct signature of EBs has been found in the He I 10 830 Å
line, and conclusive observations of EBs in He I D3 have
never been reported. Aims: We aim to study the signature of
EBs in neutral helium triplet lines. Methods: The observations
consisted of ten consecutive SST/TRIPPEL raster scans close to the
limb, featuring the Hβ, He I D3 and He I 10 830 Å spectral
regions. We also obtained raster scans with IRIS and made use of the
SDO/AIA 1700 Å channel. We used Hazel to invert the neutral helium
triplet lines. Results: Three EBs in our data show distinct
emission signatures in neutral helium triplet lines, most prominently
visible in the He I D3 line. The helium lines have two
components: a broad and blueshifted emission component associated with
the EB, and a narrower absorption component formed in the overlying
chromosphere. One of the EBs in our data shows evidence of strong
velocity gradients in its emission component. The emission component of
the other two EBs could be fitted using a constant slab. Our analysis
hints towards thermal Doppler motions having a large contribution to
the broadening for helium and IRIS lines. We conclude that the EBs
must have high temperatures to exhibit emission signals in neutral
helium triplet lines. An order of magnitude estimate places our
observed EBs in the range of T 2 × 104-105
K. Movies associated to Figs. 3-5 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629266/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Properties of Supersonic Evershed Downflows
Authors: Pozuelo, S. Esteban; Bellot Rubio, L. R.; de la Cruz
Rodríguez, J.
2016ApJ...832..170P Altcode: 2016ApJ...832..170E; 2016arXiv160901106E
We study supersonic Evershed downflows in a sunspot penumbra by means
of high spatial resolution spectropolarimetric data acquired in the
Fe I 617.3 nm line with the CRISP instrument at the Swedish 1 m Solar
Telescope. Physical observables, such as Dopplergrams calculated from
line bisectors and Stokes V zero-crossing wavelengths, and Stokes V
maps in the far red-wing, are used to find regions where supersonic
Evershed downflows may exist. We retrieve the line-of-sight velocity
and the magnetic field vector in these regions using two-component
inversions of the observed Stokes profiles with the help of the SIR
code. We follow these regions during their lifetime to study their
temporal behavior. Finally, we carry out a statistical analysis
of the detected supersonic downflows to characterize their physical
properties. Supersonic downflows are contained in compact patches moving
outward, which are located in the mid- and outer penumbra. They are
observed as bright, roundish structures at the outer end of penumbral
filaments that resemble penumbral grains. The patches may undergo
fragmentations and mergings during their lifetime; some of them are
recurrent. Supersonic downflows are associated with strong and rather
vertical magnetic fields with a reversed polarity compared to that of
the sunspot. Our results suggest that downflows returning back to the
solar surface with supersonic velocities are abruptly stopped in dense
deep layers and produce a shock. Consequently, this shock enhances
the temperature and is detected as a bright grain in the continuum
filtergrams, which could explain the existence of outward-moving grains
in the mid- and outer penumbra.

---------------------------------------------------------
Title: Non-LTE Inversions of the Mg II h &amp; k and UV Triplet Lines
Authors: de la Cruz Rodríguez, Jaime; Leenaarts, Jorrit; Asensio
Ramos, Andrés
2016ApJ...830L..30D Altcode: 2016arXiv160909527D
The Mg II h &amp; k lines are powerful diagnostics for studying the
solar chromosphere. They have become particularly popular with the
launch of the Interface Region Imaging Spectrograph (IRIS) satellite,
and a number of studies that include these lines have lead to great
progress in understanding chromospheric heating, in many cases thanks
to the support from 3D MHD simulations. In this study, we utilize
another approach to analyze observations: non-LTE inversions of
the Mg II h &amp; k and UV triplet lines including the effects of
partial redistribution. Our inversion code attempts to construct a
model atmosphere that is compatible with the observed spectra. We have
assessed the capabilities and limitations of the inversions using the
FALC atmosphere and a snapshot from a 3D radiation-MHD simulation. We
find that Mg II h &amp; k allow reconstructing a model atmosphere from
the middle photosphere to the transition region. We have also explored
the capabilities of a multi-line/multi-atom setup, including the Mg
II h &amp; k, the Ca II 854.2 nm, and the Fe I 630.25 lines to recover
the full stratification of physical parameters, including the magnetic
field vector, from the photosphere to the chromosphere. Finally, we
present the first inversions of observed IRIS spectra from quiet-Sun,
plage, and sunspot, with very promising results.

---------------------------------------------------------
Title: On the Magnetism and Dynamics of Prominence Legs Hosting
Tornadoes
Authors: Martínez González, M. J.; Asensio Ramos, A.; Arregui, I.;
Collados, M.; Beck, C.; de la Cruz Rodríguez, J.
2016ApJ...825..119M Altcode: 2016arXiv160501183M
Solar tornadoes are dark vertical filamentary structures observed
in the extreme ultraviolet associated with prominence legs and
filament barbs. Their true nature and relationship to prominences
requires an understanding of their magnetic structure and dynamic
properties. Recently, a controversy has arisen: is the magnetic field
organized forming vertical, helical structures or is it dominantly
horizontal? And concerning their dynamics, are tornadoes really rotating
or is it just a visual illusion? Here we analyze four consecutive
spectro-polarimetric scans of a prominence hosting tornadoes on its
legs, which helps us shed some light on their magnetic and dynamical
properties. We show that the magnetic field is very smooth in all the
prominence, which is probably an intrinsic property of the coronal
field. The prominence legs have vertical helical fields that show
slow temporal variation that is probably related to the motion of
the fibrils. Concerning the dynamics, we argue that (1) if rotation
exists, it is intermittent, lasting no more than one hour, and (2)
the observed velocity pattern is also consistent with an oscillatory
velocity pattern (waves).

---------------------------------------------------------
Title: Emergence of Granular-sized Magnetic Bubbles Through the
Solar Atmosphere. III. The Path to the Transition Region
Authors: Ortiz, Ada; Hansteen, Viggo H.; Bellot Rubio, Luis Ramón;
de la Cruz Rodríguez, Jaime; De Pontieu, Bart; Carlsson, Mats;
Rouppe van der Voort, Luc
2016ApJ...825...93O Altcode: 2016arXiv160400302O
We study, for the first time, the ascent of granular-sized magnetic
bubbles from the solar photosphere through the chromosphere into the
transition region and above. Such events occurred in a flux emerging
region in NOAA 11850 on 2013 September 25. During that time, the
first co-observing campaign between the Swedish 1-m Solar Telescope
(SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft
was carried out. Simultaneous observations of the chromospheric Hα
656.28 nm and Ca II 854.2 nm lines, plus the photospheric Fe I 630.25
nm line, were made with the CRISP spectropolarimeter at the Spitzer
Space Telescope (SST) reaching a spatial resolution of 0.″14. At
the same time, IRIS was performing a four-step dense raster of the
emerging flux region, taking slit jaw images at 133 (C II, transition
region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper
chromosphere), and 283.2 nm (Mg II k, wing, photosphere). Spectroscopy
of several lines was performed by the IRIS spectrograph in the far-
and near-ultraviolet, of which we have used the Si IV 140.3 and the
Mg II k 279.6 nm lines. Coronal images from the Atmospheric Imaging
Assembly of the Solar Dynamics Observatory were used to investigate
the possible coronal signatures of the flux emergence events. The
photospheric and chromospheric properties of small-scale emerging
magnetic bubbles have been described in detail in Ortiz et al. Here
we are able to follow such structures up to the transition region. We
describe the properties, including temporal delays, of the observed
flux emergence in all layers. We believe this may be an important
mechanism of transporting energy and magnetic flux from subsurface
layers to the transition region and corona.

---------------------------------------------------------
Title: Spectropolarimetric capabilities of Ca II 8542 Å line
Authors: Quintero Noda, C.; Shimizu, T.; de la Cruz Rodríguez, J.;
Katsukawa, Y.; Ichimoto, K.; Anan, T.; Suematsu, Y.
2016MNRAS.459.3363Q Altcode: 2016MNRAS.tmp..667Q; 2016arXiv160404957Q
The next generation of space- and ground-based solar missions aim
to study the magnetic properties of the solar chromosphere using the
infrared Ca II lines and the He I 10830 Å line. The former seem to be
the best candidates to study the stratification of magnetic fields in
the solar chromosphere and their relation to the other thermodynamical
properties underlying the chromospheric plasma. The purpose of this
work is to provide a detailed analysis of the diagnostic capabilities
of the Ca II 8542 Å line, anticipating forthcoming observational
facilities. We study the sensitivity of the Ca II 8542 Å line
to perturbations applied to the physical parameters of reference
semi-empirical 1D model atmospheres using response functions and we
make use of 3D magnetohydrodynamics simulations to examine the expected
polarization signals for moderate magnetic field strengths. Our results
indicate that the Ca II 8542 Å line is mostly sensitive to the layers
enclosed in the range log τ = [0, -5.5], under the physical conditions
that are present in our model atmospheres. In addition, the simulated
magnetic flux tube generates strong longitudinal signals in its centre
and moderate transversal signals, due to the vertical expansion of
magnetic field lines, in its edge. Thus, observing the Ca II 8542 Å
line we will be able to infer the 3D geometry of moderate magnetic
field regions.

---------------------------------------------------------
Title: Fan-shaped jets above the light bridge of a sunspot driven
by reconnection
Authors: Robustini, Carolina; Leenaarts, Jorrit; de la Cruz Rodriguez,
Jaime; Rouppe van der Voort, Luc
2016A&A...590A..57R Altcode: 2015arXiv150807927R
We report on a fan-shaped set of high-speed jets above a strongly
magnetized light bridge (LB) of a sunspot observed in the Hα line. We
study the origin, dynamics, and thermal properties of the jets using
high-resolution imaging spectroscopy in Hα from the Swedish 1m Solar
Telescope and data from the Solar Dynamics Observatory and Hinode. The
Hα jets have lengths of 7-38 Mm, are impulsively accelerated to a speed
of ~100 km s-1 close to photospheric footpoints in the LB,
and exhibit a constant deceleration consistent with solar effective
gravity. They are predominantly launched from one edge of the light
bridge, and their footpoints appear bright in the Hα wings. Atmospheric
Imaging Assembly data indicates elongated brightenings that are nearly
co-spatial with the Hα jets. We interpret them as jets of transition
region temperatures. The magnetic field in the light bridge has a
strength of 0.8-2 kG and it is nearly horizontal. All jet properties
are consistent with magnetic reconnection as the driver. Movies
associated to Figs. 1 and 2 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201528022/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Inversion of Stokes profiles with systematic effects
Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.; Martínez
González, M. J.; Pastor Yabar, A.
2016A&A...590A..87A Altcode: 2016arXiv160405470A
Quantitative thermodynamical, dynamical and magnetic properties of the
solar and stellar plasmas are obtained by interpreting their emergent
non-polarized and polarized spectrum. This inference requires the
selection of a set of spectral lines that are particularly sensitive to
the physical conditions in the plasma and a suitable parametric model
of the solar/stellar atmosphere. Nonlinear inversion codes are then
used to fit the model to the observations. However, the presence of
systematic effects, like nearby or blended spectral lines, telluric
absorption, or incorrect correction of the continuum, among others,
can strongly affect the results. We present an extension to current
inversion codes that can deal with these effects in a transparent
way. The resulting algorithm is very simple and can be applied to any
existing inversion code with the addition of a few lines of code as
an extra step in each iteration.

---------------------------------------------------------
Title: Small-scale magnetic flux emergence in a sunspot light bridge
Authors: Louis, Rohan E.; Bellot Rubio, Luis R.; de la Cruz Rodríguez,
Jaime; Socas-Navarro, Héctor; Ortiz, Ada
2015A&A...584A...1L Altcode: 2015arXiv150900741L
Context. Light bridges are convective intrusions in sunspots that
often show enhanced chromospheric activity. Aims: We seek
to determine the nature of flux emergence in a light bridge and the
processes related to its evolution in the solar atmosphere. 
Methods: We analyse a sequence of high-resolution spectropolarimetric
observations of a sunspot taken at the Swedish 1-m Solar Telescope. The
data consist of spectral scans of the photospheric Fe i line pair at
630 nm and the chromospheric Ca ii 854.2 nm line. Bisectors were used
to construct Dopplergrams from the Fe i 630.15 nm measurements. We
employed LTE and non-LTE inversions to derive maps of physical
parameters in the photosphere and chromosphere, respectively. 
Results: We observe the onset of blueshifts of about 2 km s-1
near the entrance of a granular light bridge on the limbward side of
the spot. The blueshifts lie immediately next to a strongly redshifted
patch that appeared six minutes earlier. Both patches can be seen for
25 min until the end of the sequence. The blueshifts coincide with
an elongated emerging granule, while the redshifts appear at the end
of the granule. In the photosphere, the development of the blueshifts
is accompanied by a simultaneous increase in field strength of about
400 G. The field inclination increases by some 25°, becoming nearly
horizontal. At the position of the redshifts, the magnetic field is
equally horizontal but of opposite polarity. An intense brightening
is seen in the Ca ii filtergrams over the blueshifts and redshifts,
about 17 min after their detection in the photosphere. The brightening
is due to emission in the blue wing of the Ca ii 854.2 nm line, close
to its knee. Non-LTE inversions reveal that this kind of asymmetric
emission is caused by a temperature enhancement of ~700 K between -5.0
≤ log τ ≤ -3.0 and a blueshift of 3 km s-1 at log τ
= -2.3 that decreases to zero at log τ = -6.0 Conclusions:
The photospheric blueshifts and redshifts observed in a granular
light bridge seem to be caused by the emergence of a small-scale,
flat Ω-loop with highly inclined footpoints of opposite polarity that
brings new magnetic field to the surface. The gas motions detected in
the two footpoints are reminiscent of a siphon flow. The rising loop
is probably confined to the lower atmosphere by the overlying sunspot
magnetic field and the interaction between the two flux systems may be
responsible for temperature enhancements in the upper photosphere/lower
chromosphere. This is the first time that magnetic flux is observed
to emerge in the strongly magnetised environment of sunspots, pushed
upwards by the convective flows of a granular light bridge. The
movie associated to Fig. 2 is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526854/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: New generation Stokes inversion codes
Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.
2015IAUS..305..225A Altcode: 2015IAUS..305..225R
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 to
exploit the presence of spatial correlation on the maps of physical
parameters. Sparsity also regularizes the solution by reducing the
number of unknowns. The solution has an increased robustness.

---------------------------------------------------------
Title: Magnetic Upflow Events in the Quiet-Sun
Photosphere. I. Observations
Authors: Jafarzadeh, S.; Rouppe van der Voort, L.; de la Cruz
Rodríguez, J.
2015ApJ...810...54J Altcode: 2015arXiv150707355J
Rapid magnetic upflows in the quiet-Sun photosphere were recently
uncovered from both Sunrise/IMaX and Hinode/SOT observations. Here, we
study magnetic upflow events (MUEs) from high-quality, high- (spatial,
temporal, and spectral) resolution, and full Stokes observations
in four photospheric magnetically sensitive Fe i lines centered at
5250.21, 6173.34, 6301.51, and 6302.50 Å acquired with the Swedish
Solar Telescope (SST)/CRISP. We detect MUEs by subtracting in-line
Stokes V signals from those in the far blue wing whose signal-to-noise
ratio (S/N) ≥slant 7. We find a larger number of MUEs at any given
time (2.0× {10}-2 arcsec-2), larger by one to
two orders of magnitude, than previously reported. The MUEs appear
to fall into four classes presenting different shapes of Stokes V
profiles with (I) asymmetric double lobes, (II) single lobes, (III)
double-humped (two same-polarity lobes), and (IV) three lobes (an
extra blueshifted bump in addition to double lobes), of which less
than half are single-lobed. We also find that MUEs are almost equally
distributed in network and internetwork areas and they appear in the
interior or at the edge of granules in both regions. Distributions
of physical properties, except for horizontal velocity, of the MUEs
(namely, Stokes V signal, size, line-of-sight velocity, and lifetime)
are almost identical for the different spectral lines in our data. A
bisector analysis of our spectrally resolved observations shows that
these events host modest upflows and do not show a direct indication of
the presence of supersonic upflows reported earlier. Our findings reveal
that the numbers, types (classes), and properties determined for MUEs
can strongly depend on the detection techniques used and the properties
of the employed data, namely, S/Ns, resolutions, and wavelengths.

---------------------------------------------------------
Title: Emergence of Granular-sized Magnetic Bubbles through the
Solar Atmosphere. II. Non-LTE Chromospheric Diagnostics and Inversions
Authors: de la Cruz Rodríguez, Jaime; Hansteen, Viggo; Bellot-Rubio,
Luis; Ortiz, Ada
2015ApJ...810..145D Altcode: 2015arXiv150303846D
Magnetic flux emergence into the outer layers of the Sun is a
fundamental mechanism for releasing energy into the chromosphere and
the corona. In this paper, we study the emergence of granular-sized
flux concentrations and the structuring of the corresponding physical
parameters and atmospheric diagnostics in the upper photosphere and
in the chromosphere. We make use of a realistic 3D MHD simulation of
the outer layers of the Sun to study the formation of the Ca ii 8542
line. We also derive semi-empirical 3D models from non-LTE inversions of
our observations. These models contain information on the line-of-sight
stratifications of temperature, velocity, and the magnetic field. Our
analysis explains the peculiar Ca ii 8542 Å profiles observed in the
flux emerging region. Additionally, we derive detailed temperature
and velocity maps describing the ascent of a magnetic bubble from the
photosphere to the chromosphere. The inversions suggest that, in active
regions, granular-sized bubbles emerge up to the lower chromosphere
where the existing large-scale field hinders their ascent. We report
hints of heating when the field reaches the chromosphere.

---------------------------------------------------------
Title: An open-source, massively parallel code for non-LTE synthesis
and inversion of spectral lines and Zeeman-induced Stokes profiles
Authors: Socas-Navarro, H.; de la Cruz Rodríguez, J.; Asensio Ramos,
A.; Trujillo Bueno, J.; Ruiz Cobo, B.
2015A&A...577A...7S Altcode: 2014arXiv1408.6101S
With the advent of a new generation of solar telescopes and
instrumentation, interpreting chromospheric observations (in
particular, spectropolarimetry) requires new, suitable diagnostic
tools. This paper describes a new code, NICOLE, that has been
designed for Stokes non-LTE radiative transfer, for synthesis and
inversion of spectral lines and Zeeman-induced polarization profiles,
spanning a wide range of atmospheric heights from the photosphere
to the chromosphere. The code features a number of unique features
and capabilities and has been built from scratch with a powerful
parallelization scheme that makes it suitable for application on
massive datasets using large supercomputers. The source code is
written entirely in Fortran 90/2003 and complies strictly with the
ANSI standards to ensure maximum compatibility and portability. It
is being publicly released, with the idea of facilitating future
branching by other groups to augment its capabilities. The
source code is currently hosted at the following repository: <A
href="http://github.com/hsocasnavarro/NICOLE">https://github.com/hsocasnavarro/NICOLE</A>

---------------------------------------------------------
Title: Sparse inversion of Stokes profiles. I. Two-dimensional
Milne-Eddington inversions
Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.
2015A&A...577A.140A Altcode: 2015arXiv150307666A
Context. Inversion codes are numerical tools used to infer physical
properties from observations. Despite their success, the quality of
current spectropolarimetric observations and those expected in the
near future presents a challenge to current inversion codes. 
Aims: The pixel-by-pixel strategy of inverting spectropolarimetric
data that we currently use 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. Methods: We used the concept of
sparsity or compressibility to develop a 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 exploiting
the spatial correlation on the maps of physical parameters. Sparsity
also regularizes the solution by reducing the number of unknowns. Results: We compare the results of the new inversion code with
pixel-by-pixel inversions to demonstrate the increased 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.

---------------------------------------------------------
Title: Lateral Downflows in Sunspot Penumbral Filaments and their
Temporal Evolution
Authors: Esteban Pozuelo, S.; Bellot Rubio, L. R.; de la Cruz
Rodríguez, J.
2015ApJ...803...93E Altcode: 2015arXiv150202981E
We study the temporal evolution of downflows observed at the lateral
edges of penumbral filaments in a sunspot located very close to
the disk center. Our analysis is based on a sequence of nearly
diffraction-limited scans of the Fe i 617.3 nm line taken with the
CRisp Imaging Spectro-Polarimeter instrument at the Swedish 1 m
Solar Telescope. We compute Dopplergrams from the observed intensity
profiles using line bisectors and filter the resulting velocity maps
for subsonic oscillations. Lateral downflows appear everywhere in
the center-side penumbra as small, weak patches of redshifts next to
or along the edges of blueshifted flow channels. These patches have
an intermittent life and undergo mergings and fragmentations quite
frequently. The lateral downflows move together with the hosting
filaments and react to their shape variations, very much resembling
the evolution of granular convection in the quiet Sun. There is a
good relation between brightness and velocity in the center-side
penumbra, with downflows being darker than upflows on average, which
is again reminiscent of quiet Sun convection. These results point to
the existence of overturning convection in sunspot penumbrae, with
elongated cells forming filaments where the flow is upward but very
inclined, and weak lateral downward flows. In general, the circular
polarization profiles emerging from the lateral downflows do not show
sign reversals, although sometimes we detect three-lobed profiles that
are suggestive of opposite magnetic polarities in the pixel.

---------------------------------------------------------
Title: Spectro-Polarimetric Imaging Reveals Helical Magnetic Fields
in Solar Prominence Feet
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
A.; Beck, C.; de la Cruz Rodríguez, J.; Díaz, A. J.
2015ApJ...802....3M Altcode: 2015arXiv150103295M
Solar prominences are clouds of cool plasma levitating above
the solar surface and insulated from the million-degree corona by
magnetic fields. They form in regions of complex magnetic topology,
characterized by non-potential fields, which can evolve abruptly,
disintegrating the prominence and ejecting magnetized material
into the heliosphere. However, their physics is not yet fully
understood because mapping such complex magnetic configurations
and their evolution is extremely challenging, and must often be
guessed by proxy from photometric observations. Using state-of-the-art
spectro-polarimetric data, we reconstruct the structure of the magnetic
field in a prominence. We find that prominence feet harbor helical
magnetic fields connecting the prominence to the solar surface below.

---------------------------------------------------------
Title: CRISPRED: A data pipeline for the CRISP imaging
spectropolarimeter
Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.;
Hillberg, T.; Rouppe van der Voort, L.
2015A&A...573A..40D Altcode: 2014arXiv1406.0202D
The production of science-ready data from major solar telescopes
requires expertise beyond that of the typical observer. This is
a consequence of the increasing complexity of instruments and
observing sequences, which require calibrations and corrections
for instrumental and seeing effects that are not only difficult to
measure, but are also coupled in ways that require careful analysis
in the design of the correction procedures. Modern space-based
telescopes have data-processing pipelines capable of routinely
producing well-characterized data products. High resolution imaging
spectropolarimeters at ground-based telescopes need similar data
pipelines.We present new methods for flat-fielding spectropolarimetric
data acquired with telecentric Fabry-Perot instruments and a new
approach for accurate camera co-alignment for image restoration. We
document a procedure that forms the basis of current state-of-the-art
processing of data from the CRISP imaging spectropolarimeter at the
Swedish 1 m Solar Telescope (SST). By collecting, implementing, and
testing a suite of computer programs, we have defined a data reduction
pipeline for this instrument. This pipeline, CRISPRED, streamlines the
process of making science-ready data.It is implemented and operated
in IDL, with time-consuming steps delegated to C.CRISPRED will also be
the basis for the data pipeline of the forthcoming CHROMIS instrument.

---------------------------------------------------------
Title: Dynamic Properties along the Neutral Line of a Delta Spot
Inferred from High-resolution Observations
Authors: Cristaldi, A.; Guglielmino, S. L.; Zuccarello, F.; Romano,
P.; Falco, M.; Rouppe van der Voort, L.; de la Cruz Rodríguez, J.;
Ermolli, I.; Criscuoli, S.
2014ApJ...789..162C Altcode:
Delta (δ) spots are complex magnetic configurations of sunspots
characterized by umbrae of opposite polarity sharing a common
penumbra. In order to investigate the fine structure of the region
separating the two magnetic polarities of a δ spot, we studied the
morphology, the magnetic configuration, and the velocity field in
such a region using observations of active region (AR) NOAA 11267
obtained with the CRisp Imaging SpectroPolarimeter (CRISP) at the
Swedish Solar Telescope on 2011 August 6. The analysis of CRISP data
shows upflows and downflows of ~ ± 3 km s-1 in proximity
of the δ spot polarity inversion line (PIL), and horizontal motions
along the PIL of the order of ~1 km s-1. The results
obtained from the SIR inversion of CRISP data also indicate that the
transverse magnetic field in the brighter region separating the two
opposite magnetic polarities of the δ spot is tilted about ~45°
with respect to the PIL. Solar Dynamics Observatory/Helioseismic and
Magnetic Imager observations confirm the presence of motions of ~
± 3 km s-1 in proximity of the PIL, which were observed
to last 15 hr. From the data analyzed, we conclude that the steady,
persistent, and subsonic motions observed along the δ spot PIL can be
interpreted as being due to Evershed flows occurring in the penumbral
filaments that show a curved, wrapped configuration. The fluting of
the penumbral filaments and their bending, continuously increased by
the approaching motion of the negative umbra toward the positive one,
give rise to the complex line-of-sight velocity maps that we observed.

---------------------------------------------------------
Title: The Effect of Isotopic Splitting on the Bisector and Inversions
of the Solar Ca II 854.2 nm Line
Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Kochukhov,
Oleg; Carlsson, Mats
2014ApJ...784L..17L Altcode: 2014arXiv1401.5019L
The Ca II 854.2 nm spectral line is a common diagnostic of the solar
chromosphere. The average line profile shows an asymmetric core,
and its bisector shows a characteristic inverse-C shape. The line
actually consists of six components with slightly different wavelengths
depending on the isotope of calcium. This isotopic splitting of the
line has been taken into account in studies of non-solar stars, but
never for the Sun. We performed non-LTE radiative transfer computations
from three models of the solar atmosphere and show that the line-core
asymmetry and inverse C-shape of the bisector of the 854.2 nm line
can be explained by isotopic splitting. We confirm this finding by
analyzing observations and showing that the line asymmetry is present
irrespective of conditions in the solar atmosphere. Finally, we show
that inversions based on the Ca II 854.2 nm line should take the
isotopic splitting into account, otherwise the inferred atmospheres
will contain erroneous velocity gradients and temperatures.

---------------------------------------------------------
Title: Emergence of Granular-sized Magnetic Bubbles through the
Solar Atmosphere. I. Spectropolarimetric Observations and Simulations
Authors: Ortiz, Ada; Bellot Rubio, Luis R.; Hansteen, Viggo H.;
de la Cruz Rodríguez, Jaime; Rouppe van der Voort, Luc
2014ApJ...781..126O Altcode: 2013arXiv1312.5735O
We study a granular-sized magnetic flux emergence event that occurred
in NOAA 11024 in 2009 July. The observations were made with the CRISP
spectropolarimeter at the Swedish 1 m Solar Telescope achieving a
spatial resolution of 0.”14. Simultaneous full Stokes observations of
the two photospheric Fe I lines at 630.2 nm and the chromospheric Ca
II 854.2 nm line allow us to describe in detail the emergence process
across the solar atmosphere. We report here on three-dimensional
(3D) semi-spherical bubble events, where instead of simple magnetic
footpoints, we observe complex semi-circular feet straddling a few
granules. Several phenomena occur simultaneously, namely, abnormal
granulation, separation of opposite-polarity legs, and brightenings at
chromospheric heights. However, the most characteristic signature in
these events is the observation of a dark bubble in filtergrams taken
in the wings of the Ca II 854.2 nm line. There is a clear coincidence
between the emergence of horizontal magnetic field patches and the
formation of the dark bubble. We can infer how the bubble rises through
the solar atmosphere as we see it progressing from the wings to the
core of Ca II 854.2 nm. In the photosphere, the magnetic bubble shows
mean upward Doppler velocities of 2 km s-1 and expands at a
horizontal speed of 4 km s-1. In about 3.5 minutes it travels
some 1100 km to reach the mid chromosphere, implying an average ascent
speed of 5.2 km s-1. The maximum separation attained by the
magnetic legs is 6.”6. From an inversion of the observed Stokes spectra
with the SIR code, we find maximum photospheric field strengths of 480 G
and inclinations of nearly 90° in the magnetic bubble interior, along
with temperature deficits of up to 250 K at log τ = -2 and above. To
aid the interpretation of the observations, we carry out 3D numerical
simulations of the evolution of a horizontal, untwisted magnetic flux
sheet injected in the convection zone, using the Bifrost code. The
computational domain spans from the upper convection zone to the lower
corona. In the modeled chromosphere, the rising flux sheet produces a
large, cool, magnetized bubble. We compare this bubble with the observed
ones and find excellent agreement, including similar field strengths
and velocity signals in the photosphere and chromosphere, temperature
deficits, ascent speeds, expansion velocities, and lifetimes.

---------------------------------------------------------
Title: Inversion and Inter-Comparison of Photospheric and
Chromospheric Magnetic Fields
Authors: De la Cruz Rodriguez, Jaime
2014cosp...40E.649D Altcode:
The structure of the solar chromosphere is believed to be governed
by magnetic fields, even in quiet Sun regions with a relatively
weak field. However, measuring the magnetic field of the solar
chromosphere is an outstanding challenge for observers. The reason for
this is threefold: there are not many lines available in the optical
spectrum with a sufficiently high opacity to place the formation in
the chromosphere, the polarimetric response is limited, and the few
lines available have a non-local component to their formation. Despite
those difficulties, non-LTE inversions are computationally affordable
nowadays and allow to retrieve physical quantities in the chromosphere,
including the magnetic field. We study the structuring of chromospheric
magnetic fields and their connection with the underlaying photosphere
in different solar scenarios such as sunspots, active regions and
the quiet-Sun.

---------------------------------------------------------
Title: Short Dynamic Fibrils in Sunspot Chromospheres
Authors: Rouppe van der Voort, L.; de la Cruz Rodríguez, J.
2013ApJ...776...56R Altcode: 2013arXiv1308.3893R
Sunspot chromospheres display vigorous oscillatory signatures when
observed using chromospheric diagnostics such as the strong Ca II lines
and Hα. New high-resolution sunspot observations from the Swedish
1 m Solar Telescope show the ubiquitous presence of small-scale,
periodic, jet-like features that move up and down. This phenomenon
has not been described before. The typical width of these features is
about 0.”3 and they display clear parabolic trajectories in space-time
diagrams. The maximum extension of the top of the jets is lowest in
the umbra, a few 100 km, and progressively longer further away from
the umbra in the penumbra, with the longest extending more than 1000
km. These jets resemble the dynamic fibrils found in plage regions but
at smaller extensions. Local thermodynamic equilibrium inversion of
spectropolarimetric Ca II 8542 observations enabled a comparison of the
magnetic field inclination and properties of these short jets. We find
that the most extended of these jets also have longer periods and tend
to be located in regions with more horizontal magnetic fields. These
results are direct observational confirmation of the mechanism of
long-period waves propagating along inclined magnetic fields into the
solar chromosphere. This mechanism was identified earlier as the driver
of dynamic fibrils in plage, part of the mottles in the quiet Sun, and
the type I spicules at the limb. The sunspot dynamic fibrils that we
report here represent a new class of manifestation of this mechanism,
distinct from the transient penumbral and umbral micro-jets reported
earlier.

---------------------------------------------------------
Title: Physical properties of a sunspot chromosphere with umbral
flashes
Authors: de la Cruz Rodríguez, J.; Rouppe van der Voort, L.;
Socas-Navarro, H.; van Noort, M.
2013A&A...556A.115D Altcode: 2013arXiv1304.0752D
We present new high-resolution spectro-polarimetric Ca IIλ8542
observations of umbral flashes in sunspots. At nearly 0.18 arcsec,
and spanning about one hour of continuous observation, this is the
most detailed dataset published thus far. Our study involves both LTE
and non-LTE inversions (but includes also a weak field analysis as a
sanity check) to quantify temperatures, mass flows and the full magnetic
field vector geometry. We confirm earlier reports that UFs have very
fine structure with hot and cool material intermixed at sub-arcsecond
scales. The shock front is roughly 1000 K hotter than the surrounding
material. We do not observe significant fluctuations of the field in the
umbra. In the penumbra, however, the passage of the running penumbral
waves alter the magnetic field strength by some 200 G (peak-to-peak
amplitude) but it does not change the field orientation (at least not
significantly within our sensitivity of a few degrees). From a fast
Fourier transform analysis, we find a trend of decreasing power at high
temporal frequencies at those locations with more horizontal magnetic
fields, for the line-of-sight velocity and magnetic field strength. In
the outer penumbra we find an absence of high frequency power while
there is increasingly more power at high frequencies towards the
umbra. Movie and Appendices A and B are available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Magnetic tornadoes and chromospheric swirls - Definition
and classification
Authors: Wedemeyer, Sven; Scullion, Eamon; Steiner, Oskar; de la Cruz
Rodriguez, Jaime; Rouppe van der Voort, L. H. M.
2013JPhCS.440a2005W Altcode: 2013arXiv1303.0179W
Chromospheric swirls are the observational signatures of rotating
magnetic field structures in the solar atmosphere, also known as
magnetic tornadoes. Swirls appear as dark rotating features in the core
of the spectral line of singly ionized calcium at a wavelength of 854.2
nm. This signature can be very subtle and difficult to detect given
the dynamic changes in the solar chromosphere. Important steps towards
a systematic and objective detection method are the compilation and
characterization of a statistically significant sample of observed
and simulated chromospheric swirls. Here, we provide a more exact
definition of the chromospheric swirl phenomenon and also present a
first morphological classification of swirls with three types: (I) Ring,
(II) Split, (III) Spiral. We also discuss the nature of the magnetic
field structures connected to tornadoes and the influence of limited
spatial resolution on the appearance of their photospheric footpoints.

---------------------------------------------------------
Title: Opposite polarity field with convective downflow and its
relation to magnetic spines in a sunspot penumbra
Authors: Scharmer, G. B.; de la Cruz Rodriguez, J.; Sütterlin, P.;
Henriques, V. M. J.
2013A&A...553A..63S Altcode: 2012arXiv1211.5776S
We discuss NICOLE inversions of Fe i 630.15 nm and 630.25 nm Stokes
spectra from a sunspot penumbra recorded with the CRISP imaging
spectropolarimeter on the Swedish 1-m Solar Telescope at a spatial
resolution close to 0.15 arcsec. We report on narrow, radially extended
lanes of opposite polarity field, located at the boundaries between
areas of relatively horizontal magnetic field (the intra-spines) and
much more vertical field (the spines). These lanes harbor convective
downflows of about 1 km s-1. The locations of these downflows
close to the spines agree with predictions from the convective gap
model (the "gappy penumbra") proposed six years ago, and more recent
three-dimensional magnetohydrodynamic simulations. We also confirm the
existence of strong convective flows throughout the entire penumbra,
showing the expected correlation between temperature and vertical
velocity, and having vertical root mean square velocities of about
1.2 km s-1.

---------------------------------------------------------
Title: DELO-Bezier Formal Solutions of the Polarized Radiative
Transfer Equation
Authors: de la Cruz Rodríguez, J.; Piskunov, N.
2013ApJ...764...33D Altcode: 2012arXiv1212.2737D
We present two new accurate and efficient methods to compute the
formal solution of the polarized radiative transfer equation. In this
work, the source function and the absorption matrix are approximated
using quadratic and cubic Bezier spline interpolants. These schemes
provide second- and third-order approximations, respectively, and
do not suffer from erratic behavior of the polynomial approximation
(overshooting). The accuracy and the convergence of the new method are
studied along with other popular solutions of the radiative transfer
equation, using stellar atmospheres with strong gradients in the
line-of-sight velocity and in the magnetic-field vector.

---------------------------------------------------------
Title: Heating of the Magnetic Chromosphere: Observational Constraints
from Ca II λ8542 Spectra
Authors: de la Cruz Rodríguez, J.; De Pontieu, B.; Carlsson, M.;
Rouppe van der Voort, L. H. M.
2013ApJ...764L..11D Altcode: 2013arXiv1301.3141D
The heating of the Sun's chromosphere remains poorly understood. While
progress has been made on understanding what drives the quiet-Sun
internetwork chromosphere, chromospheric heating in strong magnetic
field regions continues to present a difficult challenge, mostly
because of a lack of observational constraints. We use high-resolution
spectropolarimetric data from the Swedish 1 m Solar Telescope to
identify the location and spatio-temporal properties of heating in
the magnetic chromosphere. In particular, we report the existence of
raised-core spectral line profiles in the Ca II λ8542 line. These
profiles are characterized by the absence of an absorption line
core, showing a quasi-flat profile between λ ≈ ±0.5 Å, and are
abundant close to magnetic bright points and plage. Comparison with
three-dimensional MHD simulations indicates that such profiles occur
when the line of sight goes through an "elevated temperature canopy"
associated with the expansion with height of the magnetic field of
flux concentrations. This temperature canopy in the simulations is
caused by ohmic dissipation where there are strong magnetic field
gradients. The raised-core profiles are thus indicators of locations
of increased chromospheric heating. We characterize the location and
temporal and spatial properties of such profiles in our observations,
thus providing much stricter constraints on theoretical models of
chromospheric heating mechanisms than before.

---------------------------------------------------------
Title: Chromospheric Magnetic Fields: Observations, Simulations and
their Interpretation
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.;
Leenaarts, J.
2012ASPC..463...15D Altcode: 2012arXiv1203.4577D
The magnetic field of the quiet-Sun chromosphere remains a mystery
for solar physicists. The reduced number of chromospheric lines are
intrinsically hard to model and only a few of them are magnetically
sensitive. In this work, we use a 3D numerical simulation of the outer
layers of the solar atmosphere, to asses the reliability of non-LTE
inversions, in this case applied to the Ca II λ8542 Å line. We show
that NLTE inversions provide realistic estimates of physical quantities
from synthetic observations.

---------------------------------------------------------
Title: Temporal Evolution of Velocity and Magnetic Field in and
around Umbral Dots
Authors: Watanabe, Hiroko; Bellot Rubio, Luis R.; de la Cruz
Rodríguez, Jaime; Rouppe van der Voort, Luc
2012ApJ...757...49W Altcode: 2012arXiv1207.6006W
We study the temporal evolution of umbral dots (UDs) using measurements
from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar
Telescope. Scans of the magnetically sensitive 630 nm iron lines
were performed under stable atmospheric conditions for 71 minutes
with a cadence of 63 s. These observations allow us to investigate
the magnetic field and velocity in and around UDs at a resolution
approaching 0farcs13. From the analysis of 339 UDs, we draw the
following conclusions: (1) UDs show clear hints of upflows, as predicted
by magnetohydrodynamic simulations. By contrast, we could not find
systematic downflow signals. Only in very deep layers, we detect
localized downflows around UDs, but they do not persist in time. (2)
We confirm that UDs exhibit weaker and more inclined fields than their
surroundings, as reported previously. However, UDs that have strong
fields above 2000 G or are in the decay phase show enhanced and more
vertical fields. (3) There are enhanced fields at the migration front
of UDs detached from penumbral grains, as if their motion were impeded
by the ambient field. (4) Long-lived UDs travel longer distances with
slower proper motions. Our results appear to confirm some aspects of
recent numerical simulations of magnetoconvection in the umbra (e.g.,
the existence of upflows in UDs), but not others (e.g., the systematic
weakening of the magnetic field at the position of UDs).

---------------------------------------------------------
Title: Non-local thermodynamic equilibrium inversions from a 3D
magnetohydrodynamic chromospheric model
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.;
Leenaarts, J.
2012A&A...543A..34D Altcode: 2012arXiv1205.3171D
Context. The structure of the solar chromosphere is believed to
be governed by magnetic fields, even in quiet-Sun regions that
have a relatively weak photospheric field. During the past decade
inversion methods have emerged as powerful tools for analyzing the
chromosphere of active regions. The applicability of inversions to
infer the stratification of the physical conditions in a dynamic 3D
solar chromosphere has not yet been studied in detail. Aims:
This study aims to establish the diagnostic capabilities of non-local
thermodynamical equilibrium (NLTE) inversion techniques of Stokes
profiles induced by the Zeeman effect in the Ca ii λ8542 Å line. Methods: We computed the Ca ii atomic level populations in a snapshot
from a 3D radiation-MHD simulation of the quiet solar atmosphere in
non-LTE using the 3D radiative transfer code Multi3d. These populations
were used to compute synthetic full-Stokes profiles in the Ca ii
λ8542 Å line using 1.5D radiative transfer and the inversion code
Nicole. The profiles were then spectrally degraded to account for
finite filter width, and Gaussian noise was added to account for
finite photon flux. These profiles were inverted using Nicole and
the results were compared with the original model atmosphere. 
Results: Our NLTE inversions applied to quiet-Sun synthetic observations
provide reasonably good estimates of the chromospheric magnetic field,
line-of-sight velocities and somewhat less accurate, but still very
useful, estimates of the temperature. Three-dimensional scattering
of photons cause cool pockets in the chromosphere to be invisible in
the line profile and consequently they are also not recovered by the
inversions. To successfully detect Stokes linear polarization in this
quiet snapshot, a noise level below 10-3.5 is necessary.

---------------------------------------------------------
Title: Magnetic tornadoes as energy channels into the solar corona
Authors: Wedemeyer-Böhm, Sven; Scullion, Eamon; Steiner, Oskar;
Rouppe van der Voort, Luc; de La Cruz Rodriguez, Jaime; Fedun, Viktor;
Erdélyi, Robert
2012Natur.486..505W Altcode:
Heating the outer layers of the magnetically quiet solar atmosphere to
more than one million kelvin and accelerating the solar wind requires
an energy flux of approximately 100 to 300 watts per square metre,
but how this energy is transferred and dissipated there is a puzzle and
several alternative solutions have been proposed. Braiding and twisting
of magnetic field structures, which is caused by the convective flows
at the solar surface, was suggested as an efficient mechanism for
atmospheric heating. Convectively driven vortex flows that harbour
magnetic fields are observed to be abundant in the photosphere
(the visible surface of the Sun). Recently, corresponding swirling
motions have been discovered in the chromosphere, the atmospheric
layer sandwiched between the photosphere and the corona. Here we
report the imprints of these chromospheric swirls in the transition
region and low corona, and identify them as observational signatures
of rapidly rotating magnetic structures. These ubiquitous structures,
which resemble super-tornadoes under solar conditions, reach from
the convection zone into the upper solar atmosphere and provide an
alternative mechanism for channelling energy from the lower into the
upper solar atmosphere.

---------------------------------------------------------
Title: NLTE inversions from a 3D MHD Chromospheric simulation
Authors: de la Cruz Rodriguez, J.; Socas-Navarro, H.; Carlsson, M.;
Leenaarts, J.
2012decs.confE..80D Altcode:
The structure of the solar chromosphere is believed to be governed
by magnetic fields, even in quiet Sun regions with a relatively weak
field. Measuring the magnetic field of the solar chromosphere is an
outstanding challenge for observers. Inversion codes allow for detailed
interpretation of full-Stokes data from spectral lines formed in the
chromosphere. However, the applicability of non-LTE inversions to infer
physical conditions in the dynamic 3D solar chromosphere, has not yet
been studied in detail. In this study, we use a snapshot from a 3D MHD
simulation of quiet-sun, extending from the photosphere to the corona,
to asses the reliability of non-LTE inversions to infer chromospheric
quantities, especially the magnetic field.

---------------------------------------------------------
Title: Small-scale rotating magnetic flux structures as alternative
energy channels into the low corona
Authors: Wedemeyer-Böhm; , Sven; Scullion; , Eamon; Steiner; , Oskar;
Rouppe van der Voort, Luc; de la Cruz Rodriguez, Jaime; Erdelyi,
Robertus; Fedun, Viktor
2012decs.confE..67W Altcode:
Vortex flows are frequently observed in the downflow areas in the lanes
between granules. The magnetic field is advected and trapped by these
flows in the low photosphere. Consequently, the rotation of a vortex
flow is transferred to the atmospheric layers above by means of the
magnetic flux structure. This effect results in so-called swirls, which
are observed in the chromosphere. New simultaneous observations with
the Swedish Solar Telescope and the Solar Dynamics Observatory reveal
that chromospheric swirls can have a coronal counterpart. This finding
implies that the rotating flux structure couples the layers of the solar
atmosphere from the photosphere to the (low) corona. Three-dimensional
numerical simulations confirm this picture and reproduce the swirl
signature. A combined analysis of the simulations and observations
implies that such small-scale rotating flux structures could provide
an alternative mechanism for channeling substantial energy from the
photosphere into the upper solar atmosphere.

---------------------------------------------------------
Title: Type-II spicules: Heating and magnetic field properties from
aligned CRISP/SST and SDO observations
Authors: Scullion, E.; Rouppe van der Voort, L.; de la Cruz Rodriguez,
J.
2012decs.confE..44S Altcode:
Over the past decade there has been a resurgence in the study of
small-scale chromospheric jets known, classically, as spicules. Recent
observations have lead us to conclude that there are two distinct
varieties of spicule, namely, slower type-I (i.e. mottles, dynamic
fibrils, H-alpha spicules etc.) and faster type-II (RBEs: Rapid
Blue-shift Excursions on-disk). Such events dominate the dynamics of
the chromosphere. Joint SDO (Solar Dynamics Observatory) and Hinode
observations have revealed that fast spicules are the source of hot
plasma channelling into the corona. Here we report on the properties
of this widespread heating with observations from the high resolution
CRISP (CRisp Imaging SpectroPolarimeter) instrument at the SST (1-m
Swedish Solar Telescope, La Palma) and co-aligned SDO data. Furthermore,
we reveal new insight into the formation of type-II spicules through
considering the distribution of RBEs with respect to the photospheric
magnetic field (via CRISP).

---------------------------------------------------------
Title: Stokes imaging polarimetry using image restoration: a
calibration strategy for Fabry-Pérot based instruments
Authors: Schnerr, R. S.; de La Cruz Rodríguez, J.; van Noort, M.
2011A&A...534A..45S Altcode: 2010arXiv1012.1225S
Context. The combination of image restoration and a Fabry-Pérot
interferometer (FPI) based instrument in solar observations results
in specific calibration issues. FPIs generally show variations over
the field-of-view, while in the image restoration process, the 1-to-1
relation between pixel space and image space is lost, thus complicating
any correcting for such variations. Aims: We develop a data
reduction method that takes these issues into account and minimizes the
resulting errors. Methods: By accounting for the time variations
in the telescope's Mueller matrix and using separate calibration data
optimized for the wavefront sensing in the MOMFBD image restoration
process and for the final deconvolution of the data, we have removed
most of the calibration artifacts from the resulting data. 
Results: Using this method to reduce full Stokes data from CRISP at
the SST, we find that it drastically reduces the instrumental and
image restoration artifacts resulting from cavity errors, reflectivity
variations, and the polarization dependence of flatfields. The results
allow for useful scientific interpretation. Inversions of restored
data from the δ sunspot AR11029 using the Nicole inversion code,
reveal strong (~10 km s-1) downflows near the disk center
side of the umbra. Conclusions: The use of image restoration
in combination with an FPI-based instrument leads to complications in
the calibrations and intrinsic limitations to the accuracy that can
be achieved. We find that for CRISP, the resulting errors can be kept
mostly below the polarimetric accuracy of ~10-3. Similar
instruments aiming for higher polarimetric and high spectroscopic
accuracy, will, however, need to take these problems into account.

---------------------------------------------------------
Title: Detection of Convective Downflows in a Sunspot Penumbra
Authors: Scharmer, G. B.; Henriques, V. M. J.; Kiselman, D.; de la
Cruz Rodríguez, J.
2011Sci...333..316S Altcode:
The fine structure and dynamics of sunspots and the strong outflow in
their outer filamentary part—the penumbra—have puzzled astronomers
for more than a century. Recent theoretical models and three-dimensional
numerical simulations explain the penumbral filaments and their
radiative energy output as the result of overturning convection. Here,
we describe the detection of ubiquitous, relatively dark downward
flows of up to 1 kilometer per second (km/s) in the interior penumbra,
using imaging spectropolarimetric data from the Swedish 1-meter Solar
Telescope. The dark downflows are omnipresent in the interior penumbra,
distinguishing them from flows in arched flux tubes, and are associated
with strong (3 to 3.5 km/s) radial outflows. They are thus part of a
penumbral convective flow pattern, with the Evershed flow representing
the horizontal component of that convection.

---------------------------------------------------------
Title: Solar velocity references from 3D HD photospheric models
Authors: de la Cruz Rodríguez, J.; Kiselman, D.; Carlsson, M.
2011A&A...528A.113D Altcode: 2011arXiv1101.2671D
Context. The measurement of Doppler velocities in spectroscopic solar
observations requires a reference for the local frame of rest. The
rotational and radial velocities of the Earth and the rotation of
the Sun introduce velocity offsets in the observations. Normally,
good references for velocities are missing (e.g. telluric lines),
especially in filter-based spectropolarimetric observations. 
Aims: We determine an absolute reference for line-of-sight velocities
measured from solar observations for any heliocentric angle, calibrating
the convective line shift of spatially-averaged profiles on quiet sun
from a 3D hydrodynamical simulation. This method works whenever there
is quiet sun in the field-of-view, and it has the advantage of being
relatively insensitive to uncertainties in the atomic data. 
Methods: We carry out radiative transfer computations in LTE for
selected C i and Fe i lines, whereas the Ca ii infrared lines are
synthesized in non-LTE. Radiative transfer calculations are done with
a modified version of Multi, using the snapshots of a non-magnetic
3D hydrodynamical simulation of the photosphere. Results:
The resulting synthetic profiles show the expected C-shaped bisector
at disk center. The degree of asymmetry and the line shifts, however,
show a clear dependence on the heliocentric angle and the properties of
the lines. The profiles at μ = 1 are compared with observed profiles
to prove their reliability, and they are tested against errors induced
by the LTE calculations, inaccuracies in the atomic data and the 3D
simulation. Conclusions: Theoretical quiet-sun profiles of lines
commonly used by solar observers are provided to the community. Those
can be used as absolute references for line-of-sight velocities. The
limb effect is produced by the projection of the 3D atmosphere along the
line of sight. Non-LTE effects on Fe i lines are found to have a small
impact on the convective shifts of the lines, reinforcing the usability
of the LTE approximation in this case. We estimate the precision of the
disk-center line shifts to be approximately 50 m s-1, but
the off-center profiles remain to be tested against observations. The spectral profiles are only available in electronic form at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/528/A113">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/528/A113</A>

---------------------------------------------------------
Title: Are solar chromospheric fibrils tracing the magnetic field?
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.
2011A&A...527L...8D Altcode: 2011arXiv1101.3551D
Fibrils are thin elongated features visible in the solar chromosphere
in and around magnetized regions. Because of their visual appearance,
they have been traditionally considered a tracer of the magnetic field
lines. For the first time, we challenge that notion by comparing their
orientation to that of the magnetic field, obtained via high-resolution
spectropolarimetric observations of Ca ii lines. The short answer to
the question posed in the title is that mostly yes, but not always.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Synthetic lines in the Sun (de
la Cruz Rodríguez+, 2011)
Authors: de la Cruz Rodríguez, J.; Kiselman, D.; Carlsson, M.
2011yCat..35289113D Altcode:
We have computed synthetic spectra from a realistic 3D numerical
simulation of the solar photosphere. We provide the spatially
averaged spectra for selected lines that are commonly used on solar
applications. These data can be used to calibrate Doppler velocity
measurements in the solar photosphere. The calculations are carried
out along the solar disk from heliocentric angle mu=1.0 to mu=0.3. (11 data files).

---------------------------------------------------------
Title: Measuring the solar atmosphere
Authors: de la Cruz Rodriguez, Jaime
2010PhDT.......219D Altcode: 2012PhDT.........8D; 2012arXiv1204.4448D
The new CRISP filter at the Swedish 1-m Solar Telescope provides
opportunities for observing the solar atmosphere with unprecedented
spatial resolution and cadence. In order to benefit from the
high quality of observational data from this instrument, we have
developed methods for calibrating and restoring polarized Stokes
images, obtained at optical and near infrared wavelengths, taking
into account field-of-view variations of the filter properties. In order to facilitate velocity measurements, a time series from a
3D hydrodynamical granulation simulation is used to compute quiet Sun
spectral line profiles at different heliocentric angles. The synthetic
line profiles, with their convective blueshifts, can be used as
absolute references for line-of-sight velocities. Observations
of the Ca II 8542 Å line are used to study magnetic fields in
chromospheric fibrils. The line wings show the granulation pattern at
mid-photospheric heights whereas the overlying chromosphere is seen
in the core of the line. Using full Stokes data, we have attempted
to observationally verify the alignment of chromospheric fibrils with
the magnetic field. Our results suggest that in most cases fibrils are
aligned along the magnetic field direction, but we also find examples
where this is not the case. Detailed interpretation of Stokes
data from spectral lines formed in the chromospheric data can be made
using non-LTE inversion codes. For the first time, we use a realistic
3D MHD chromospheric simulation of the quiet Sun to assess how well
NLTE inversions recover physical quantities from spectropolarimetric
observations of Ca II 8542 Å. We demonstrate that inversions provide
realistic estimates of depth-averaged quantities in the chromosphere,
although high spectral resolution and high sensitivity are needed to
measure quiet Sun chromospheric magnetic fields.

---------------------------------------------------------
Title: Are chromospheric fibrils tracing the magnetic field?
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.
2010iac..talk..221D Altcode: 2010iac..talk..178D
No abstract at ADS

---------------------------------------------------------
Title: High-order aberration compensation with multi-frame blind
deconvolution and phase diversity image restoration techniques
Authors: Scharmer, G. B.; Löfdahl, M. G.; van Werkhoven, T. I. M.;
de la Cruz Rodríguez, J.
2010A&A...521A..68S Altcode: 2010arXiv1007.1236S
Context. For accurately measuring intensities and determining magnetic
field strengths of small-scale solar (magnetic) structure, knowledge of
and compensation for the point spread function is crucial. For images
recorded with the Swedish 1-meter Solar Telescope (SST), restoration
with multi-frame blind deconvolution (MFBD) and joint phase diverse
speckle (JPDS) methods lead to remarkable improvements in image quality
but granulation contrasts that are too low, indicating additional stray
light. Aims: We propose a method to compensate for stray light
from high-order atmospheric aberrations not included in MFBD and JPDS
processing. Methods: To compensate for uncorrected aberrations,
a reformulation of the image restoration process is proposed that allows
the average effect of hundreds of high-order modes to be compensated for
by relying on Kolmogorov statistics for these modes. The applicability
of the method requires simultaneous measurements of Fried's parameter
r0. The method is tested with simulations as well as real
data and extended to include compensation for conventional stray
light. Results: We find that only part of the reduction of
granulation contrast in SST images is due to uncompensated high-order
aberrations. The remainder is still unaccounted for and attributed
to stray light from the atmosphere, the telescope with its re-imaging
system and to various high-altitude seeing effects. Conclusions:
We conclude that statistical compensation of high-order modes is a
viable method to reduce the loss of contrast occurring when a limited
number of aberrations is explicitly compensated for with MFBD and JPDS
processing. We show that good such compensation is possible with only
10 recorded frames. The main limitation of the method is that already
MFBD and JPDS processing introduces high-order compensation that,
if not taken into account, can lead to over-compensation.

---------------------------------------------------------
Title: Observation and analysis of chromospheric magnetic fields .
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; van Noort,
M.; Rouppe van der Voort, L.
2010MmSAI..81..716D Altcode: 2010arXiv1004.0698D
The solar chromosphere is a vigorously dynamic region of the sun,
where waves and magnetic fields play an important role. To improve
chromospheric diagnostics, we present new observations in Ca II 8542
carried out with the SST/CRISP on La Palma, working in full-Stokes
mode. We measured Stokes line profiles in active regions. The line
profiles observed close to the solar limb show signals in all four
Stokes parameters, while profiles observed close to disk center only
show signals above the noise level in Stokes I and V. We used the NLTE
inversion code 'NICOLE' to derive atmospheric parameters in umbral
flashes present in a small round sunspot without penumbra.

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Title: Measuring the solar atmosphere
Authors: de la Cruz Rodríguez, Jaime
2010PhDT.......246D Altcode:
No abstract at ADS

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Title: The magnetic SW Sextantis star RXJ1643.7+3402
Authors: Rodríguez-Gil, P.; Martínez-Pais, I. G.; de la Cruz
Rodríguez, J.
2009MNRAS.395..973R Altcode: 2009arXiv0902.1799R; 2009MNRAS.tmp..437R
We present time-resolved spectroscopy and circular spectropolarimetry of
the SW Sex star RXJ1643.7+3402. We find significant polarization levels
exhibiting a variability at a period of 19.38 +/- 0.39 min. In addition,
emission-line flaring is found predominantly at twice the polarimetric
period. These two findings are strong evidences in favour of the
presence of a magnetic white dwarf in the system. We interpret the
measured periodicities in the context of our magnetic accretion model
for SW Sex stars. In contrast with LS Pegasi - the first SW Sex star
discovered to have modulated circular polarization - the polarization in
RXJ1643.7+3402 is suggested to vary at 2(ω - Ω), while the emission
lines flare at (ω - Ω). However, a 2ω/ω interpretation cannot be
ruled out. Together with LS Peg and V795 Her, RXJ1643.7+3402 is the
third SW Sex star known to exhibit modulated circular polarization.

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Title: CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
Authors: Scharmer, G. B.; Narayan, G.; Hillberg, T.; de la Cruz
Rodriguez, J.; Löfdahl, M. G.; Kiselman, D.; Sütterlin, P.; van
Noort, M.; Lagg, A.
2008ApJ...689L..69S Altcode: 2008arXiv0806.1638S
We discuss penumbral fine structure in a small part of a pore,
observed with the CRISP imaging spectropolarimeter at the Swedish
1-m Solar Telescope (SST), close to its diffraction limit of
0.16”. Milne-Eddington inversions applied to these Stokes data
reveal large variations of field strength and inclination angle over
dark-cored penumbral intrusions and a dark-cored light bridge. The
mid-outer part of this penumbra structure shows ~0.3” wide spines,
separated by ~1.6” (1200 km) and associated with 30° inclination
variations. Between these spines, there are no small-scale magnetic
structures that easily can be identified with individual flux tubes. A
structure with nearly 10° more vertical and weaker magnetic field is
seen midway between two spines. This structure is cospatial with the
brightest penumbral filament, possibly indicating the location of a
convective upflow from below.

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Title: Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution
Authors: Scharmer, G.; Henriques, V.; Hillberg, T.; Kiselman, D.;
Löfdahl, M.; Narayan, G.; Sütterlin, P.; van Noort, M.; de la Cruz
Rodríguez, J.
2008ESPM...12..2.5S Altcode:
We present first observations of sunspots with the imaging
spectropolarimeter CRISP, recently installed at the Swedish 1-m
Solar Telescope (SST) on La Palma. This spectropolarimeter is based
on a high-fidelity dual Fabry-Perot filter system. Two liquid
crystals and a polarizing beam splitter are used to reduce seeing
induced I,Q,U,V crosstalk by simultaneously recording images with
two 1kx1k back-illuminated Sarnoff CCD's. A third CCD simultaneously
records broadband images through the pre-filter of the FPI filter
system, allowing image reconstruction and co-alignment of images of
different polarization states and at different wavelengths in Zeeman
sensitive spectral lines. The first data, recorded in April 2008,
demonstrate the capability of this system to record high cadence,
high S/N polarimetric data with a spatial resolution at or close to
the diffraction limit of the SST at 630 nm, 0.16 arcsec. We discuss
the analysis of first spectropolarimetric data for sunspots, based on
Milne-Eddington inversion techniques.

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Title: SST/CRISP observations of Ca II 854.2 nm
Authors: de la Cruz Rodríguez, J.; van Noort, M.
2008ESPM...12.2.77D Altcode:
We present observations of the Ca II 854.2 nm line using the new
Crisp Imaging Spectropolarimeter (CRISP) at the SST. CRISP allows
high-cadence, high-spatial resolution scans through the line profile
with full Stokes polarimetry. We discuss the performance of the
instrument and the diagnostic potential of the Ca II IR triplet lines.