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

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.
Bibcode: 2022A&A...665A..50Y
Altcode: 2022arXiv220702840Y
  Context. Solar flares release an enormous amount of energy
  (∼10<SUP>32</SUP> 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. <BR />
  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. <BR /> 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. <BR /> 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<SUP>−2</SUP>). 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<SUP>−2</SUP>)
  in the selected limits of the chromosphere. The temporal evolution
  of total integrated radiative losses exhibits sharply rising losses
  (0.4 kW m<SUP>−2</SUP> s<SUP>−1</SUP>) and a relatively slow decay
  (0.23 kW m<SUP>−2</SUP> s<SUP>−1</SUP>). The maximum value of total
  radiative losses is reached around the flare peak time, and can go up to
  175 kW m<SUP>−2</SUP> 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 × 10<SUP>10</SUP> erg s<SUP>−1</SUP> cm<SUP>−2</SUP>.

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.
Bibcode: 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. <BR /> 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. <BR /> 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. <BR /> 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 amp̅<SUB>Q</SUB> = 7.6 kW m<SUP>−2</SUP>
  and a static (or very slowly evolving) component with a mean value of
  −26.1 kW m<SUP>−2</SUP>. The period of the modulation present in
  the net radiative losses matches that of the line-of-sight velocity
  of the model. <BR /> 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.

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
Bibcode: 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.

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
Bibcode: 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.

Title: Quiet-Sun radiative losses: contribution to chromospheric
    heating
Authors: Pastor Yabar, Adur; De la Cruz Rodriguez, Jaime
Bibcode: 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.

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.
Bibcode: 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. <BR /> 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. <BR
  /> 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. <BR /> 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. <BR /> 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.

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.
Bibcode: 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. <BR />
  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). <BR /> 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. <BR />
  Results: We find enhanced heating rates in the upper chromosphere of up
  to ∼5 kW m<SUP>−2</SUP>, 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 ∼10<SUP>4</SUP> 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. <BR /> 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. <P />The movie is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202243191/olm">https://www.aanda.org</A>

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.
Bibcode: 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. <BR /> 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. <BR
  /> 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. <BR /> 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<SUP>−11</SUP> g cm<SUP>−3</SUP>. <BR /> Conclusions: The total
  mass observed in Hα was found to be 3.9 ± 0.7 × 10<SUP>13</SUP>
  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.

Title: Bayesian Stokes inversion with normalizing flows
Authors: Díaz Baso, C. J.; Asensio Ramos, A.; de la Cruz Rodríguez,
   J.
Bibcode: 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.

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
Bibcode: 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.

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
Bibcode: 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.

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
Bibcode: 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. <BR /> 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. <BR />
  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. <BR /> 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.

Title: Line formation of He I D<SUB>3</SUB> 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
Bibcode: 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 D<SUB>3</SUB> and He I 10 830 Å
  lines, suggesting that their temperatures are higher than previously
  expected. <BR /> Aims: We aim to explain the line formation of He I
  D<SUB>3</SUB> and He I 10 830 Å in small-scale reconnection events. <BR
  /> Methods: We used a simulated EB in a Bifrost-generated radiative
  magnetohydrodynamics snapshot. The resulting He I D<SUB>3</SUB> 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 D<SUB>3</SUB> obtained at the Swedish Solar Telescope
  with the TRI-Port Polarimetric Echelle-Littrow Spectrograph. <BR />
  Results: Emission in He I D<SUB>3</SUB> 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
  D<SUB>3</SUB> 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 × 10<SUP>3</SUP> K
  to 10<SUP>6</SUP> 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 D<SUB>3</SUB>
  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 × 10<SUP>4</SUP> − 2 × 10<SUP>6</SUP>
  K and electron densities between 10<SUP>11</SUP> and 10<SUP>13</SUP>
  cm<SUP>−3</SUP>. The synthetic emission signals are a result of
  coupling to local conditions in a thin shell around the EB, with
  temperatures between 7 × 10<SUP>3</SUP> and 10<SUP>4</SUP> K and
  electron densities ranging from ∼10<SUP>12</SUP> to 10<SUP>13</SUP>
  cm<SUP>−3</SUP>. This shows that both strong non-LTE and thermal
  processes play a role in the formation of He I D<SUB>3</SUB> and
  He I 10 830 Å in the synthetic EB/UV burst that we studied. <BR />
  Conclusions: In conclusion, the synthetic He I D<SUB>3</SUB> and He I 10
  830 Å emission signatures are an indicator of temperatures of at least
  2 × 10<SUP>4</SUP> K; in this case, as high as ∼10<SUP>6</SUP> K.

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.
Bibcode: 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 τ<SUB>500</SUB> ∼ −2.5
  to −3.5 is heated up to 7 kK, whereas from log τ<SUB>500</SUB>
  ∼ −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 τ<SUB>500</SUB>
  ∼ −3) of the flaring atmosphere, whereas for the non-flaring
  atmosphere they are mainly sensitive around log τ<SUB>500</SUB> ∼
  −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.
Bibcode: 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. <BR /> Aims:
  We aim to study the heating caused by a strong reconnection event
  that was triggered by magnetic flux cancelation. <BR /> 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. <BR /> 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<SUP>−1</SUP> 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<SUP>−1</SUP>
  and a blueshift of 100 km s<SUP>−1</SUP>, 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<SUP>−2</SUP>,
  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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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. <BR />
  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. <BR /> 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. <BR />
  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. <BR /> 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.
Bibcode: 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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). <P />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. <P
  />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.
Bibcode: 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.
Bibcode: 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. <BR /> 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. <BR /> 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<SUP>-3</SUP> I<SUB>c</SUB>. We
  then used STiC, a non-local thermodynamic equilibrium inversion
  code to infer the atmospheric structure and magnetic field pixel by
  pixel. <BR /> 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.
Bibcode: 2020A&A...643A..41D
Altcode: 2020arXiv200614564D
  <BR /> 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). <BR /> 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. <BR /> 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<SUP>-1</SUP>) 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. <P />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
Bibcode: 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. <BR /> 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. <BR /> 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. <BR /> 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<SUB>∥</SUB>⟩ ≈ 417 G. <BR /> 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
Bibcode: 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<SUB>.</SUB><SUP>″</SUP>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. <P
  />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.
Bibcode: 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
Bibcode: 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. <BR /> 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. <BR /> 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. <BR />
  Results: The line profiles in bright fibrils have a higher intensity in
  their K<SUB>2</SUB> 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(τ<SUB>500 nm</SUB>)
  = -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<SUP>-1</SUP> 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. <P
  />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
Bibcode: 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). <BR /> 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. <BR /> 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). <BR /> 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. <BR /> 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. <P />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
Bibcode: 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.

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.
Bibcode: 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. <BR /> 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. <BR />
  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. <BR />
  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<SUP>-1</SUP> in plage compared
  to ∼4-7 km s<SUP>-1</SUP> 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.

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
Bibcode: 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<SUP>-1</SUP> appearing near the foot points
  of loops connecting two pores of opposite polarity, whereas strong
  upflows of 22 km s<SUP>-1</SUP> appear near the apex of the loops. At
  the location of supersonic downflows in the chromosphere, we observed
  downflows of 3 km s<SUP>-1</SUP> 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.

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.
Bibcode: 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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.

Title: A method for global inversion of multi-resolution solar data
Authors: de la Cruz Rodríguez, J.
Bibcode: 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.

Title: Solar image denoising with convolutional neural networks
Authors: Díaz Baso, C. J.; de la Cruz Rodríguez, J.; Danilovic, S.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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<SUP>-1</SUP> 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.

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.
Bibcode: 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.

Title: The Dark Side of Penumbral Microjets: Observations in Hα
Authors: Buehler, D.; Esteban Pozuelo, S.; de la Cruz Rodriguez, J.;
   Scharmer, G. B.
Bibcode: 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<SUP>-1</SUP>, 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<SUP>-1</SUP> 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.

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
Bibcode: 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 10<SUP>5</SUP> -
  10<SUP>6</SUP>. 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.

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.
Bibcode: 2019A&A...623A.178D
Altcode: 2019arXiv190206574D
  <BR /> 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. <BR />
  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. <BR /> 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.

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.
Bibcode: 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.

Title: Chromospheric observations and magnetic configuration of a
    supergranular structure
Authors: Robustini, Carolina; Esteban Pozuelo, Sara; Leenaarts,
   Jorrit; de la Cruz Rodríguez, Jaime
Bibcode: 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. <BR /> 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. <BR /> 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
  K<SUB>3</SUB> 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. <BR /> 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. <P />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>

Title: Chromospheric condensations and magnetic field in a C3.6-class
    flare studied via He I D<SUB>3</SUB> spectro-polarimetry
Authors: Libbrecht, Tine; de la Cruz Rodríguez, Jaime; Danilovic,
   Sanja; Leenaarts, Jorrit; Pazira, Hiva
Bibcode: 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. <BR /> Aims:
  We aim to study the chromosphere during a C3.6 class flare via
  spectro-polarimetric observations of the He I D<SUB>3</SUB> line. <BR
  /> Methods: We present the first SST/CRISP spectro-polarimetric
  observations of He I D<SUB>3</SUB>. We analyzed the data using the
  inversion code HAZEL, and estimate the line-of-sight velocity and
  the magnetic field vector. <BR /> Results: Strong He I D<SUB>3</SUB>
  emission at the flare footpoints, as well as strong He I D<SUB>3</SUB>
  absorption profiles tracing the flaring loops are observed during the
  flare. The He I D<SUB>3</SUB> traveling emission kernels at the flare
  footpoints exhibit strong chromospheric condensations of up to ∼60
  km s<SUP>-1</SUP> at their leading edge. Our observations suggest that
  such condensations result in shocking the deep chromosphere, causing
  broad and modestly blueshifted He I D<SUB>3</SUB> 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 D<SUB>3</SUB> 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 D<SUB>3</SUB> inversions. We propose a
  fan-spine configuration as the flare magnetic field topology. <BR />
  Conclusions: The He I D<SUB>3</SUB> 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.

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.
Bibcode: 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.
Bibcode: 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. <BR />
  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. <BR /> 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. <BR /> 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.
Bibcode: 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
Bibcode: 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. <BR /> 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. <BR /> 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. <BR /> Results: Our results indicate
  that the Ca II 8542 Å line in sunspots is greatly sensitive to
  magnetic fields at log τ<SUB>500</SUB> = -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). <BR /> 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.
Bibcode: 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
Bibcode: 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<SUP>-1</SUP>. 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 2018A&A...612A..28L
Altcode: 2017arXiv171200474L
  Context. The radiative losses in the solar chromosphere vary from
  4 kW m<SUP>-2</SUP> in the quiet Sun, to 20 kW m<SUP>-2</SUP> 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. <BR /> 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. <BR /> 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 τ<SUB>500</SUB> = -3 to a more
  space-filling and time-persistent heating above log τ<SUB>500</SUB>
  = -4. The inferred gas temperature at log τ<SUB>500</SUB> = -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. <P />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.
Bibcode: 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. <BR /> 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. <BR /> 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
Bibcode: 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. <BR /> 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. <BR />
  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. <BR />
  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 τ<SUB>500</SUB> = -2 and log
  τ<SUB>500</SUB> = -3. The magnetic field vector and the atmospheric
  temperature maps show a stationary configuration during the whole
  observation. <P />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: Radiative Diagnostics in the Solar Photosphere and Chromosphere
Authors: de la Cruz Rodríguez, J.; van Noort, M.
Bibcode: 2018smf..book..109D
Altcode:
  No abstract at ADS

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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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. <BR /> Aims: Use statistical analysis to test
  whether the association between fibrils and magnetic field lines is
  justified. <BR /> 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. <BR /> 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.
Bibcode: 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
    D<SUB>3</SUB> and He I 10 830 Å
Authors: Libbrecht, Tine; Joshi, Jayant; de la Cruz Rodríguez, Jaime;
   Leenaarts, Jorrit; Ramos, Andrés Asensio
Bibcode: 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 D<SUB>3</SUB> have
  never been reported. <BR /> Aims: We aim to study the signature of
  EBs in neutral helium triplet lines. <BR /> Methods: The observations
  consisted of ten consecutive SST/TRIPPEL raster scans close to the
  limb, featuring the Hβ, He I D<SUB>3</SUB> 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. <BR /> Results: Three EBs in our data show distinct
  emission signatures in neutral helium triplet lines, most prominently
  visible in the He I D<SUB>3</SUB> 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 × 10<SUP>4</SUP>-10<SUP>5</SUP>
  K. <P />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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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<SUP>-1</SUP> 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. <P />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.
Bibcode: 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
Bibcode: 2015A&A...584A...1L
Altcode: 2015arXiv150900741L
  Context. Light bridges are convective intrusions in sunspots that
  often show enhanced chromospheric activity. <BR /> 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. <BR />
  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. <BR />
  Results: We observe the onset of blueshifts of about 2 km s<SUP>-1</SUP>
  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<SUP>-1</SUP> at log τ
  = -2.3 that decreases to zero at log τ = -6.0 <BR /> 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. <P />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.
Bibcode: 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.
Bibcode: 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}<SUP>-2</SUP> arcsec<SUP>-2</SUP>), 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
Bibcode: 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.
Bibcode: 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. <P />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.
Bibcode: 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. <BR />
  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. <BR /> 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. <BR
  /> 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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<SUP>-1</SUP> in proximity
  of the δ spot polarity inversion line (PIL), and horizontal motions
  along the PIL of the order of ~1 km s<SUP>-1</SUP>. 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<SUP>-1</SUP> 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
Bibcode: 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
Bibcode: 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<SUP>-1</SUP> and expands at a
  horizontal speed of 4 km s<SUP>-1</SUP>. 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<SUP>-1</SUP>. 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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. <P />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.
Bibcode: 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.
Bibcode: 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<SUP>-1</SUP>. 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<SUP>-1</SUP>.

Title: DELO-Bezier Formal Solutions of the Polarized Radiative
    Transfer Equation
Authors: de la Cruz Rodríguez, J.; Piskunov, N.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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. <BR /> 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. <BR
  /> 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. <BR />
  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<SUP>-3.5</SUP> 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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. <BR /> Aims: We develop a data
  reduction method that takes these issues into account and minimizes the
  resulting errors. <BR /> 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. <BR />
  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<SUP>-1</SUP>) downflows near the disk center
  side of the umbra. <BR /> 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<SUP>-3</SUP>. 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.
Bibcode: 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.
Bibcode: 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. <BR />
  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. <BR />
  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. <BR /> 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. <BR /> 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<SUP>-1</SUP>, but
  the off-center profiles remain to be tested against observations. <P
  />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.
Bibcode: 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.
Bibcode: 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. <P
  />(11 data files).

Title: Measuring the solar atmosphere
Authors: de la Cruz Rodriguez, Jaime
Bibcode: 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. <P
  />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. <P />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. <P />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.
Bibcode: 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.
Bibcode: 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. <BR /> Aims: We propose a method to compensate for stray light
  from high-order atmospheric aberrations not included in MFBD and JPDS
  processing. <BR /> 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
  r<SUB>0</SUB>. The method is tested with simulations as well as real
  data and extended to include compensation for conventional stray
  light. <BR /> 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. <BR /> 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.
Bibcode: 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.

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

Title: The magnetic SW Sextantis star RXJ1643.7+3402
Authors: Rodríguez-Gil, P.; Martínez-Pais, I. G.; de la Cruz
   Rodríguez, J.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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. <P />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. <P />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.

Title: SST/CRISP observations of Ca II 854.2 nm
Authors: de la Cruz Rodríguez, J.; van Noort, M.
Bibcode: 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.

