ruiz-cobo

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Author name code: ruiz-cobo
ADS astronomy entries on 2022-09-14
author:"Ruiz Cobo, Basilio"

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Title: The umbral filament of active region NOAA 12529
Authors: Guglielmino, Salvatore Luigi; Romano, Paolo; Zuccarello,
Francesca; Murabito, Mariarita; Ruiz Cobo, Basilio
2022cosp...44.2453G Altcode:
Umbral filaments are elongated, filamentary bright structures inside
sunspot umbrae, which differ in morphology, magnetic configuration,
and evolution from light bridges usually observed in sunspots. We
investigated an umbral filament (UF) observed in the umbra of the giant
leading sunspot of active region NOAA 12529, analyzing high-resolution
observations taken in the photosphere with the spectropolarimeter
aboard the \textit{Hinode} satellite and in the upper chromosphere and
transition region with the \textit{IRIS} telescope. We recognize in
the UF the presence of a strong horizontal field larger than 2500 G,
a significant portion of the UF with opposite polarity with respect
to the surroundings, and filaments in the upper atmospheric layers
corresponding to the UF in the photosphere. These findings suggest
that this UF is the photospheric manifestation of a flux rope hanging
above the sunspot and forming penumbral-like filaments within the
umbra via magneto-convection.

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Title: DeSIRe: Departure coefficient aided Stokes Inversion based
on Response functions
Authors: Ruiz Cobo, B.; Quintero Noda, C.; Gafeira, R.; Uitenbroek,
H.; Orozco Suárez, D.; Páez Mañá, E.
2022A&A...660A..37R Altcode: 2022arXiv220202226R
Future ground-based telescopes, such as the 4-metre class facilities
DKIST and EST, will dramatically improve on current capabilities for
simultaneous multi-line polarimetric observations in a wide range of
wavelength bands, from the near-ultraviolet to the near-infrared. As a
result, there will be an increasing demand for fast diagnostic tools,
i.e., inversion codes, that can infer the physical properties of the
solar atmosphere from the vast amount of data these observatories
will produce. The advent of substantially larger apertures,
with the concomitant increase in polarimetric sensitivity, will
drive an increased interest in observing chromospheric spectral
lines. Accordingly, pertinent inversion codes will need to take
account of line formation under general non-local thermodynamic
equilibrium (NLTE) conditions. Several currently available codes can
already accomplish this, but they have a common practical limitation
that impairs the speed at which they can invert polarised spectra,
namely that they employ numerical evaluation of the so-called response
functions to changes in the atmospheric parameters, which makes them
less suitable for the analysis of very large data volumes. Here we
present DeSIRe (Departure coefficient aided Stokes Inversion based on
Response functions), an inversion code that integrates the well-known
inversion code SIR with the NLTE radiative transfer solver RH. The
DeSIRe runtime benefits from employing analytical response functions
computed in local thermodynamic equilibrium (through SIR), modified
with fixed departure coefficients to incorporate NLTE effects in
chromospheric spectral lines. This publication describes the operating
fundamentals of DeSIRe and describes its behaviour, robustness,
stability, and speed. The code is ready to be used by the solar
community and is being made publicly available.

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Title: Inference of electric currents in the solar photosphere
Authors: Pastor Yabar, A.; Borrero, J. M.; Quintero Noda, C.; Ruiz
Cobo, B.
2021A&A...656L..20P Altcode: 2021arXiv211204356P
Context. Despite their importance, routine and direct measurements
of electric currents, j, in the solar atmosphere have generally not
been possible. Aims: We aim at demonstrating the capabilities
of a newly developed method for determining electric currents in
the solar photosphere. Methods: We employ three-dimensional
radiative magneto-hydrodynamic (MHD) simulations to produce synthetic
Stokes profiles in several spectral lines with a spatial resolution
similar to what the newly operational 4-meter Daniel K. Inouye Solar
Telescope solar telescope should achieve. We apply a newly developed
inversion method of the polarized radiative transfer equation with
magneto-hydrostatic (MHS) constraints to infer the magnetic field
vector in the three-dimensional Cartesian domain, B(x, y, z), from the
synthetic Stokes profiles. We then apply Ampere's law to determine
the electric currents, j, from the inferred magnetic field, B(x, y,
z), and compare the results with the electric currents present in
the original MHD simulation. Results: We show that the method
employed here is able to attain reasonable reliability (close to 50%
of the cases are within a factor of two, and this increases to 60%-70%
for pixels with B ≥ 300 G) in the inference of electric currents for
low atmospheric heights (optical depths at 500 nm τ5∈[1,
0.1]) regardless of whether a small or large number of spectral lines
are inverted. Above these photospheric layers, the method's accuracy
strongly deteriorates as magnetic fields become weaker and as the MHS
approximation becomes less accurate. We also find that the inferred
electric currents have a floor value that is related to low-magnetized
plasma, where the uncertainty in the magnetic field inference prevents
a sufficiently accurate determination of the spatial derivatives. 
Conclusions: We present a method that allows the inference of the three
components of the electric current vector at deep atmospheric layers
(photospheric layers) from spectropolarimetric observations.

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Title: Multiple Stokes I inversions for inferring magnetic fields
in the spectral range around Cr I 5782 Å
Authors: Kuckein, C.; Balthasar, H.; Quintero Noda, C.; Diercke, A.;
Trelles Arjona, J. C.; Ruiz Cobo, B.; Felipe, T.; Denker, C.; Verma,
M.; Kontogiannis, I.; Sobotka, M.
2021A&A...653A.165K Altcode: 2021arXiv210711116K
 Aims: In this work, we explore the spectral window containing
Fraunhofer lines formed in the solar photosphere, around the
magnetically sensitive Cr I lines at 5780.9, 5781.1, 5781.7, 5783.0,
and 5783.8 Å, with Landé g-factors between 1.6 and 2.5. The goal is
to simultaneously analyze 15 spectral lines, comprising Cr I, Cu I,
Fe I, Mn I, and Si I lines, without the use of polarimetry, to infer
the thermodynamic and magnetic properties in strongly magnetized
plasmas using an inversion code. Methods: Our study is based
on a new setup at the Vacuum Tower Telescope (VTT, Tenerife), which
includes fast spectroscopic scans in the wavelength range around
the Cr I 5781.75 Å line. The oscillator strengths log(gf) of all
spectral lines, as well as their response functions to temperature,
magnetic field, and Doppler velocity, were determined using the Stokes
Inversion based on Response functions (SIR) code. Snapshot 385 of the
enhanced network simulation from the Bifrost code serves to synthesize
all the lines, which are, in turn, inverted simultaneously with SIR to
establish the best inversion strategy. We applied this strategy to VTT
observations of a sunspot belonging to NOAA 12723 on 2018 September
30 and compared the results to full-disk vector field data obtained
with the Helioseismic and Magnetic Imager (HMI). Results: The
15 simultaneously inverted intensity profiles (Stokes I) delivered
accurate temperatures and Doppler velocities when compared with the
simulations. The derived magnetic fields and inclinations achieve
the best level of accuracy when the fields are oriented along the
line-of-sight (LOS) and less accurate when the fields are transverse to
the LOS. In general, the results appear similar to what is reported in
the HMI vector-field data, although some discrepancies exist. 
Conclusions: The analyzed spectral range has the potential to deliver
thermal, dynamic, and magnetic information for strongly magnetized
features on the Sun, such as pores and sunspots, even without the use
of polarimetry. The highest sensitivity of the lines is found in the
lower photosphere, on average, around log τ = −1. The multiple-line
inversions provide smooth results across the whole field of view
(FOV). The presented spectral range and inversion strategy will be
used for future VTT observing campaigns.

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Title: Diagnostic capabilities of spectropolarimetric observations for
understanding solar phenomena. I. Zeeman-sensitive photospheric lines
Authors: Quintero Noda, C.; Barklem, P. S.; Gafeira, R.; Ruiz Cobo,
B.; Collados, M.; Carlsson, M.; Martínez Pillet, V.; Orozco Suárez,
D.; Uitenbroek, H.; Katsukawa, Y.
2021A&A...652A.161Q Altcode: 2021arXiv210605084Q
Future ground-based telescopes will expand our capabilities for
simultaneous multi-line polarimetric observations in a wide range of
wavelengths, from the near-ultraviolet to the near-infrared. This
creates a strong demand to compare candidate spectral lines to
establish a guideline of the lines that are most appropriate for each
observation target. We focused in this first work on Zeeman-sensitive
photospheric lines in the visible and infrared. We first examined their
polarisation signals and response functions using a 1D semi-empirical
atmosphere. Then we studied the spatial distribution of the line core
intensity and linear and circular polarisation signals using a realistic
3D numerical simulation. We ran inversions of synthetic profiles, and
we compared the heights at which we obtain a high correlation between
the input and the inferred atmosphere. We also used this opportunity
to revisit the atomic information we have on these lines and computed
the broadening cross-sections due to collisions with neutral hydrogen
atoms for all the studied spectral lines. The results reveal that
four spectral lines stand out from the rest for quiet-Sun and network
conditions: Fe I 5250.2, 6302, 8468, and 15 648 Å. The first three
form higher in the atmosphere, and the last line is mainly sensitive to
the atmospheric parameters at the bottom of the photosphere. However,
as they reach different heights, we strongly recommend using at least
one of the first three candidates together with the Fe I 15 648 Å line
to optimise our capabilities for inferring the thermal and magnetic
properties of the lower atmosphere.

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Title: Machine learning initialization to accelerate Stokes profile
inversions
Authors: Gafeira, R.; Orozco Suárez, D.; Milić, I.; Quintero Noda,
C.; Ruiz Cobo, B.; Uitenbroek, H.
2021A&A...651A..31G Altcode: 2021arXiv210309651G
Context. At present, an exponential growth in scientific data
from current and upcoming solar observatories is expected. Most of
the data consist of high spatial and temporal resolution cubes of
Stokes profiles taken in both local thermodynamic equilibrium (LTE)
and non-LTE spectral lines. The analysis of such solar observations
requires complex inversion codes. Hence, it is necessary to develop
new tools to boost the speed and efficiency of inversions and reduce
computation times and costs. Aims: In this work we discuss
the application of convolutional neural networks (CNNs) as a tool to
advantageously initialize Stokes profile inversions. Methods:
To demonstrate the usefulness of CNNs, we concentrate in this paper on
the inversion of LTE Stokes profiles. We use observations taken with
the spectropolarimeter on board the Hinode spacecraft as a test bench
mark. First, we carefully analyse the data with the SIR inversion code
using a given initial atmospheric model. The code provides a set of
atmospheric models that reproduce the observations well. These models
are then used to train a CNN. Afterwards, the same data are again
inverted with SIR but using the trained CNN to provide the initial
guess atmospheric models for SIR. Results: The CNNs allow us
to significantly reduce the number of inversion cycles when used to
compute initial guess model atmospheres (`assisted inversions'),
therefore decreasing the computational time for LTE inversions by
a factor of two to four. CNNs alone are much faster than assisted
inversions, but the latter are more robust and accurate. CNNs also
help to automatically cluster pixels with similar physical properties,
allowing the association with different solar features on the solar
surface, which is useful when inverting huge datasets where completely
different regimes are present. The advantages and limitations of machine
learning techniques for estimating optimum initial atmospheric models
for spectral line inversions are discussed. Finally, we describe a
python wrapper for the SIR and DeSIRe codes that allows for the easy
setup of parallel inversions. The tool implements the assisted inversion
method described in this paper. The parallel wrapper can also be used
to synthesize Stokes profiles with the RH code. Conclusions:
The assisted inversions can speed up the inversion process, but the
efficiency and accuracy of the inversion results depend strongly on
the solar scene and the data used for the CNN training. This method
(assisted inversions) will not obviate the need for analysing individual
events with the utmost care but will provide solar scientists with
a much better opportunity to sample large amounts of inverted data,
which will undoubtedly broaden the physical discovery space.

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Title: Mapping the Hidden Magnetic Field of the Quiet Sun
Authors: Trelles Arjona, J. C.; Martínez González, M. J.; Ruiz
Cobo, B.
2021ApJ...915L..20T Altcode: 2021arXiv210610546T
The Sun is the only star where we can resolve the intricate magnetism
that all convective stars harbor. Yet, more than 99% of its visible
surface along the solar cycle (the so-called quiet Sun) is filled with
a tangled, unresolved magnetism. These "hidden" fields are thought
to store enough magnetic energy to play a role in the heating of
the Sun's outer atmosphere, but its field strength is still not
constrained. Previous investigations based on the Hanle effect in
atomic lines claim a strong magnetization of about 100 G, while
the same effect in molecules show a factor of 10 weaker fields. The
discrepancy disappears if the magnetic field strength of the hidden
field is not homogeneous in the solar surface. In this Letter, we
prove using magnetohydrodynamical simulations that it is possible
to infer the average field strength of the hidden quiet-Sun magnetic
fields using multiline inversions of intensity profiles in the Zeeman
regime. Using this technique with 15 spectral lines in the 1.5 μm
spectral range, we reveal that the spatial distribution of the hidden
field is strongly correlated with convection motions, and that the
average magnetization is about 46 G. Reconciling our findings with
the Hanle ones is not obvious and will require future work on both
sides, since it implies an increase of the field strength with height,
something that is physically questionable.

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Title: Empirical determination of atomic line parameters of the 1.5
μm spectral region
Authors: Trelles Arjona, J. C.; Ruiz Cobo, B.; Martínez González,
M. J.
2021A&A...648A..68T Altcode: 2021arXiv210304160T
Context. Both the quality and amount of astrophysical data are steadily
increasing over time owing to the improvement of telescopes and their
instruments. This requires corresponding evolution of the techniques
used for obtaining and analyzing the resulting data. The infrared
spectral range at 1.56 μm usually observed by the GRegor Infrared
Spectrograph (GRIS) at the GREGOR solar telescope has a width of
around 30 Å and includes at least 15 spectral lines. Normally, only a
handful of spectral lines (five at most) are used in studies using GRIS
because of the lack of atomic parameters for the others. Including more
spectral lines may alleviate some of the known ambiguities between
solar atmospheric parameters. Aims: We used high-precision
spectropolarimetric data for the quiet Sun at 1.56 μm observed with
GRIS on the GREGOR along with the SIR inversion code in order to obtain
accurate atomic parameters for 15 spectral lines in this spectral
range. Methods: We used inversion techniques to infer both solar
atmospheric models and the atomic parameters of spectral lines which,
under the local thermodynamic equilibrium approximation, reproduce
spectropolarimetric observations. Results: We present accurate
atomic parameters for 15 spectral lines within the spectral range from
15 644 to 15 674 Å. This spectral range is commonly used in solar
studies because it enables the study of the low photosphere. Moreover,
the infrared spectral lines are better tracers of the magnetic fields
than the optical ones.

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Title: Combining magneto-hydrostatic constraints with Stokes profiles
inversions. II. Application to Hinode/SP observations
Authors: Borrero, J. M.; Pastor Yabar, A.; Ruiz Cobo, B.
2021A&A...647A.190B Altcode: 2021arXiv210104394B
Context. Inversion techniques applied to the radiative transfer equation
for polarized light are capable of inferring the physical parameters in
the solar atmosphere (temperature T, magnetic field B, and line-of-sight
velocity vlos) from observations of the Stokes vector
(i.e., spectropolarimetric observations) in spectral lines. Inferences
are usually performed in the (x, y, τc) domain, where
τc refers to the optical-depth scale. Generally, their
determination in the (x, y, z) volume is not possible due to the lack
of a reliable estimation of the gas pressure, particularly in regions
of the solar surface harboring strong magnetic fields. Aims:
We aim to develop a new inversion code capable of reliably inferring
the physical parameters in the (x, y, z) domain. Methods: We
combine, in a self-consistent way, an inverse solver for the radiative
transfer equation (Firtez-DZ) with a solver for the magneto-hydrostatic
equilibrium, which derives realistic values of the gas pressure
by taking the magnetic pressure and tension into account. 
Results: We test the correct behavior of the newly developed code with
spectropolarimetric observations of two sunspots recorded with the
spectropolarimeter (SP) instrument on board the Hinode spacecraft,
and we show how the physical parameters are inferred in the (x, y,
z) domain, with the Wilson depression of the sunspots arising as a
natural consequence of the force balance. In particular, our approach
significantly improves upon previous determinations that were based
on semiempirical models. Conclusions: Our results open the door
for the possibility of calculating reliable electric currents in three
dimensions, j(x, y, z), in the solar photosphere. Further consistency
checks would include a comparison with other methods that have recently
been proposed and which achieve similar goals.

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Title: Magnetoacoustic wave energy dissipation in the atmosphere of
solar pores
Authors: Gilchrist-Millar, Caitlin A.; Jess, David B.; Grant, Samuel
D. T.; Keys, Peter H.; Beck, Christian; Jafarzadeh, Shahin; Riedl,
Julia M.; Van Doorsselaere, Tom; Ruiz Cobo, Basilio
2021RSPTA.37900172G Altcode: 2020arXiv200711594G
The suitability of solar pores as magnetic wave guides has been a key
topic of discussion in recent years. Here, we present observational
evidence of propagating magnetohydrodynamic wave activity in a group
of five photospheric solar pores. Employing data obtained by the
Facility Infrared Spectropolarimeter at the Dunn Solar Telescope,
oscillations with periods of the order of 5 min were detected at
varying atmospheric heights by examining Si ɪ 10827 Å line bisector
velocities. Spectropolarimetric inversions, coupled with the spatially
resolved root mean square bisector velocities, allowed the wave energy
fluxes to be estimated as a function of atmospheric height for each
pore. We find propagating magnetoacoustic sausage mode waves with
energy fluxes on the order of 30 kW m-2 at an atmospheric
height of 100 km, dropping to approximately 2 kW m-2
at an atmospheric height of around 500 km. The cross-sectional
structuring of the energy fluxes reveals the presence of both body-
and surface-mode sausage waves. Examination of the energy flux decay
with atmospheric height provides an estimate of the damping length,
found to have an average value across all five pores of Ld
≈ 268 km, similar to the photospheric density scale height. We find
the damping lengths are longer for body mode waves, suggesting that
surface mode sausage oscillations are able to more readily dissipate
their embedded wave energies. This work verifies the suitability of
solar pores to act as efficient conduits when guiding magnetoacoustic
wave energy upwards into the outer solar atmosphere. This article
is part of the Theo Murphy meeting issue `High-resolution wave dynamics
in the lower solar atmosphere'.

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Title: On the Magnetic Nature of an Exploding Granule as Revealed
by Sunrise/IMaX
Authors: Guglielmino, Salvo L.; Martínez Pillet, Valentín; Ruiz
Cobo, Basilio; Bellot Rubio, Luis R.; del Toro Iniesta, José Carlos;
Solanki, Sami K.; Riethmüller, Tino L.; Zuccarello, Francesca
2020ApJ...896...62G Altcode: 2020arXiv200503371G
We study the photospheric evolution of an exploding granule
observed in the quiet Sun at high spatial (∼0"3) and temporal
(31.5 s) resolution by the imaging magnetograph Sunrise/IMaX in 2009
June. These observations show that the exploding granule is cospatial
to a magnetic flux emergence event occurring at mesogranular scale
(up to ∼12 Mm2 area). Using a modified version of the
SIR code for inverting the IMaX spectropolarimetric measurements, we
obtain information about the magnetic configuration of this photospheric
feature. In particular, we find evidence of highly inclined emerging
fields in the structure, carrying a magnetic flux content up to ∼4
× 1018 Mx. The balance between gas and magnetic pressure
in the region of flux emergence, compared with a very quiet region of
the Sun, indicates that the additional pressure carried by the emerging
flux increases the total pressure by about 5% and appears to allow the
granulation to be modified, as predicted by numerical simulations. The
overall characteristics suggest that a multipolar structure emerges
into the photosphere, resembling an almost horizontal flux sheet. This
seems to be associated with exploding granules. Finally, we discuss
the origin of such flux emergence events.

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Title: Capabilities of bisector analysis of the Si I 10 827 Å line
for estimating line-of-sight velocities in the quiet Sun
Authors: González Manrique, S. J.; Quintero Noda, C.; Kuckein, C.;
Ruiz Cobo, B.; Carlsson, M.
2020A&A...634A..19G Altcode: 2020arXiv200100508G
We examine the capabilities of a fast and simple method to infer
line-of-sight (LOS) velocities from observations of the photospheric
Si I 10 827 Å line. This spectral line is routinely observed together
with the chromospheric He I 10 830 Å triplet as it helps to constrain
the atmospheric parameters. We study the accuracy of bisector analysis
and a line core fit of Si I 10 827 Å. We employ synthetic profiles
starting from the Bifrost enhanced network simulation. The profiles are
computed solving the radiative transfer equation, including non-local
thermodynamic equilibrium effects on the determination of the atomic
level populations of Si I. We found a good correlation between the
inferred velocities from bisectors taken at different line profile
intensities and the original simulation velocity at given optical
depths. This good correlation means that we can associate bisectors
taken at different line-profile percentages with atmospheric layers
that linearly increase as we scan lower spectral line intensities. We
also determined that a fit to the line-core intensity is robust and
reliable, providing information about atmospheric layers that are
above those accessible through bisectors. Therefore, by combining
both methods on the Si I 10 827 Å line, we can seamlessly trace the
quiet-Sun LOS velocity stratification from the deep photosphere to
higher layers until around logτ = -3.5 in a fast and straightforward
way. This method is ideal for generating quick-look reference images
for future missions like the Daniel K. Inoue Solar Telescope and the
European Solar Telescope, for example.

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Title: Combining magnetohydrostatic constraints with Stokes profiles
inversions. I. Role of boundary conditions
Authors: Borrero, J. M.; Pastor Yabar, A.; Rempel, M.; Ruiz Cobo, B.
2019A&A...632A.111B Altcode:
Context. Inversion codes for the polarized radiative transfer
equation, when applied to spectropolarimetric observations (i.e.,
Stokes vector) in spectral lines, can be used to infer the temperature
T, line-of-sight velocity vlos, and magnetic field B as
a function of the continuum optical-depth τc. However,
they do not directly provide the gas pressure Pg or density
ρ. In order to obtain these latter parameters, inversion codes rely
instead on the assumption of hydrostatic equilibrium (HE) in addition
to the equation of state (EOS). Unfortunately, the assumption of HE is
rather unrealistic across magnetic field lines, causing estimations
of Pg and ρ to be unreliable. This is because the role
of the Lorentz force, among other factors, is neglected. Unreliable
gas pressure and density also translate into an inaccurate conversion
from optical depth τc to geometrical height z. Aims:
We aim at improving the determination of the gas pressure and density
via the application of magnetohydrostatic (MHS) equilibrium instead of
HE. Methods: We develop a method to solve the momentum equation
under MHS equilibrium (i.e., taking the Lorentz force into account)
in three dimensions. The method is based on the iterative solution of
a Poisson-like equation. Considering the gas pressure Pg
and density ρ from three-dimensional magnetohydrodynamic (MHD)
simulations of sunspots as a benchmark, we compare the results from the
application of HE and MHS equilibrium using boundary conditions with
different degrees of realism. Employing boundary conditions that can
be applied to actual observations, we find that HE retrieves the gas
pressure and density with an error smaller than one order of magnitude
(compared to the MHD values) in only about 47% of the grid points in
the three-dimensional domain. Moreover, the inferred values are within
a factor of two of the MHD values in only about 23% of the domain. This
translates into an error of about 160 - 200 km in the determination of
the z - τc conversion (i.e., Wilson depression). On the
other hand, the application of MHS equilibrium with similar boundary
conditions allows determination of Pg and ρ with an error
smaller than an order of magnitude in 84% of the domain. The inferred
values are within a factor of two in more than 55% of the domain. In
this latter case, the z - τc conversion is obtained with an
accuracy of 30 - 70 km. Inaccuracies are due in equal part to deviations
from MHS equilibrium and to inaccuracies in the boundary conditions. Results: Compared to HE, our new method, based on MHS equilibrium,
significantly improves the reliability in the determination of the
density, gas pressure, and conversion between geometrical height z and
continuum optical depth τc. This method could be used in
conjunction with the inversion of the radiative transfer equation for
polarized light in order to determine the thermodynamic, kinematic,
and magnetic parameters of the solar atmosphere.

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Title: Combining magneto-hydrostatic constraints with Stokes profiles
inversions
Authors: Borrero, J. M.; Pastor Yabar, A.; Rempel, M.; Ruiz Cobo, B.
2019arXiv191014131B Altcode:
Inversion codes for the polarized radiative transfer equation can
be used to infer the temperature $T$, line-of-sight velocity $v_{\rm
los}$, and magnetic field $\rm{\bf B}$ as a function of the continuum
optical-depth $\tau_{\rm c}$. However, they do not directly provide
the gas pressure $P_{\rm g}$ or density $\rho$. In order to obtain
these latter parameters, inversion codes rely instead on the assumption
of hydrostatic equilibrium (HE) in addition to the equation of state
(EOS). Unfortunately, the assumption of HE is rather unrealistic across
magnetic field lines. This is because the role of the Lorentz force,
among other factors, is neglected. This translates into an inaccurate
conversion from optical depth $\tau_{\rm c}$ to geometrical height
$z$. We aim at improving this conversion via the application of
magneto-hydrostatic (MHS) equilibrium instead of HE. We develop a
method to solve the momentum equation under MHS equilibrium (i.e.,
taking the Lorentz force into account) in three dimensions. The method
is based on the solution of a Poisson-like equation. Considering the
gas pressure $P_{\rm g}$ and density $\rho$ from three-dimensional
magneto-hydrodynamic (MHD) simulations of sunspots as a benchmark, we
compare the results from the application of HE and MHS equilibrium. We
find that HE retrieves the gas pressure and density within an order
of magnitude of the MHD values in only about 47 \% of the domain. This
translates into an error of about $160-200$ km in the determination of
the $z-\tau_{\rm c}$ conversion. On the other hand, the application of
MHS equilibrium allows determination of $P_{\rm g}$ and $\rho$ within
an order of magnitude in 84 \% of the domain. In this latter case, the
$z-\tau_{\rm c}$ conversion is obtained with an accuracy of $30-70$ km.

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Title: FIRTEZ-dz. A forward and inverse solver of the polarized
radiative transfer equation under Zeeman regime in geometrical scale
Authors: Pastor Yabar, A.; Borrero, J. M.; Ruiz Cobo, B.
2019A&A...629A..24P Altcode: 2019arXiv190808075P
We present a numerical code that solves the forward and inverse problem
of the polarized radiative transfer equation in geometrical scale under
the Zeeman regime. The code is fully parallelized, making it able to
easily handle large observational and simulated datasets. We checked
the reliability of the forward and inverse modules through different
examples. In particular, we show that even when properly inferring
various physical parameters (temperature, magnetic field components,
and line-of-sight velocity) in optical depth, their reliability in
height-scale depends on the accuracy with which the gas-pressure or
density are known. The code is made publicly available as a tool
to solve the radiative transfer equation and perform the inverse
solution treating each pixel independently. An important feature of
this code, that will be exploited in the future, is that working
in geometrical-scale allows for the direct calculation of spatial
derivatives, which are usually required in order to estimate the gas
pressure and/or density via the momentum equation in a three-dimensional
volume, in particular the three-dimensional Lorenz force.

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

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Title: Properties of the Umbral Filament Observed in Active Region
NOAA 12529
Authors: Guglielmino, Salvo L.; Romano, Paolo; Ruiz Cobo, Basilio;
Zuccarello, Francesca; Murabito, Mariarita
2019ApJ...880...34G Altcode: 2019arXiv190600065G
Recent observations of the solar photosphere revealed the presence
of elongated filamentary bright structures inside sunspot umbrae,
called umbral filaments (UFs). These features differ in morphology,
magnetic configuration, and evolution from light bridges (LBs) that are
usually observed to intrude in sunspots. To characterize a UF observed
in the umbra of the giant leading sunspot of active region NOAA 12529,
we analyze high-resolution observations taken in the photosphere with
the spectropolarimeter on board the Hinode satellite and in the upper
chromosphere and transition region with the IRIS telescope. The results
of this analysis definitely rule out the hypothesis that the UF might
be a kind of LB. In fact, we find no field-free or low-field strength
region cospatial to the UF. Conversely, we recognize the presence of
a strong horizontal field larger than 2500 G, a significant portion
of the UF with opposite polarity with respect to the surroundings,
and filaments in the upper atmospheric layers corresponding to the
UF in the photosphere. These findings suggest that this structure
is the photospheric manifestation of a flux rope hanging above the
sunspot and forming penumbral-like filaments within the umbra via
magneto-convection. This reinforces a previously proposed scenario.

---------------------------------------------------------
Title: On the Magnetic Nature of Solar Exploding Granules
Authors: Guglielmino, S. L.; Martínez Pillet, V.; Ruiz Cobo,
B.; Bellot Rubio, L. R.; del Toro Iniesta, J. C.; Solanki, S. K.;
Zuccarello, F.
2019ASPC..526..299G Altcode:
We report on spectropolarimetric observations acquired by the imaging
magnetograph SUNRISE/IMaX at high spatial 0.”3 and temporal (31.5 s)
resolution during the first science flight of this balloon-borne solar
observatory. We describe the photospheric evolution of an exploding
granule observed in the quiet Sun. This granule is cospatial with
a magnetic flux emergence event occurring at mesogranular scales
(up to ∼12 Mm2 area). Using a modified version of the
SIR code, we show that we can estimate the longitudinal field also
in the presence of a residual cross-talk in these IMaX longitudinal
measurements. We determine the magnetic flux content of the structure
(∼3 ×1018 Mx), which appears to have a multipolar
configuration, and discuss the origin of such flux emergence events.

---------------------------------------------------------
Title: Helioseismic Inversion method applied to Stokes data
Authors: Agrawal, Piyush; Rast, Mark; Ruiz Cobo, Basilio
2019shin.confE.132A Altcode:
As light travels through an atmosphere, it interacts with the medium
through absorption, emission and scattering processes. Given a light
spectra, inferring the physical properties (for example T, Pg, velocity)
of the atmosphere it traversed, is called an inversion problem. To infer
the unknown atmosphere, one usually starts with a depth-dependent guess
atmospheric model and perturbs it until the synthesized spectra through
this model match the observed spectra. The desired perturbations are
computed using response functions which is a measure of the sensitivity
of spectra to changes in atmospheric variables. Due to the ill-posed
nature of inverse problems, the solutions are non-unique and highly
oscillatory. Thus, nodes are used to obtain a smooth solution. These
nodes are a small number of evenly spaced depth locations where the
perturbations are calculated. Perturbations at remaining depth points
are interpolated using these nodal values. The final model has a depth
resolution set by the number of nodes, independent of the information
content of the spectra. The solution thus obtained, most likely, does
not have the optimal depth resolution. The OLA inversion method
used in helioseismology does not suffer from the limited resolution
issues with nodes. In this method, the response functions are linearly
combined in order to obtain a highly localized, average response kernel
at a given target depth. The width of the kernel corresponds to the
vertical resolution at that depth, and its limit mostly depends on the
amount of spectral information. The inverted physical parameter then
corresponds to this kernel averaged quantity. The process is repeated
for all depths and a smooth inverted solution is obtained. In
this work, we aim to apply the OLA method to spectroscopic data. To
facilitate this, we used SIR code to synthesize spectra through the
1D smooth temperature profiles from MURaM. To this 1D model, we added
a Gaussian perturbation. The goal of the project is how well can we
invert for this perturbed atmosphere using OLA method and how do the
results compare to the SIR inversion code.

---------------------------------------------------------
Title: High-resolution observations of the umbral filament in AR
NOAA 12529
Authors: Guglielmino, S. L.; Romano, P.; Ruiz Cobo, B.; Zuccarello,
, F.; Murabito, M.
2019NCimC..42...12G Altcode:
Recent observations have shown in some sunspots the presence of
structures that have been called umbral filaments (UFs). These consist
of bright filamentary features intruding sunspot umbrae, different in
morphology, evolution, and magnetic configuration from usually observed
light bridges. We report on the properties of an UF observed inside
the umbra of the giant leading sunspot in active region NOAA 12529. We
analysed high-resolution observations taken in the photosphere with
the spectropolarimeter aboard the Hinode satellite and in the upper
chromosphere and transition region with the IRIS telescope. These
observations were complemented with data from the Solar Dynamic
Observatory satellite and from the INAF-OACT equatorial spar to study
the evolution of this structure. We find that the UF harbours a strong
horizontal component of the magnetic field and a portion with polarity
opposite with respect to that of the umbra. In the upper atmospheric
layers, the structure is cospatial to a bundle of filaments, which
appears to be rooted in the sunspot umbra. We propose that the UF is the
photospheric counterpart of a flux rope touching the sunspot and giving
rise to penumbral-like filaments in the umbra via magneto-convection.

---------------------------------------------------------
Title: Spectro-polarimetric analysis of a short lived solar active
region
Authors: Viavattene, G.; Zuccarello, F.; Collados Vera, , M.; Ruiz
Cobo, B.
2019NCimC..42...10V Altcode:
The physical processes related to the formation, evolution
and disappearance of solar active regions are not completely
clear. High-resolution solar spectro-polarimetric data are needed
to investigate these processes with unprecedented details. Here we
present the analysis of the short-lived NOAA 12549 active region using
high-resolution spectro-polarimetric data acquired with the GREGOR
solar telescope and the GRIS instrument, inverted using the SIR code.

---------------------------------------------------------
Title: Study of the polarization produced by the Zeeman effect in
the solar Mg I b lines
Authors: Quintero Noda, C.; Uitenbroek, H.; Carlsson, M.; Orozco
Suárez, D.; Katsukawa, Y.; Shimizu, T.; Ruiz Cobo, B.; Kubo, M.; Oba,
T.; Kawabata, Y.; Hasegawa, T.; Ichimoto, K.; Anan, T.; Suematsu, Y.
2018MNRAS.481.5675Q Altcode: 2018arXiv181001067Q; 2018MNRAS.tmp.2566Q
The next generation of solar observatories aim to understand the
magnetism of the solar chromosphere. Therefore, it is crucial to
understand the polarimetric signatures of chromospheric spectral
lines. For this purpose, we here examine the suitability of the three
Fraunhofer Mg I b1, b2, and b4 lines
at 5183.6, 5172.7, and 5167.3 Å, respectively. We start by describing
a simplified atomic model of only six levels and three line transitions
for computing the atomic populations of the 3p-4s (multiplet number
2) levels involved in the Mg I b line transitions assuming non-local
thermodynamic conditions and considering only the Zeeman effect using
the field-free approximation. We test this simplified atom against
more complex ones finding that, although there are differences in the
computed profiles, they are small compared with the advantages provided
by the simple atom in terms of speed and robustness. After comparing
the three Mg I lines, we conclude that the most capable one is the
b2 line as b1 forms at similar heights and always
shows weaker polarization signals, while b4 is severely
blended with photospheric lines. We also compare Mg I b2
with the K I D1 and Ca II 8542 Å lines finding that the
former is sensitive to the atmospheric parameters at heights that
are in between those covered by the latter two lines. This makes Mg I
b2 an excellent candidate for future multiline observations
that aim to seamlessly infer the thermal and magnetic properties of
different features in the lower solar atmosphere.

---------------------------------------------------------
Title: Solar polarimetry in the K I D2 line : A novel
possibility for a stratospheric balloon
Authors: Quintero Noda, C.; Villanueva, G. L.; Katsukawa, Y.; Solanki,
S. K.; Orozco Suárez, D.; Ruiz Cobo, B.; Shimizu, T.; Oba, T.; Kubo,
M.; Anan, T.; Ichimoto, K.; Suematsu, Y.
2018A&A...610A..79Q Altcode: 2018arXiv180101655Q
Of the two solar lines, K I D1 and D2, almost
all attention so far has been devoted to the D1 line, as
D2 is severely affected by an O2 atmospheric
band. This, however, makes the latter appealing for balloon and space
observations from above (most of) the Earth's atmosphere. We estimate
the residual effect of the O2 band on the K I D2
line at altitudes typical for stratospheric balloons. Our aim is to
study the feasibility of observing the 770 nm window. Specifically,
this paper serves as a preparation for the third flight of the Sunrise
balloon-borne observatory. The results indicate that the absorption
by O2 is still present, albeit much weaker, at the expected
balloon altitude. We applied the obtained O2 transmittance
to K I D2 synthetic polarimetric spectra and found that in
the absence of line-of-sight motions, the residual O2 has
a negligible effect on the K I D2 line. On the other hand,
for Doppler-shifted K I D2 data, the residual O2
might alter the shape of the Stokes profiles. However, the residual
O2 absorption is sufficiently weak at stratospheric levels
that it can be divided out if appropriate measurements are made,
something that is impossible at ground level. Therefore, for the
first time with Sunrise III, we will be able to perform polarimetric
observations of the K I D2 line and, consequently, we will
have improved access to the thermodynamics and magnetic properties of
the upper photosphere from observations of the K I lines.

---------------------------------------------------------
Title: Detection of emission in the Si I 1082.7 nm line core in
sunspot umbrae
Authors: Orozco Suárez, D.; Quintero Noda, C.; Ruiz Cobo, B.;
Collados Vera, M.; Felipe, T.
2017A&A...607A.102O Altcode: 2017arXiv170906773O
Context. Determining empirical atmospheric models for the solar
chromosphere is difficult since it requires the observation and
analysis of spectral lines that are affected by non-local thermodynamic
equilibrium (NLTE) effects. This task is especially difficult in sunspot
umbrae because of lower continuum intensity values in these regions
with respect to the surrounding brighter granulation. Umbral data is
therefore more strongly affected by the noise and by the so-called
scattered light, among other effects. Aims: The purpose of this
study is to analyze spectropolarimetric sunspot umbra observations
taken in the near-infrared Si I 1082.7 nm line taking NLTE effects into
account. Interestingly, we detected emission features at the line core
of the Si I 1082.7 nm line in the sunspot umbra. Here we analyze the
data in detail and offer a possible explanation for the Si I 1082.7 nm
line emission. Methods: Full Stokes measurements of a sunspot
near disk center in the near-infrared spectral range were obtained
with the GRIS instrument installed at the German GREGOR telescope. A
point spread function (PSF) including the effects of the telescope,
the Earth's atmospheric seeing, and the scattered light was constructed
using prior Mercury observations with GRIS and the information provided
by the adaptive optics system of the GREGOR telescope during the
observations. The data were then deconvolved from the PSF using a
principal component analysis deconvolution method and were analyzed
via the NICOLE inversion code, which accounts for NLTE effects in the
Si I 1082.7 nm line. The information of the vector magnetic field was
included in the inversion process. Results: The Si I 1082.7 nm
line seems to be in emission in the umbra of the observed sunspot after
the effects of scattered light (stray light coming from wide angles)
are removed. We show how the spectral line shape of umbral profiles
changes dramatically with the amount of scattered light. Indeed, the
continuum levels range, on average, from 44% of the quiet Sun continuum
intensity to about 20%. Although very low, the inferred levels are in
line with current model predictions and empirical umbral models. The Si
I 1082.7 nm line is in emission after adding more that 30% of scattered
light so that it is very sensitive to a proper determination of the
PSF. Additionally, we have thoroughly investigated whether the emission
is a byproduct of the particular deconvolution technique but have not
found any evidence to the contrary. Only the circular polarization
signals seem to be more sensitive to the deconvolution strategy
because of the larger amount of noise in the umbra. Interestingly,
current umbral empirical models are not able to reproduce the emission
in the deconvolved umbral Stokes profiles. The results of the NLTE
inversions suggests that to obtain the emission in the Si I 1082.7 nm
line, the temperature stratification should first have a hump located
at about log τ = -2 and start rising at lower heights when moving into
the transition region. Conclusions: This is, to our knowledge,
the first time the Si I 1082.7 nm line is seen in emission in sunspot
umbrae. The results show that the temperature stratification of current
umbral models may be more complex than expected with the transition
region located at lower heights above sunspot umbrae. Our finding might
provide insights into understanding why the sunspot umbra emission in
the millimeter spectral range is less than that predicted by current
empirical umbral models.

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

---------------------------------------------------------
Title: Solar polarimetry through the K I lines at 770 nm
Authors: Quintero Noda, C.; Uitenbroek, H.; Katsukawa, Y.; Shimizu,
T.; Oba, T.; Carlsson, M.; Orozco Suárez, D.; Ruiz Cobo, B.; Kubo,
M.; Anan, T.; Ichimoto, K.; Suematsu, Y.
2017MNRAS.470.1453Q Altcode: 2017arXiv170510002Q
We characterize the K I D1 &amp; D2 lines in
order to determine whether they could complement the 850 nm window,
containing the Ca II infrared triplet lines and several Zeeman sensitive
photospheric lines, that was studied previously. We investigate the
effect of partial redistribution on the intensity profiles, their
sensitivity to changes in different atmospheric parameters, and
the spatial distribution of Zeeman polarization signals employing a
realistic magnetohydrodynamic simulation. The results show that these
lines form in the upper photosphere at around 500 km, and that they
are sensitive to the line-of-sight velocity and magnetic field strength
at heights where neither the photospheric lines nor the Ca II infrared
lines are. However, at the same time, we found that their sensitivity
to the temperature essentially comes from the photosphere. Then, we
conclude that the K I lines provide a complement to the lines in the
850 nm window for the determination of atmospheric parameters in the
upper photosphere, especially for the line-of-sight velocity and the
magnetic field.

---------------------------------------------------------
Title: Spectropolarimetric Evidence for a Siphon Flow along an
Emerging Magnetic Flux Tube
Authors: Requerey, Iker S.; Ruiz Cobo, B.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Blanco Rodríguez, J.; Solanki, S. K.; Barthol,
P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.;
van Noort, M.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229...15R Altcode: 2016arXiv161106732R
We study the dynamics and topology of an emerging magnetic flux
concentration using high spatial resolution spectropolarimetric data
acquired with the Imaging Magnetograph eXperiment on board the sunrise
balloon-borne solar observatory. We obtain the full vector magnetic
field and the line of sight (LOS) velocity through inversions of
the Fe I line at 525.02 nm with the SPINOR code. The derived vector
magnetic field is used to trace magnetic field lines. Two magnetic flux
concentrations with different polarities and LOS velocities are found
to be connected by a group of arch-shaped magnetic field lines. The
positive polarity footpoint is weaker (1100 G) and displays an upflow,
while the negative polarity footpoint is stronger (2200 G) and shows
a downflow. This configuration is naturally interpreted as a siphon
flow along an arched magnetic flux tube.

---------------------------------------------------------
Title: Inversion of the radiative transfer equation for polarized
light
Authors: del Toro Iniesta, Jose Carlos; Ruiz Cobo, Basilio
2016LRSP...13....4D Altcode: 2016arXiv161010039D
Since the early 1970s, inversion techniques have become the most
useful tool for inferring the magnetic, dynamic, and thermodynamic
properties of the solar atmosphere. Inversions have been proposed
in the literature with a sequential increase in model complexity:
astrophysical inferences depend not only on measurements but also on
the physics assumed to prevail both on the formation of the spectral
line Stokes profiles and on their detection with the instrument. Such
an intrinsic model dependence makes it necessary to formulate specific
means that include the physics in a properly quantitative way. The core
of this physics lies in the radiative transfer equation (RTE), where the
properties of the atmosphere are assumed to be known while the unknowns
are the four Stokes profiles. The solution of the (differential)
RTE is known as the direct or forward problem. From an observational
point of view, the problem is rather the opposite: the data are made
up of the observed Stokes profiles and the unknowns are the solar
physical quantities. Inverting the RTE is therefore mandatory. Indeed,
the formal solution of this equation can be considered an integral
equation. The solution of such an integral equation is called the
inverse problem. Inversion techniques are automated codes aimed at
solving the inverse problem. The foundations of inversion techniques
are critically revisited with an emphasis on making explicit the many
assumptions underlying each of them.

---------------------------------------------------------
Title: Analysis of a spatially deconvolved solar pore
Authors: Quintero Noda, C.; Shimizu, T.; Ruiz Cobo, B.; Suematsu,
Y.; Katsukawa, Y.; Ichimoto, K.
2016MNRAS.460.1476Q Altcode: 2016arXiv160501796Q; 2016MNRAS.tmp..847Q
Solar pores are active regions with large magnetic field strengths
and apparent simple magnetic configurations. Their properties
resemble the ones found for the sunspot umbra although pores do
not show penumbra. Therefore, solar pores present themselves as an
intriguing phenomenon that is not completely understood. We examine
in this work a solar pore observed with Hinode/SP using two state
of the art techniques. The first one is the spatial deconvolution
of the spectropolarimetric data that allows removing the stray
light contamination induced by the spatial point spread function
of the telescope. The second one is the inversion of the Stokes
profiles assuming local thermodynamic equilibrium that let us to
infer the atmospheric physical parameters. After applying these
techniques, we found that the spatial deconvolution method does not
introduce artefacts, even at the edges of the magnetic structure,
where large horizontal gradients are detected on the atmospheric
parameters. Moreover, we also describe the physical properties of
the magnetic structure at different heights finding that, in the
inner part of the solar pore, the temperature is lower than outside,
the magnetic field strength is larger than 2 kG and unipolar, and
the line-of-sight velocity is almost null. At neighbouring pixels,
we found low magnetic field strengths of same polarity and strong
downward motions that only occur at the low photosphere, below the
continuum optical depth log τ = -1. Finally, we studied the spatial
relation between different atmospheric parameters at different heights
corroborating the physical properties described before.

---------------------------------------------------------
Title: Analysis of spatially deconvolved polar faculae
Authors: Quintero Noda, C.; Suematsu, Y.; Ruiz Cobo, B.; Shimizu,
T.; Asensio Ramos, A.
2016MNRAS.460..956Q Altcode: 2016MNRAS.tmp..838Q; 2016arXiv160500330Q
Polar faculae are bright features that can be detected in
solar limb observations and they are related to magnetic field
concentrations. Although there are a large number of works studying
them, some questions about their nature as their magnetic properties
at different heights are still open. Thus, we aim to improve the
understanding of solar polar faculae. In that sense, we infer
the vertical stratification of the temperature, gas pressure,
line-of-sight velocity and magnetic field vector of polar faculae
regions. We performed inversions of the Stokes profiles observed
with Hinode/Spectropolarimeter after removing the stray light
contamination produced by the spatial point spread function of the
telescope. Moreover, after solving the azimuth ambiguity, we transform
the magnetic field vector to local solar coordinates. The obtained
results reveal that the polar faculae are constituted by hot plasma with
low line-of-sight velocities and single polarity magnetic fields in the
kilogauss range that are nearly perpendicular to the solar surface. We
also found that the spatial location of these magnetic fields is
slightly shifted respect to the continuum observations towards the
disc centre. We believe that this is due to the hot wall effect that
allows detecting photons that come from deeper layers located closer
to the solar limb.

---------------------------------------------------------
Title: Future of Inversion Tools
Authors: del Toro Iniesta, J. C.; Ruiz Cobo, B.
2015AGUFMSH21C..02D Altcode:
Since the early 1970's, inversion techniques have become the most
useful tool for inferring the magnetic, dynamic and thermodynamic
properties of the solar atmosphere. They have evolved with model
dependence as a driver: astrophysical inferences do not only depend
on measurements but also on the physics assumed to prevail both on the
formation of the spectral line Stokes profiles and on their detection
with the instrument. Such an intrinsic model dependence makes it
necessary to formulate specific means that include the physics in a
proper quantitative way. The core of this physics is in the radiative
transfer equation (RTE), where the properties of the atmosphere are
assumed to be known while the unknowns are the four Stokes profiles. The
solution of the (differential) RTE is known as the direct or forward
problem. From an observational point of view, the problem is rather
the opposite: the data are made up of the observed Stokes profiles and
the unknowns are the solar physical quantities. Inverting the RTE is
therefore mandatory. Indeed, the formal solution of this equation can
be considered an integral equation. The solution of such an integral
equation is called the inverse problem. Inversion techniques are
automated codes aimed at solving the inverse problem. The foundations
of inversion techniques are critically revisited with an emphasis
on making explicit the many assumptions underlying each of them. An
incremental complexity procedure is advised for the implementation in
practice. Coarse details of the profiles or coarsely sampled profiles
should be reproduced first with simple model atmospheres (with, for
example, a few physical quantities that are constant with optical
depth). If the Stokes profiles are well sampled and differences
between synthetic and observed ones are larger than the noise, then
the inversion should proceed by using more complex models (that is,
models where physical quantities vary with depth or, eventually, with
more than one component). Significant improvements are expected as
well from the use of new inversion techniques that take the spatial
degradation by the instruments into account.

---------------------------------------------------------
Title: Spatial deconvolution of spectropolarimetric data: an
application to quiet Sun magnetic elements
Authors: Quintero Noda, C.; Asensio Ramos, A.; Orozco Suárez, D.;
Ruiz Cobo, B.
2015A&A...579A...3Q Altcode: 2015arXiv150503219Q
Context. One of the difficulties in extracting reliable information
about the thermodynamical and magnetic properties of solar plasmas from
spectropolarimetric observations is the presence of light dispersed
inside the instruments, known as stray light. Aims: We aim
to analyze quiet Sun observations after the spatial deconvolution
of the data. We examine the validity of the deconvolution process
with noisy data as we analyze the physical properties of quiet Sun
magnetic elements. Methods: We used a regularization method
that decouples the Stokes inversion from the deconvolution process,
so that large maps can be quickly inverted without much additional
computational burden. We applied the method on Hinode quiet Sun
spectropolarimetric data. We examined the spatial and polarimetric
properties of the deconvolved profiles, comparing them with the
original data. After that, we inverted the Stokes profiles using the
Stokes Inversion based on Response functions (SIR) code, which allow
us to obtain the optical depth dependence of the atmospheric physical
parameters. Results: The deconvolution process increases
the contrast of continuum images and makes the magnetic structures
sharper. The deconvolved Stokes I profiles reveal the presence of
the Zeeman splitting while the Stokes V profiles significantly change
their amplitude. The area and amplitude asymmetries of these profiles
increase in absolute value after the deconvolution process. We inverted
the original Stokes profiles from a magnetic element and found that
the magnetic field intensity reproduces the overall behavior of
theoretical magnetic flux tubes, that is, the magnetic field lines
are vertical in the center of the structure and start to fan when we
move far away from the center of the magnetic element. The magnetic
field vector inferred from the deconvolved Stokes profiles also mimic a
magnetic flux tube but in this case we found stronger field strengths
and the gradients along the line-of-sight are larger for the magnetic
field intensity and for its inclination. Moreover, the discontinuity
between the magnetic and non magnetic environment in the flux tube
gets sharper. Conclusions: The deconvolution process used
in this paper reveals information that the smearing induced by the
point spread function (PSF) of the telescope hides. Additionally,
the deconvolution is done with a low computational load, making it
appealing for its use on the analysis of large data sets. A
copy of the IDL code is available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/579/A3">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/579/A3</A>

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

---------------------------------------------------------
Title: VizieR Online Data Catalog: Spatial deconvolution code
(Quintero Noda+, 2015)
Authors: Quintero Noda, C.; Asensio Ramos, A.; Orozco Suarez, D.;
Ruiz Cobo, B.
2015yCat..35790003Q Altcode:
This deconvolution method follows the scheme presented in Ruiz Cobo
&amp; Asensio Ramos (2013A&amp;A...549L...4R) The Stokes parameters
are projected onto a few spectral eigenvectors and the ensuing maps
of coefficients are deconvolved using a standard Lucy-Richardson
algorithm. This introduces a stabilization because the PCA filtering
reduces the amount of noise. (1 data file).

---------------------------------------------------------
Title: High speed magnetized flows in the quiet Sun
Authors: Quintero Noda, C.; Borrero, J. M.; Orozco Suárez, D.;
Ruiz Cobo, B.
2014A&A...569A..73Q Altcode: 2014arXiv1407.7477Q
Context. We analyzed spectropolarimetric data recorded with
Hinode/SP in quiet-Sun regions located at the disk center. We found
single-lobed Stokes V profiles showing highly blue- and red-shifted
signals. Oftentimes both types of events appear to be related to
each other. Aims: We aim to set constraints on the nature and
physical causes of these highly Doppler-shifted signals, as well as to
study their spatial distribution, spectropolarimetric properties, size,
and rate of occurrence. Also, we plan to retrieve the variation of the
physical parameters with optical depth through the photosphere. 
Methods: We have examined the spatial and polarimetric properties of
these events using a variety of data from the Hinode spacecraft. We
have also inferred the atmospheric stratification of the physical
parameters by means of the inversion of the observed Stokes profiles
employing the Stokes Inversion based on Response functions (SIR)
code. Finally, we analyzed their evolution using a time series from
the same instrument. Results: Blue-shifted events tend to appear
over bright regions at the edge of granules, while red-shifted events
are seen predominantly over dark regions on intergranular lanes. Large
linear polarization signals can be seen in the region that connects
them. The magnetic structure inferred from the time series revealed that
the structure corresponds to a Ω-loop, with one footpoint always over
the edge of a granule and the other inside an intergranular lane. The
physical parameters obtained from the inversions of the observed
Stokes profiles in both events show an increase with respect to the
Harvard-Smithonian reference atmosphere in the temperature at log
τ500 ∈ (-1, -3) and a strong magnetic field, B ≥ 1 kG,
at the bottom of the atmosphere that quickly decreases upward until
vanishing at log τ500 ≈ -2. In the blue-shifted events,
the LOS velocities change from upflows at the bottom to downflows at
the top of the atmosphere. Red-shifted events display the opposite
velocity stratification. The change of sign in LOS velocity happens at
the same optical depth in which the magnetic field becomes zero. Conclusions: The physical mechanism that best explains the inferred
magnetic field configuration and flow motions is a siphon flow along an
arched magnetic flux tube. Further investigation is required, however,
as the expected features of a siphon flow cannot be unequivocally
identified.

---------------------------------------------------------
Title: Photospheric downward plasma motions in the quiet Sun
Authors: Quintero Noda, C.; Ruiz Cobo, B.; Orozco Suárez, D.
2014A&A...566A.139Q Altcode: 2014arXiv1405.1561Q
Context. We analyze spectropolarimetric data taken with the Hinode
spacecraft in quiet solar regions at the disk center. Distorted
redshifted Stokes V profiles are found that show a characteristic
evolution that always follows the same sequence of phases. Aims:
We aim to characterize the statistical properties of these events
and recover the stratification of the relevant physical quantities
to understand the nature of the mechanism behind them. 
Methods: We studied the statistical properties of these events using
spectropolarimetric data from Hinode/SP. We also examined the upper
photosphere and the low chromosphere using Mg i b2 and Ca
ii h data from Hinode. Finally, we applied the SIRGAUSS inversion code
to the polarimetric data to infer the atmospheric stratification of the
physical parameters. We also obtained these physical parameters taking
into account dynamical terms in the equation of motion. Results:
The Stokes V profiles display a bump that evolves in four different
time steps, and the total process lasts 108 seconds. The Stokes I shows
a strongly bent red wing and the continuum signal exhibits a bright
point inside an intergranular lane. This bright point is correlated
with a strong redshift in the Mg i b2 line and a bright
feature in Ca ii h images. The model obtained from the inversion of
the Stokes profiles is hotter than the average quiet-Sun model, with a
vertical magnetic field configuration and field strengths in the range
of kG values. It also presents a line of sight velocity stratification
with a Gaussian perturbation, the center of which is moving to deeper
layers with time. The Gaussian perturbation is also found in the gas
pressure and density stratification obtained taking into account
dynamical terms in the equation of motion. Conclusions: We
have examined a particular type of event that can be described as a
plasmoid of hot plasma that is moving downward from the top of the
photosphere, placed over intergranular lanes and always related to
strong magnetic field concentrations. We argue that the origin of this
plasmoid might be magnetic reconnection that is taking place in the
chromosphere. Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201423461/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Returning magnetic flux in sunspot penumbrae
Authors: Ruiz Cobo, B.; Asensio Ramos, A.
2013A&A...549L...4R Altcode: 2012arXiv1211.6335R
 Aims: We study the presence of reversed polarity magnetic flux in
sunspot penumbra. Methods: We applied a new regularized method to
deconvolve spectropolarimetric data observed with the spectropolarimeter
SP onboard Hinode. The new regularization is based on a principal
component decomposition of the Stokes profiles. The resulting Stokes
profiles were inverted to infer the magnetic field vector using SIR. Results: We find, for the first time, reversed polarity fields at
the border of many bright penumbral filaments in the whole penumbra.

---------------------------------------------------------
Title: Inversions of L12-2 IMaX data of an emerging flux mantle
Authors: Guglielmino, S. L.; Martínez Pillet, V.; Ruiz Cobo,
B.; del Toro Iniesta, J. C.; Bellot Rubio, L. R.; Solanki, S. K.;
Sunrise/IMaX Team
2013MmSAI..84..355G Altcode:
We present the analysis of a flux emergence event observed with
the IMaX magnetograph flown aboard the SUNRISE balloon. IMaX took a
15' sequence with cadence of 31 s along the Fe I line at 525.0 nm,
acquiring only Stokes I and V at 12 line positions (L12-2 mode). This
sequence shows the emergence of a flux mantle at mesogranular scale,
cospatial with a large exploding granule. An undesired cross-talk
between Stokes U and V was found in such L12-2 data. We show that the
use of a modified version of the SIR inversion code is able to remove
such effect in inferring the physical quantities of interest.

---------------------------------------------------------
Title: SIR: Stokes Inversion based on Response functions
Authors: Ruiz Cobo, B.; del Toro Iniesta, J. C.
2012ascl.soft12008R Altcode:
SIR is a general-purpose code capable of dealing with gradients of
the physical quantities with height. It admits one and two-component
model atmospheres. It allows the recovery of the stratification of the
temperature, the magnetic field vector, and the line of sight velocity
through the atmosphere, and the micro- and macroturbulence velocities
- which are assumed to be constant with depth. It is based on the
response functions, which enter a Marquardt nonlinear least-squares
algorithm in a natural way. Response functions are calculated at the
same time as the full radiative transfer equation for polarized light
is integrated, which determines values of many free parameters in a
reasonable computation time. SIR demonstrates high stability, accuracy,
and uniqueness of results, even when simulated observations present
signal-to-noise ratios of the order of the lowest acceptable values
in real observations.

---------------------------------------------------------
Title: Twist, Writhe, and Helicity in the Inner Penumbra of a Sunspot
Authors: Ruiz Cobo, B.; Puschmann, K. G.
2012ApJ...745..141R Altcode: 2011arXiv1111.3881R
The aim of this work is the determination of the twist, writhe, and
self-magnetic helicity of penumbral filaments located in an inner
sunspot penumbra. For this purpose, we inverted data taken with the
spectropolarimeter on board Hinode with the SIR (Stokes Inversion based
on Response function) code. For the construction of a three-dimensional
geometrical model we applied a genetic algorithm minimizing
the divergence of \vec{B} and the net magnetohydrodynamic force,
consequently a force-free solution would be reached if possible. We
estimated two proxies to the magnetic helicity frequently used in
literature: the force-free parameter α z and the current
helicity term h_{c_{z}}. We show that both proxies are only qualitative
indicators of the local twist as the magnetic field in the area under
study significantly departs from a force-free configuration. The
local twist shows significant values only at the borders of bright
penumbral filaments with opposite signs on each side. These locations
are precisely correlated to large electric currents. The average twist
(and writhe) of penumbral structures is very small. The spines (dark
filaments in the background) show a nearly zero writhe. The writhe
per unit length of the intraspines diminishes with increasing length
of the tube axes. Thus, the axes of tubes related to intraspines are
less wrung when the tubes are more horizontal. As the writhe of the
spines is very small, we can conclude that the writhe reaches only
significant values when the tube includes the border of an intraspine.

---------------------------------------------------------
Title: The Electrical Current Density Vector in the Inner Penumbra
of a Sunspot
Authors: Puschmann, K. G.; Ruiz Cobo, B.; Martínez Pillet, V.
2010ApJ...721L..58P Altcode: 2010arXiv1008.2131P
We determine the entire electrical current density vector in a
geometrical three-dimensional volume of the inner penumbra of a
sunspot from an inversion of spectropolarimetric data obtained with
Hinode/SP. Significant currents are seen to wrap around the hotter,
more elevated regions with lower and more horizontal magnetic fields
that harbor strong upflows and radial outflows (the intraspines). The
horizontal component of the current density vector is 3-4 times larger
than the vertical; nearly all previous studies only obtain the vertical
component Jz , thus strongly underestimating the current
density. The current density \vec{J} and the magnetic field \vec{B}
form an angle of about 20°. The plasma β at the 0 km level is larger
than 1 in the intraspines and is one order of magnitude lower in the
background component of the penumbra (spines). At the 200 km level,
the plasma β is below 0.3, nearly everywhere. The plasma β surface
as well as the surface optical depth unity is very corrugated. At the
borders of intraspines and inside, \vec{B} is not force-free at deeper
layers and nearly force-free at the top layers. The magnetic field of
the spines is close to being potential everywhere. The dissipated ohmic
energy is five orders of magnitudes smaller than the solar energy flux
and thus negligible for the energy balance of the penumbra.

---------------------------------------------------------
Title: A Geometrical Height Scale for Sunspot Penumbrae
Authors: Puschmann, K. G.; Ruiz Cobo, B.; Martínez Pillet, V.
2010ApJ...720.1417P Altcode: 2010arXiv1007.2779P
Inversions of spectropolarimetric observations of penumbral filaments
deliver the stratification of different physical quantities in an
optical depth scale. However, without establishing a geometrical
height scale, their three-dimensional geometrical structure cannot
be derived. This is crucial in understanding the correct spatial
variation of physical properties in the penumbral atmosphere and to
provide insights into the mechanism capable of explaining the observed
penumbral brightness. The aim of this work is to determine a global
geometrical height scale in the penumbra by minimizing the divergence of
the magnetic field vector and the deviations from static equilibrium as
imposed by a force balance equation that includes pressure gradients,
gravity, and the Lorentz force. Optical depth models are derived from
the inversion of spectropolarimetric data of an active region observed
with the Solar Optical Telescope on board the Hinode satellite. We
use a genetic algorithm to determine the boundary condition for the
inference of geometrical heights. The retrieved geometrical height
scale permits the evaluation of the Wilson depression at each pixel and
the correlation of physical quantities at each height. Our results fit
into the uncombed penumbral scenario, i.e., a penumbra composed of flux
tubes with channeled mass flow and with a weaker and more horizontal
magnetic field as compared with the background field. The ascending
material is hotter and denser than their surroundings. We do not find
evidence of overturning convection or field-free regions in the inner
penumbral area analyzed. The penumbral brightness can be explained by
the energy transfer of the ascending mass carried by the Evershed flow,
if the physical quantities below z = -75 km are extrapolated from the
results of the inversion.

---------------------------------------------------------
Title: Structure and Dynamics of Penumbral Filaments
Authors: Ruiz Cobo, B.; Bellot Rubio, L. R.
2010ASSP...14..461R Altcode: 2010hsa5.conf..461R; 2010ASSP...14..461C; 2008arXiv0810.2463R
High-resolution observations of sunspots have revealed the existence
of dark cores inside the bright filaments of the penumbra. Here we
present the stationary solution of the heat transfer equation in a
stratified penumbra consisting of nearly horizontal magnetic flux
tubes embedded in a stronger and more vertical field. The tubes and
the external medium are in horizontal mechanical equilibrium. This
model produces bright filaments with dark cores as a consequence of
the higher density of the plasma inside the flux tube, which shifts
the surface of optical depth unity toward higher (cooler) layers. Our
results suggest that the surplus brightness of the penumbra is a natural
consequence of the Evershed flow, and that magnetic flux tubes about
250 km in diameter can explain the morphology of sunspot penumbra.

---------------------------------------------------------
Title: Supersonic Continuation of the Evershed Flow Outside a Sunspot
as Observed with Hinode
Authors: Martínez Pillet, V.; Katsukawa, Y.; Puschmann, K. G.;
Ruiz Cobo, B.
2009ApJ...701L..79M Altcode: 2009arXiv0907.3835M
We report on the discovery of mostly horizontal field channels just
outside sunspot penumbrae (in the so-called "moat" region) that are
seen to sustain supersonic flows (line-of-sight component of 6 km
s-1). The spectral signature of these supersonic flows
corresponds to circular polarization profiles with an additional,
satellite, third lobe of the same sign as the parent sunspot' Stokes
V blue lobe, for both downflows and upflows. This is consistent with
an outward directed flow that we interpret as the continuation of the
magnetized Evershed flow outside sunspots at supersonic speeds. In
Stokes Q and U, a clear signature of a transverse field connecting
the two flow streams is observed. Such an easily detectable spectral
signature should allow for a clear identification of these horizontal
field channels in other spectropolarimetric sunspot data. For the spot
analyzed in this paper, a total of five channels with this spectral
signature have been unambiguously found.

---------------------------------------------------------
Title: Preliminary results on the contribution of the convection
motions to the Doppler velocity signal
Authors: Simoniello, R.; Ruiz-Cobo, B.; Jiménez-Reyes, S. J.; García,
R. A.; Pallé, P. L.
2008JPhCS.118a2089S Altcode:
This investigation aims to study the correlation of the solar
background with atmosphere. We used high resolution observations of
the NaDl spectral line. In fact the large span in formation heights
of this spectral line allowed us to infer the signal from photosphere
to chromosphere. We analyzed the data by applying the SIR code (Stokes
Inversion based on response functions). It is an inversion method of the
RTE (Radiative transfer equation) that provides physical information
on the region where the spectral lines are sensitive to changes in
thermal and dynamical parameters of the atmosphere. The survey has
been divided in two different steps: 1) identification of a
model of the atmosphere that reproduce our observations; 2)
determination of the degree of correlation of convective motions with
the solar atmosphere. Here we present the results of the first step:
we identified in the model presented by Vernazza the best model that
matches our observations.

---------------------------------------------------------
Title: Heat transfer in sunspot penumbrae. Origin of dark-cored
penumbral filaments
Authors: Ruiz Cobo, B.; Bellot Rubio, L. R.
2008A&A...488..749R Altcode: 2008arXiv0806.0804R
Context: Observations at 0.1 arcsec have revealed the existence of
dark cores in the bright filaments of sunspot penumbrae. Expectations
are high that such dark-cored filaments are the basic building blocks
of the penumbra, but their nature remains unknown. Aims: We
investigate the origin of dark cores in penumbral filaments and the
surplus brightness of the penumbra. To that end we use an uncombed
penumbral model. Methods: The 2D stationary heat transfer equation
is solved in a stratified atmosphere consisting of nearly horizontal
magnetic flux tubes embedded in a stronger and more vertical field. The
tubes carry an Evershed flow of hot plasma. Results: This model
produces bright filaments with dark cores as a consequence of the higher
density of the plasma inside the tubes, which shifts the surface of
optical depth unity toward higher (cooler) layers. Our calculations
suggest that the surplus brightness of the penumbra is a natural
consequence of the Evershed flow, and that magnetic flux tubes about
250 km in diameter can explain the morphology of sunspot penumbrae.

---------------------------------------------------------
Title: Multiline Spectropolarimetry of the Quiet Sun at 5250 and
6302 Å
Authors: Socas-Navarro, H.; Borrero, J. M.; Asensio Ramos, A.;
Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez
González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez
Almeida, J.
2008ApJ...674..596S Altcode:
The reliability of quiet-Sun magnetic field diagnostics based on the
Fe I lines at 6302 Å has been questioned by recent work. Here we
present the results of a thorough study of high-resolution multiline
observations taken with the new spectropolarimeter SPINOR, comprising
the 5250 and 6302 Å spectral domains. The observations were analyzed
using several inversion algorithms, including Milne-Eddington,
LTE with 1 and 2 components, and MISMA codes. We find that the
line-ratio technique applied to the 5250 Å lines is not sufficiently
reliable to provide a direct magnetic diagnostic in the presence
of thermal fluctuations and variable line broadening. In general,
one needs to resort to inversion algorithms, ideally with realistic
magnetohydrodynamic constrains. When this is done, the 5250 Å lines
do not seem to provide any significant advantage over those at 6302
Å. In fact, our results point toward a better performance with the
latter (in the presence of turbulent line broadening). In any case,
for very weak flux concentrations, neither spectral region alone
provides sufficient constraints to fully disentangle the intrinsic
field strengths. Instead, we advocate for a combined analysis of both
spectral ranges, which yields a better determination of the quiet-Sun
magnetic properties. Finally, we propose the use of two other Fe I
lines (at 4122 and 9000 Å) with identical line opacities that seem
to work much better than the others.

---------------------------------------------------------
Title: Internetwork magnetic field distribution from simultaneous
1.56 μm and 630 nm observations
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.;
Beck, C.
2008A&A...477..953M Altcode: 2007arXiv0711.0267M
Aims:We study the contradictory magnetic field strength distributions
retrieved from independent analyses of spectropolarimetric observations
in the near-infrared (1.56 μm) and in the visible (630 nm) spectral
ranges in internetwork regions. Methods: To solve this apparent
controversy, we present simultaneous and co-spatial 1.56 μm and 630 nm
observations of an internetwork area. The properties of the circular
and linear polarization signals, as well as the Stokes V area and
amplitude asymmetries, are discussed. As a complement, we also used
inversion techniques to infer the physical parameters of the solar
atmosphere. As a first step, the infrared and visible observations
are analysed separately to check their compatibility. Finally, the
simultaneous inversion of the two data sets is performed. Results:
The magnetic flux densities retrieved from the individual analysis
of the infrared and visible data sets are strongly correlated. The
polarity of the Stokes V profiles is the same at co-spatial pixels
in both wavelength ranges. This indicates that both 1.56 μm and
630 nm observations trace the same magnetic structures on the solar
surface. The simultaneous inversion of the two pairs of lines reveals
an internetwork full of sub-kG structures that fill only 2% of the
resolution element. A correlation is found between the magnetic field
strength and the continuum intensity: equipartition fields (B∼ 500
G) tend to be located in dark intergranular lanes, whereas weaker
field structures are found inside granules. The most probable unsigned
magnetic flux density is 10 Mx/cm^2. The net magnetic flux density in
the whole field of view is nearly zero. This means that both polarities
cancel out almost exactly in our observed internetwork area.

---------------------------------------------------------
Title: Supersonic Downflows in the Photosphere Discovered in Sunspot
Moat Regions
Authors: Shimizu, T.; Martinez-Pillet, V.; Collados, M.; Ruiz-Cobo,
B.; Centeno, R.; Beck, C.; Katsukawa, Y.
2007ASPC..369..113S Altcode:
This paper reports on our new findings from the International
Time Program observations at the Canaries islands, Spain, in July
2005. We have found small-scale photospheric events with extremely
red-shifted Stokes V signals in sunspot moat regions. A preliminary
estimate of the physical conditions for an observed Stokes V profile
indicates the presence of a downward motion with a supersonic speed
in the order of 10 km/s. With the currently evaluated observational
information, we interprete the supersonic flows as downward motion from
magnetic reconnection occurring at the upper chromosphere or lower
photosphere. With coordinated observations of the Solar-B onboard
telescopes, Stokes measurements by the SOT spectro-polarimeter would
give new information for further understanding the nature of these
events with strongly red-shifted Stokes V, and for discussing the
physical conditions involving in possible magnetic reconnections in
the lower solar atmosphere.

---------------------------------------------------------
Title: Multi-Line Quiet Sun Spectro-Polarimetry at 5250 and 6302 Å
Authors: Socas-Navarro, H.; Borrero, J.; Asensio Ramos, A.; Collados,
M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González,
M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J.
2007arXiv0710.1099S Altcode:
The reliability of quiet Sun magnetic field diagnostics based on the
\ion{Fe}{1} lines at 6302 Åhas been questioned by recent work. We
present here the results of a thorough study of high-resolution
multi-line observations taken with the new spectro-polarimeter SPINOR,
comprising the 5250 and 6302 Åspectral domains. The observations were
analyzed using several inversion algorithms, including Milne-Eddington,
LTE with 1 and 2 components, and MISMA codes. We find that the
line-ratio technique applied to the 5250 Ålines is not sufficiently
reliable to provide a direct magnetic diagnostic in the presence
of thermal fluctuations and variable line broadening. In general,
one needs to resort to inversion algorithms, ideally with realistic
magneto-hydrodynamical constrains. When this is done, the 5250 Ålines
do not seem to provide any significant advantage over those at 6302
Å. In fact, our results point towards a better performance with the
latter (in the presence of turbulent line broadening). In any case,
for very weak flux concentrations, neither spectral region alone
provides sufficient constraints to fully disentangle the intrinsic field
strengths. Instead, we advocate for a combined analysis of both spectral
ranges, which yields a better determination of the quiet Sun magnetic
properties. Finally, we propose the use of two other \ion{Fe}{1} lines
(at 4122 and 9000 Å) with identical line opacities that seem to work
much better than the others.

---------------------------------------------------------
Title: Low-lying magnetic loops in the solar internetwork
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.;
Solanki, S. K.
2007A&A...469L..39M Altcode: 2007arXiv0705.1319M
Aims:We study the structure of the magnetic field vector in the
internetwork and search for the presence of small-scale loops. Methods: We invert 1.56 μm spectropolarimetric observations of
internetwork regions at disc centre by applying the SIR code. This
allows us to recover the atmospheric parameters that play a role in
the formation of these spectral lines. We are mainly interested in the
structure of the magnetic field vector. Results: We find that
many opposite polarity elements of the internetwork are connected by
short (2-6´´), low-lying (photospheric) loops. These loops connect
at least the 10-20% of the internetwork flux visible in our data. Also
we have some evidence that points towards a dynamic scenario that can
be produced by the emergence of internetwork magnetic flux.

---------------------------------------------------------
Title: Inversion techniques: From observations to atmospheres
Authors: Ruiz Cobo, B.
2007msfa.conf..287R Altcode:
Inversion techniques are presently the most powerful tools to deduce
magnetic, dynamic, and thermal properties of the solar plasma from
polarization line profiles. The reliability and robustness of Stokes
inversions have been confirmed many times by means of numerical
tests. Part of the solar physics community, however, is still worried
about the uniqueness of the obtained solutions. In this presentation
I clarify the scope and the limitations of different Stokes inversion
techniques and discuss the challenges for inversion techniques for
the near future.

---------------------------------------------------------
Title: Internetwork magnetic fields
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.
2007msfa.conf..157M Altcode:
Spectropolarimetric observations of the internetwork obtained in the
1.56 [mμ]m Fe I doublet are used to calculate the distributions of
magnetic field strength, filling factor and magnetic flux density
at different positions on the solar surface. We go one step further
and describe what the observations show about the three-dimensional
structure of the magnetic fields at the internetwork quiet Sun.

---------------------------------------------------------
Title: Internetwork Magnetic Field Distribution from Simultaneous
Fe I 1.5 µ and Fe I 630 nm Observations
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.
2006ASPC..358...36M Altcode:
We took simultaneous observations in Fe I 1.5 µ and Fe I 630 nm of
an internetwork region. We discuss the compatibility of the data in
the two spectral ranges, and present results for the magnetic field
distribution. We show that the larger contribution comes from hG
field strengths.

---------------------------------------------------------
Title: On the validity of the 630 nm Fe I lines for magnetometry of
the internetwork quiet Sun
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.
2006A&A...456.1159M Altcode: 2006astro.ph..5446M
Aims.The purpose of this work is to analyze the reliability of the
magnetic field strengths inferred from the 630 nm pair of Fe i lines
in internetwork quiet Sun regions. Methods: .Some numerical
experiments have been performed that demonstrate the inability
of these lines to recover the magnetic field strength in such low
flux solar regions. Results: .It is shown how different model
atmospheres, with magnetic field strengths ranging from a few hundred
Gauss to kiloGauss, give rise to Stokes profiles that cannot be
distinguished. The reasons for this degeneracy are discussed.

---------------------------------------------------------
Title: A Detailed Analysis of an Ephemeral Region .
Authors: Guglielmino, S. L.; Martínez Pillet, V.; Ruiz Cobo, B.;
Zuccarello, F.; Lites, B. W.
2006MSAIS...9..103G Altcode:
In order to improve the understanding of the process of emergence of
magnetic flux on the solar surface, we studied the temporal evolution of
an ephemeral region using \emph{Advanced Stokes Polarimeter} data. We
adopted two different approaches: first, we used a Milne-Eddington
inversion to obtain mean parameters of the emerging bipole magnetic
configuration. Then, we considered the full radiative transfer
equation, and we studied the trend of all the previous parameters as
a function of the optical depth tau . We pointed out peculiar flows,
such as an initial upflow of 1.5 ;textrm {km s}-1 where the
zenith angle is essentially horizontal, and downflows decreasing in
time in footpoints, characterized by a vertical field. These results
seem to confirm the emerging bipole topology, due to magnetic flux
tube emergence. The results obtained with this inversion confirm the
structure found with Milne-Eddington code. However we found regions
in which the presence of two distinct magnetic components is highly
significant. It also seems very interesting the trend of the temperature
with optical depth: the plasma temperature appears to grow up in the
high photosphere above the emerging bipole.

---------------------------------------------------------
Title: Time series of high resolution photospheric spectra in a
quiet region of the Sun. II. Analysis of the variation of physical
quantities of granular structures
Authors: Puschmann, K. G.; Ruiz Cobo, B.; Vázquez, M.; Bonet, J. A.;
Hanslmeier, A.
2005A&A...441.1157P Altcode: 2012arXiv1205.1725P
From the inversion of a time series of high resolution slit spectrograms
obtained from the quiet sun, the spatial and temporal distribution
of the thermodynamical quantities and the vertical flow velocity is
derived as a function of optical depth (logτ) and geometrical height
(z). Spatial coherence and phase shift analyses between temperature
and vertical velocity depict the height variation of these physical
quantities for structures of different size. An average granular
cell model is presented, showing the granule-intergranular lane
stratification of temperature, vertical velocity, gas pressure and
density as a function of logτ and z. Studies of a specific small
and a specific large granular cell complement these results. A strong
decay of the temperature fluctuations with increasing height together
with a less efficient penetration of smaller cells is revealed. The
T-T coherence at all granular scales is broken already at logτ = -1
or z ~ 170 km. At the layers beyond, an inversion of the temperature
contrast at granular scales &gt;1.5 arcsec is revealed, both in logτ
and z. At deeper layers the temperature sensitivity of the H-
opacity leeds to much smaller temperature fluctuations at equal logτ
than at equal z, in concordance with Stein &amp; Nordlund (1998, ApJ,
499, 914). Vertical velocities are in phase throughout the photosphere
and penetrate into the highest layers under study. Velocities at the
largest granular scales (~ 4´´) are still found even at logτ ~ -2.8
or z ~ 370 km. Again a less efficient height penetration of smaller
cells concerning convective velocities is revealed, although still
at logτ ~ -2 or z ~ 280 km structures &gt;1.4 arcsec are detected. A
similar size distribution of velocity and temperature structures with
height provides observational evidence for substantial overshoot into
the photosphere. At deep photospheric layers, the behaviour of the
vertical velocities reflected in simulations is for the first time
qualitatively reproduced by observations: intergranular velocities
are larger than the granular ones and, both reach extrema, where the
granular one is shifted towards higher layers.

---------------------------------------------------------
Title: Magnetic flux in the internetwork quiet Sun
Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.;
Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C.
2005A&A...436L..27K Altcode:
We report a direct comparison of the amplitudes of Stokes spectra of the
Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations
with simultaneous observations in the same spectral regions. The
Stokes spectra were synthesized in snapshots with a mixed polarity
magnetic field having a spatially averaged strength, &lt; B &gt;,
between 10 and 30 G. The distribution of Stokes V amplitudes depends
sensitively on &lt; B &gt;. A quiet inter-network region was observed
at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630
nm). We find that the Stokes V amplitudes of both infrared and visible
observations are best reproduced by the simulation snapshot with &lt;
B &gt; = 20 G. In observations with 1 resolution, up to 2/3 of the
magnetic flux can remain undetected.

---------------------------------------------------------
Title: The imaging magnetograph eXperiment for the SUNRISE balloon
Antarctica project
Authors: Martinez Pillet, Valentin; Bonet, Jose A.; Collados, Manuel
V.; Jochum, Lieselotte; Mathew, S.; Medina Trujillo, J. L.; Ruiz Cobo,
B.; del Toro Iniesta, Jose Carlos; Lopez Jimenez, A. C.; Castillo
Lorenzo, J.; Herranz, M.; Jeronimo, J. M.; Mellado, P.; Morales, R.;
Rodriguez, J.; Alvarez-Herrero, Alberto; Belenguer, Tomas; Heredero,
R. L.; Menendez, M.; Ramos, G.; Reina, Manuel; Pastor, C.; Sanchez,
A.; Villanueva, J.; Domingo, Vicente; Gasent, J. L.; Rodriguez, P.
2004SPIE.5487.1152M Altcode:
The SUNRISE balloon project is a high-resolution mission to study solar
magnetic fields able to resolve the critical scale of 100 km in the
solar photosphere, or about one photon mean free path. The Imaging
Magnetograph eXperiment (IMaX) is one of the three instruments that
will fly in the balloon and will receive light from the 1m aperture
telescope of the mission. IMaX should take advantage of the 15 days
of uninterrupted solar observations and the exceptional resolution
to help clarifying our understanding of the small-scale magnetic
concentrations that pervade the solar surface. For this, IMaX should
act as a diffraction limited imager able to carry out spectroscopic
analysis with resolutions in the 50.000-100.000 range and capable
to perform polarization measurements. The solutions adopted by the
project to achieve all these three demanding goals are explained in this
article. They include the use of Liquid Crystal Variable Retarders for
the polarization modulation, one LiNbO3 etalon in double pass
and two modern CCD detectors that allow for the application of phase
diversity techniques by slightly changing the focus of one of the CCDs.

---------------------------------------------------------
Title: Time series of high resolution photospheric spectra in a quiet
region of the sun. I. Analysis of global and spatial variations of
line parameters
Authors: Puschmann, K.; Vázquez, M.; Bonet, J. A.; Ruiz Cobo, B.;
Hanslmeier, A.
2003A&A...408..363P Altcode: 2012arXiv1205.1723P
A 50 min time series of one-dimensional slit-spectrograms, taken in
quiet sun at disk centre, observed at the German Vacuum Tower Telescope
(Observatorio del Teide), was used to study the global and spatial
variations of different line parameters. In order to determine the
vertical structure of the photosphere two lines with well separated
formation heights have been considered. The data have been filtered of
p-modes to isolate the pure convective phenomenon. From our studies
of global correlation coefficients and coherence and phase shift
analyses between the several line parameters, the following results
can be reported. The convective velocity pattern preserves structures
larger than 1.0 arcs to the highest layers of the photosphere (
~ 435 km). However, at these layers, in the intensity pattern only
structures larger than 2.0 arcs are still connected with those at the
continuum level although showing inverted brightness contrast. This
confirms an inversion of temperature that we have found at a height
of ~ 140 km. A possible evidence of gravity waves superimposed to
the convective motions is derived from the phase shift analysis. We
interprete the behaviour of the full width at half maximum and the
equivalent width as a function of the distance to the granular borders,
as a consequence of enhanced turbulence and/or strong velocity gradients
in the intergranular lanes.

---------------------------------------------------------
Title: Models of a Small and a Large Mean Granular Cell Obtained
from Inversion of Spectrometric Data Observed with Adaptive Optics
Authors: Puschmann, K.; Ruiz Cobo, B.
2003ANS...324...21P Altcode: 2003ANS...324..C01P
No abstract at ADS

---------------------------------------------------------
Title: Getting Photometric Spatial Resolution from 2-D
Spectropolarimetric Data
Authors: Rodríguez Hidalgo, I.; Ruiz Cobo, B.
2003ASPC..307..145R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Models of a mean granular cell
Authors: Puschmann, K.; Ruiz Cobo, B.; Vázquez, M.; Bonet, J. A.;
Hanslmeier, A.
2003AN....324..352P Altcode:
From inversion of a time series of slit spectra, observed in a quiet
region of the solar photosphere, averaged models of a granular cell
have been obtained showing the stratification of physical quantities
versus optical depth and geometrical height. Furthermore a semi-empiric
dynamic model of a mean granular cell has been derived and the results
are presented.

---------------------------------------------------------
Title: Observation of Convective Collapse and Upward-moving Shocks
in the Quiet Sun
Authors: Bellot Rubio, Luis R.; Rodríguez Hidalgo, Inés; Collados,
Manuel; Khomenko, Elena; Ruiz Cobo, Basilio
2001ApJ...560.1010B Altcode:
We present spectropolarimetric evidence of convective collapse
and destruction of magnetic flux by upward-moving fronts in the
quiet Sun. The observational material consists of time series of
the full Stokes vector of two infrared spectral lines emerging from
regions associated with Ca II K network points. The amplitude of the
circular polarization profiles of a particular spatial point is seen to
increase while the profiles are redshifted. It then decreases during
a much shorter phase characterized by large blueshifts. Inspection
of the data indicates that the blueshift occurs because of the sudden
appearance of a new, strongly displaced Stokes V profile of the same
polarity. The amplification of the magnetic signal takes place in a
time interval of about 13 minutes, while blueshifts and the concomitant
decreasing Stokes V amplitudes last for only 2 minutes. An inversion
code based on the thin flux-tube scenario has been applied to the data
in order to derive the thermal, magnetic, and dynamic structures of
the atmosphere. According to our results, the field strength undergoes
a moderate increase from 400 to 600 G at z=0 km during the phase in
which redshifts are present. The observed redshifts are produced by
internal downflows of up to 6 km s-1 at z=0 km. After ~13
minutes, the material falling down inside the tube appears to bounce
off in the deeper layers, originating an upward-propagating front whose
manifestation on the Stokes V profiles is a large blueshift. The front
moves with a speed of 2.3 km s-1 and has a downflow-to-upflow
velocity difference of about 7 km s-1 initially and some
4 km s-1 after 2 minutes. It strongly weakens the magnetic
field strength and may be responsible for the complete destruction of
the magnetic feature. The observed behavior is in general agreement
with theoretical predictions of flux expulsion, convective collapse,
and development of shocks within magnetic flux tubes.

---------------------------------------------------------
Title: Chemical Abundances from Inversions of Stellar Spectra:
Analysis of Solar-Type Stars with Homogeneous and Static Model
Atmospheres
Authors: Allende Prieto, Carlos; Barklem, Paul S.; Asplund, Martin;
Ruiz Cobo, Basilio
2001ApJ...558..830A Altcode: 2001astro.ph..5262P; 2001astro.ph..5262A
Spectra of late-type stars are usually analyzed with static model
atmospheres in local thermodynamic equilibrium (LTE) and a homogeneous
plane-parallel or spherically symmetric geometry. The energy balance
requires particular attention, as two elements that are particularly
difficult to model play an important role: line blanketing and
convection. Inversion techniques are able to bypass the difficulties
of a detailed description of the energy balance. Assuming that the
atmosphere is in hydrostatic equilibrium and LTE, it is possible
to constrain its structure from spectroscopic observations. Among
the most serious approximations still implicit in the method is a
static and homogeneous geometry. In this paper, we take advantage of a
realistic three-dimensional radiative hydrodynamical simulation of the
solar surface to check the systematic errors incurred by an inversion
assuming a plane-parallel horizontally-homogeneous atmosphere. The
thermal structure recovered resembles the spatial and time average
of the three-dimensional atmosphere. Furthermore, the abundances
retrieved are typically within 10% (0.04 dex) of the abundances used
to construct the simulation. The application to a fairly complete data
set from the solar spectrum provides further confidence in previous
analyses of the solar composition. There is only a narrow range of
one-dimensional thermal structures able to fit the absorption lines in
the spectrum of the Sun. With our carefully selected data set, random
errors are about a factor of 2 smaller than systematic errors. A small
number of strong metal lines can provide very reliable results. We
foresee no major difficulties in applying the technique to other
similar stars, and obtaining similar accuracies, using spectra with
λ/δλ~5×104 and a signal-to-noise ratio as low as 30.

---------------------------------------------------------
Title: A Time-dependent Semiempirical Model of the Chromospheric
Umbral Oscillation
Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B.
2001ApJ...550.1102S Altcode:
We present a time-dependent semiempirical model of the chromospheric
umbral oscillation in sunspots. This model has been obtained by
applying recently developed non-LTE inversion techniques to a time
series of spectropolarimetric observations. The model consists of
two optically thick unresolved atmospheric components: a “quiet”
component with downward velocities that covers most of the resolution
element and an “active” component with upward velocities as high as 10
km s-1 that covers a smaller filling factor and has a higher
temperature at the same chromospheric optical depth. This semiempirical
model accounts for all the observational signatures of the chromospheric
oscillation when the filling factor of the active component oscillates
between a few percent and 20% of the resolution element. We discuss
a plausible physical scenario in which upward-propagating waves in
a downflowing magnetized environment lead to periodic mass ejections
from the atmospheric layers where the waves become nonlinear. Based
on observations obtained with the Gregory Coudé Telescope, operated
on the island of Tenerife by the Observatory of Göttingen University
in the Spanish Observatorio del Teide of the Instituto de Astrofísica
de Canarias.

---------------------------------------------------------
Title: Optical Tomography of a Sunspot. II. Vector Magnetic Field
and Temperature Stratification
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martínez Pillet, V.; Lites, B. W.; Skumanich, A.
2001ApJ...547.1130W Altcode:
An observational determination of the three-dimensional magnetic
and thermal structure of a sunspot is presented. It has been
obtained through the application of the SIR inversion technique
(Stokes Inversion based on Response functions) on a low-noise, full
Stokes profile two-dimensional map of the sunspot as observed with
the Advanced Stokes Polarimeter. As a result of the inversion, maps
of the magnetic field strength, B, zenith angle, γ, azimuth, χ, and
temperature, T, over 25 layers at given optical depths (i.e., an optical
tomography) are obtained, of which those between logτ5=0 and
logτ5=-2.8 are considered to provide accurate information on
the physical parameters. All over the penumbra γ increases with depth,
while B is larger at the bottom layers of the inner penumbra (as in
the umbra) but larger at the top layers of the outer penumbra (as in
the canopy). The corrugation of the penumbral magnetic field already
observed by other authors has been confirmed by our different inversion
technique. Such a corrugation is especially evident in the zenith angle
maps of the intermediate layers, featuring the presence of the so-called
spines that we further characterize: spines are warmer and have a less
inclined magnetic field than the spaces between them and tend to have a
smaller gradient of γ with optical depth over the entire penumbra, but
with a field strength which is locally stronger in the middle penumbra
and locally weaker in the outer penumbra and beyond in the canopy. In
the lower layers of these external parts of the sunspot, most of the
field lines are seen to return to the solar surface, a result that is
closely connected with the Evershed effect (e.g., Westendorp et al.,
the third paper in this series). The Stokes V net area asymmetry map
as well as the average B, γ, and T radial distributions (and that
of the line-of-sight velocities; see the third paper in this series)
show a border between an inner and an outer penumbra with different
three-dimensional structure. We suggest that it is in this middle zone
where most of a new family of penumbral flux tubes (some of them with
Evershed flow) emerge interlaced (both horizontally and vertically)
among themselves and with the “background” magnetic field of the
penumbra. The interlacing along the line of sight is witnessed by
the indication of many points in the outer penumbra showing rapid
transitions with height between two structures, one with very weak
and inclined magnetic field at the bottom of the photosphere and the
other with a stronger and less inclined magnetic field. Over the whole
penumbra, and at all optical layers, a constant but weak deviation from
radiality of some 5° is detected for the azimuth of the vector magnetic
field, which may be in agreement with former detections but which is
not significantly higher than the size of the errors for this parameter.

---------------------------------------------------------
Title: Optical Tomography of a Sunspot. III. Velocity Stratification
and the Evershed Effect
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martínez Pillet, V.
2001ApJ...547.1148W Altcode:
The stratification with optical depth of the line-of-sight (LOS)
velocity of a simple, isolated, round sunspot observed with the Advanced
Stokes Polarimeter (ASP; Elmore et al.) presented here completes this
series of papers that investigates the stratification in optical depths
of such a typical sunspot. These results have been obtained through the
use of the SIR technique (Stokes Inversion based on Response functions
of Ruiz Cobo &amp; del Toro Iniesta). From these data we have confirmed
that there are strong downflowing velocities at logτ5=0
that coincide spatially with the places where the magnetic field points
downward (Westendorp Plaza et al.). Further confirmation is obtained
by the application of the same method on a different sunspot, already
analyzed with the Milne-Eddington inversion technique (Stanchfield,
Thomas, &amp; Lites). These downflows reconcile observations that have
detected Evershed velocities outside sunspots together with suggestions
of the possible return of the flow within the penumbra. The Evershed
flow seems to be concentrated in elevated channels not thicker than 1 or
2 scale heights that are mostly located in the space between magnetic
spines, i.e., in places where the magnetic field is more inclined,
weaker in the inner-middle penumbra, and stronger in the outer penumbra
and beyond the visible limits of the sunspot. This conclusion is
based upon the tight correlation found between LOS velocities and
the (reported in the second paper of this series) magnetic field
strength and zenith angle. The upstreaming material is seen in the
inner penumbra and the downstreaming in the outer penumbra. A strong
increase with optical depth has been obtained for the LOS velocities
that provides indications of the superposition of Evershed channels
along the LOS. The differential opacity effect between the center-side
and the limb-side penumbra, already reported in the second paper in
this series, is also seen in the velocity maps and has suggested the
comparison of the vertical mass flux through the upstreaming zones
(mostly seen in the center side) and the downstreaming zones (mostly
seen in the limb side), obtaining a fairly good balance between the two.

---------------------------------------------------------
Title: Spectropolarimetric Signatures of Convective Collapse
Authors: Rodrígues Hidalgo, I.; Bellot Rubio, L. R.; Collados, M.;
Ruiz Cobo, B.
2001ASPC..236..415R Altcode: 2001aspt.conf..415R
No abstract at ADS

---------------------------------------------------------
Title: New strategies on the analysis of spectral lines (Invited
Review)
Authors: Ruiz Cobo, B.
2001hsa..conf..241R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Lagrangian and Eulerian Stratifications of Acoustic
Oscillations through the Solar Photosphere
Authors: Rodríguez Hidalgo, Inés; Ruiz Cobo, Basilio; Collados,
Manuel; Bellot Rubio, Luis R.
2001ApJ...547..491R Altcode:
We evaluate the stratification of acoustic oscillations in the solar
photosphere in both the Lagrangian (comoving) frame of reference and
the Eulerian (inertial) frame of reference, from a temporal sequence
of model atmospheres in an optical depth scale obtained after a
quasi-non-LTE inversion of the radiative transfer equation applied to
spectral observations of the K I 7699 Å line. Our results suggest
that, to first order, the photosphere moves up and down as a whole
with amplitudes ranging from ~8 km in deep layers (around 0 km) to ~19
km in the upper layers (around 640 km). In Lagrangian coordinates, we
observe numerous short-lived, local temperature and velocity amplitude
enhancements in medium-high layers, together with asymmetric waveforms
in the oscillation of these two physical quantities. The Lagrangian
temperature oscillation clearly shows two nodes associated with sharp
phase jumps of about 180°, whereas the velocity amplitude shows
the well-known increase with geometrical height, at nearly constant
phase. In Eulerian coordinates, the perturbations are dominated by
the coherent oscillation of the entire photosphere.

---------------------------------------------------------
Title: Photospheric acoustic oscillations in a Langrangian reference
system
Authors: Rodríguez Hidalgo, I.; Ruiz Cobo, B.; Collados Vera, M.;
Bellot Rubio, L. R.
2001ESASP.464..239R Altcode: 2001soho...10..239R
The stratification of acoustic oscillations through the solar
photosphere in Lagrangian (comoving) coordinates has been evaluated from
a quasi-NLTE inversion of the radiative transfer equation (RTE) applied
to a temporal sequence of K I 7699 Å line spectra. The stratifications
of amplitude and phase of the temperature and line-of-sight (LOS)
velocity Lagrangian oscillations have been evaluated. Our results
suggest that, to first order, the photosphere moves up and down as a
whole with amplitudes ranging from ~8 km in deep layers (around 0 km)
to ~19 km in the upper layers (around 640 km). Numerous short-lived,
local temperature and velocity amplitude enhancements in medium-high
layers are observed, together with an asymmetric waveform in the
oscillation of these two physical quantities. Two nodes are clearly seen
in the Lagrangian temperature oscillation run, which are associated with
sharp phase jumps of about 180°. The velocity amplitude shows the well
known increase with geometrical height, keeping practically in phase.

---------------------------------------------------------
Title: Anomalous Circular Polarization Profiles in Sunspot
Chromospheres
Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B.
2000ApJ...544.1141S Altcode:
This paper presents a detailed description, analysis, and
interpretation of the spectropolarimetric observations recently
reported by Socas-Navarro, Trujillo Bueno, &amp; Ruiz Cobo. These
observations consist of time series of Stokes I and V profiles above a
sunspot umbra. The spectral lines observed simultaneously are the Ca II
chromospheric lines at 8498 and 8542 Å and the photospheric Fe I line
at 8497 Å. These spectropolarimetric observations unveil an intriguing
time-dependent behavior of the Stokes V profiles in the chromospheric
lines. This behavior should be considered as an observational reference
for future radiation magnetohydrodynamic simulations of sunspot
chromospheres. The analysis of the observed time series shows that a
“normal,” nearly antisymmetric V profile rapidly evolves toward an
“anomalous,” completely asymmetric profile, returning later to the
normal state. The occurrence of such anomalous circular polarization
profiles repeats itself with a periodicity of ~150 s. After giving
arguments to discard other scenarios, we are able to interpret the
anomalous V profiles as a consequence of the development of a second
unresolved atmospheric component. This unresolved component seems to be
the same that produces the umbral flashes observed in other sunspots,
where it is present with a larger filling factor. Based on observations
obtained with the Gregory Coudé Telescope, operated on the island of
Tenerife by the Observatory of Göttingen University, in the Spanish
Observatorio del Teide of the Instituto de Astrofísica de Canarias.

---------------------------------------------------------
Title: Structure of Plage Flux Tubes from the Inversion of Stokes
Spectra. I. Spatially Averaged Stokes I and V Profiles
Authors: Bellot Rubio, L. R.; Ruiz Cobo, B.; Collados, M.
2000ApJ...535..489B Altcode:
We present results of the inversion of spatially averaged Stokes
I and V profiles emerging from plage regions near disk center. The
observations analyzed in this work were recorded with the Advanced
Stokes Polarimeter. All atmospheric quantities determining the
radiative transfer in the thin flux-tube approximation are inferred
self-consistently with the help of the inversion code described by
Bellot Rubio et al. With regard to thermodynamics, the retrieved
model atmospheres are found to behave as expected on theoretical
grounds. For the first time, velocities inside and outside the tubes
have been derived empirically. The magnetic atmospheres resulting from
the inversion are characterized by the absence of significant motions in
high layers but show strong velocity gradients in deeper layers. These
gradients turn out to be essential for reproducing the whole shape
of the observed profiles and, in particular, the asymmetries and the
extended red tail of Stokes V. Our scenario predicts that the Stokes
V zero-crossing wavelengths of Fe I and Fe II lines are redshifted
by small but nonnegligible amounts, which is indeed confirmed by
observations made with the Fourier Transform Spectrometer. According
to recent numerical simulations, the internal downflows derived from
the inversion could be produced by the strong shear that takes place
in the intermediate layer between the magnetized interior and the
ambient medium. Another possible origin is magnetic flux undergoing
convective collapse within the resolution element.

---------------------------------------------------------
Title: Oscillations in the Photosphere of a Sunspot Umbra from the
Inversion of Infrared Stokes Profiles
Authors: Bellot Rubio, L. R.; Collados, M.; Ruiz Cobo, B.; Rodríguez
Hidalgo, I.
2000ApJ...534..989B Altcode:
We report on the detection of magnetic field strength and velocity
oscillations in the photosphere of a sunspot umbra. Our analysis is
based on the inversion of the full Stokes vector of three Fe I lines
at 15650 Å, from which the stratification with optical depth of the
different atmospheric parameters has been derived. This allows us to
estimate the amplitude of the oscillations and the phase lag between
the fluctuations in the line-of-sight velocity and field strength. Our
results suggest that the inferred magnetic field oscillations are caused
by opacity fluctuations that move upward and downward the region where
the spectral lines are sensitive to magnetic fields.

---------------------------------------------------------
Title: Anomalous Polarization Profiles in Sunspots: Possible Origin
of Umbral Flashes
Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B.
2000Sci...288.1396S Altcode: 2000Sci...288.1398S
We present time-series spectropolarimetric observations of sunspots
in the Ca II infrared triplet lines, which show a periodic occurrence
of anomalous, asymmetric, circular polarization profiles in the umbral
chromosphere. The profiles may be caused by the periodic development of
an unresolved atmospheric component in a downward flowing magnetized
environment. This active component with upward directed velocities
as high as 10 kilometers per second is connected to the umbral flash
(UF) phenomenon. We can explain the observations with a semiempirical
model of the chromospheric oscillation and of the sunspot magnetized
atmospheric plasma during a UF event.

---------------------------------------------------------
Title: Inversion of Stokes Profiles from Solar Magnetic Elements
Authors: Bellot Rubio, L. R.; Ruiz Cobo, B.; Collados, M.
2000ApJ...535..475B Altcode:
We describe a new LTE inversion code for the analysis of Stokes
profiles emerging from unresolved magnetic elements. It has been
specifically designed to obtain the thermal, dynamic, and magnetic
properties of these structures in a self-consistent manner by fitting
the whole shape of the observed spectra. The inversion code is based on
a previous scheme by Ruiz Cobo &amp; del Toro Iniesta and implements
the thin flux-tube model as a reasonable description of reality. All
physical parameters considered relevant for the problem (including
velocity fields) are retrieved by means of a Marquardt nonlinear
least-squares algorithm. We present the results of extensive tests
aimed at characterizing the behavior of the code so as to understand
its limitations for the analysis of real observations. The code is
found to produce accurate results even with only two spectral lines and
noisy Stokes I and V profiles. A detailed error treatment, in which
the covariances between parameters are explicitly included, is also
carried out in order to investigate the uniqueness and reliability of
the inferred model atmospheres.

---------------------------------------------------------
Title: Non-LTE Inversion of Stokes Profiles Induced by the Zeeman
Effect
Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B.
2000ApJ...530..977S Altcode:
This paper presents a new diagnostic tool for the inference of the
thermal, dynamic, and magnetic properties of the solar chromosphere. It
consists of a non-LTE inversion code of Stokes profiles induced by
the Zeeman effect in magnetized stellar atmospheres. This code is the
generalization, to the non-LTE Stokes transfer case, of the inversion
code for unpolarized line profiles of Socas-Navarro, Ruiz Cobo, &amp;
Trujillo Bueno. It is based upon a full non-LTE multilevel treatment
of Zeeman line transfer in which the thermal, magnetic, and dynamic
properties of the atmospheric model are adjusted automatically by
means of nonlinear least-squares-fitting techniques until a best fit
to the observed Stokes profiles is obtained. Our non-LTE inversion
approach is based on the concept of response functions, which measure
the emergent Stokes profiles' first-order reaction to changes in the
atmospheric parameters. We generalize our fixed departure coefficients
(FDC) approximation in order to allow fast computation of such response
functions in the present non-LTE Zeeman line transfer context. We
present several numerical tests showing the reliability of our inversion
method for retrieving the information about the thermodynamics and
the magnetic field vector that is contained in the polarization
state of the chosen spectral lines. We also explore the limitations
of the inversion code by applying it to simulated observations where
the physical hypotheses on which it is based on are not met. Finally,
we apply our non-LTE Stokes inversion code to real spectropolarimetric
observations of a sunspot observed in the IR triplet lines of Ca II. As
a result, a new mean model of the sunspot chromosphere is provided.

---------------------------------------------------------
Title: Model Photospheres for Late-Type Stars from the Inversion
of High-Resolution Spectroscopic Observations: Groombridge 1830 and
ɛ Eridani
Authors: Allende Prieto, Carlos; García López, Ramón J.; Lambert,
David L.; Ruiz Cobo, Basilio
2000ApJ...528..885A Altcode: 1999astro.ph..7368P; 1999astro.ph..7368A
An inversion technique to recover LTE one-dimensional model photospheres
for late-type stars, which was previously applied to the Sun by Allende
Prieto et al. in 1998, is now employed to reconstruct, semiempirically,
the photospheres of cooler dwarfs: the metal-poor Groombridge 1830 and
the active star of solar metallicity ɛ Eridani. The model atmospheres
we find reproduce satisfactorily all the considered weak-to-moderate
neutral lines of metals, satisfying in detail the excitation equilibrium
of iron, the wings of strong lines, and the slope of the optical
continuum. The retrieved models show a slightly steeper temperature
gradient than flux-constant model atmospheres in the layers where
logτ&lt;=-0.5. We argue that these differences should reflect missing
ingredients in the flux-constant models and point to granular-like
inhomogeneities as the best candidate. The iron ionization equilibrium
is well satisfied by the model for Gmb 1830, but not for ɛ Eri, for
which a discrepancy of 0.2 dex between the logarithmic iron abundance
derived from neutral and singly ionized lines may signal departures from
LTE. The chemical abundances of calcium, titanium, chromium, and iron
derived with the empirical models from neutral lines do not differ much
from previous analyses based on flux-constant atmospheric structures.

---------------------------------------------------------
Title: Formation and Destruction of a Weak Magnetic Feature in the
Solar Photosphere
Authors: Khomenko, E.; Collados, M.; Bellot Rubio, L. R.; Rodríguez
Hidalgo, I.; Ruiz Cobo, B.
1999ESASP.448..307K Altcode: 1999mfsp.conf..307K; 1999ESPM....9..307K
No abstract at ADS

---------------------------------------------------------
Title: Velocity and Temperature 5 Minute Oscillations as Functions
of Geometrical Height in the Photosphere
Authors: Rodríguez Hidalgo, I.; Ruiz Cobo, B.; Collados, M.
1999ASPC..184..176R Altcode:
The stratification with geometrical height of the velocity and
temperature 5 minute oscillation through the solar photosphere is
obtained after the inversion of a time series of KI 7699 Å line
spectra taken at disk center.

---------------------------------------------------------
Title: TIP: The Tenerife Infrared Polarimeter
Authors: Martínez Pillet, V.; Collados, M.; Bellot Rubio, L. R.;
Rodríiguez Hidalgo, I.; Ruiz Cobo, B.; Soltau, D.
1999AGAb...15...89M Altcode: 1999AGM....15..P05M
The aim of the IAC Tenerife Infrared Polarimeter is to obtain
quantitative measurements of the polarization state of the solar
radiation collected in near-infrared wavelengths by the Gregory Coudé
Telescope and the Vacuum Tower Telescope of the Observatorio del Teide
(Spain). In this contribution we describe the analyzer, instrumental
calibration, and detector subsystems of TIP, and give details concerning
the specifications of the instrument.

---------------------------------------------------------
Title: Granular and Intergranular Model Atmospheres from Inversion
of Solar Two-Dimensional Spectroscopic Data
Authors: Rodríguez Hidalgo, I.; Ruiz Cobo, B.; Collados, M.; del
Toro Iniesta, J. C.
1999ASPC..173..313R Altcode: 1999sstt.conf..313R
No abstract at ADS

---------------------------------------------------------
Title: Structure of a Facular Region From the Inversion of High
Spatial and Temporal Resolution Stokes Spectra
Authors: Bellot Rubio, L.; Ruiz Cobo, B.; Collados, M.
1999ASPC..183...61B Altcode: 1999hrsp.conf...61B
No abstract at ADS

---------------------------------------------------------
Title: An LTE code for the inversion of Stokes spectra from solar
magnetic elements
Authors: Bellot Rubio, L. R.; Ruiz Cobo, B.; Collados, M.
1999ASSL..243..271B Altcode: 1999sopo.conf..271B
No abstract at ADS

---------------------------------------------------------
Title: TIP (Tenerife Infrared Polarimeter): a near IR full Stokes
Polarimeter for the German Solar Telescopes at Observatorio del Teide
Authors: Collados, M.; Rodríguez Hidalgo, I.; Bellot Rubio, L.;
Ruiz Cobo, B.; Soltau, D.
1999AGAb...15Q..11C Altcode: 1999AGM....15..A13C
In this contribution, the main characteristics of the=20 Tenerife
Infrared Polarimeter (TIP), recently built at the Instituto de
Astrofísica de Canarias, to be operated at the German Solar
Telescopes (GCT and VTT) of the Observatorio del Teide (Tenerife)
are presented. Especial emphasis is put on the modulation scheme,
polarisation efficiencies, signal-to-noise ratio, spatial and spectral
resolution, and instrumental polarisation cross-talk. Some examples
of data recently obtained are presented, which give an idea of the
performance and capabilities of this instrument.

---------------------------------------------------------
Title: A new diagnostic tool for the solar chromosphere
Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B.
1999ASSL..243..263S Altcode: 1999sopo.conf..263S
No abstract at ADS

---------------------------------------------------------
Title: The Hermitian solution of the radiative transfer equation
for non-LTE problems
Authors: Ruiz Cobo, B.; Bellot Rubio, L. R.; Collados, M.
1999ASSL..243..231R Altcode: 1999sopo.conf..231R
No abstract at ADS

---------------------------------------------------------
Title: Probing downflows in solar magnetic elements: the Fe II test
Authors: Bellot Rubio, L. R.; Ruiz Cobo, B.; Collados, M.
1999A&A...341L..31B Altcode:
In an attempt to assess the physical realism of the plage flux tube
model derived by Bellot Rubio, Ruiz Cobo &amp; Collados (\cite{brc},
\cite{bellot}) we extend its predictions to the Stokes V zero-crossing
shifts of a number of Fe ii lines. For this analysis, accurate Fe
ii central wavelengths are required. We have devised a procedure for
bringing the available Fe ii laboratory wavelengths to the system of
accurate Fe i wavelengths of Nave et al. (\cite{nav}). It is shown that,
relative to this system, the Fe ii system of Kurucz (\cite{kur}) is
shifted by 6.4 m Angstroms/ towards longer wavelengths. Some lines,
however, are displaced by more than 10 m Angstroms/. Corrected
central wavelengths have been used to extract the observed Stokes
V zero-crossing shifts of 16 Fe ii lines. Comparison with the
values resulting from the model of Bellot Rubio et al. (\cite{brc},
\cite{bellot}) suggests that the velocity gradients derived by these
authors are esentially correct.

---------------------------------------------------------
Title: Oscillations in a sunspot umbra from the inversion of infrared
Stokes profiles
Authors: Bellot Rubio, Luis R.; Collados, Manuel; Ruiz Cobo, Basilio;
Rodríguez Hidalgo, Inés; Bogdan, Thomas J.
1999AGM....15..A07B Altcode:
We report on the detection of magnetic field strength and velocity
oscillations in the photosphere of a sunspot umbra. Our analysis is
based on the inversion of the full Stokes profiles of three Fe I lines
at 15650 Å, from which the stratification with optical depth of the
different atmospheric parameters has been derived. This allows us to
estimate the amplitude of the oscillations and the phase lag between
the fluctuations in the line-of-sight velocity and field strength. Our
results suggest that the inferred magnetic field oscillations are caused
by opacity fluctuations that move upward and donward the region where
the spectral lines are sensitive to magnetic fields.

---------------------------------------------------------
Title: Non-LTE Inversion of Line Profiles
Authors: Socas-Navarro, H.; Ruiz Cobo, B.; Trujillo Bueno, J.
1998ApJ...507..470S Altcode:
In this paper we address the problem of the non-LTE (NLTE) inversion
of line profiles by means of a nonlinear least-squares minimization
procedure combined with very efficient multilevel transfer methods. Our
approach is based on the concept of response functions, which measure
the first-order response of the emergent profiles to changes in the
atmospheric conditions. We introduce the fixed departure coefficients
(FDC) approximation in order to compute these response functions in
a fast and straightforward manner. The accuracy of this approximation
is checked comparing FDC response functions with those obtained from
full NLTE computations. An NLTE inversion code based on these response
functions has been developed and extensively tested. Reference synthetic
profiles, similar to those expected from real observations, are given
as input to the inversion algorithm and the recovered models are shown
to be compatible with the reference models within the error bars. Our
NLTE inversion code thus provides a new tool for the investigation of
the chromospheres of the Sun and other stars.

---------------------------------------------------------
Title: An Hermitian Method for the Solution of Polarized Radiative
Transfer Problems
Authors: Bellot Rubio, L. R.; Ruiz Cobo, B.; Collados, M.
1998ApJ...506..805B Altcode:
Spectral synthesis calculations in stellar (magnetized) atmospheres
are based on the solution of the radiative transfer equation (RTE)
for polarized light. The thermodynamic and magnetic properties of the
atmospheres, along with the radiation field, completely specify the
basic ingredients of the RTE, after which numerical methods have to
be employed to calculate the emergent Stokes spectra. The advent of
powerful analysis techniques for the inversion of Stokes spectra has
evidenced the need for accurate and fast solutions of the RTE. In
this paper we describe a novel Hermitian strategy to integrate the
polarized RTE that is based on the Taylor expansion of the Stokes
parameter vector to fourth order in depth. Our technique makes use
of the first derivatives of the absorption matrix and source vector
with respect to the coordinate measured along the ray path. Both
analytical and numerical results indicate that the new strategy is
superior to other methods in terms of speed and accuracy. It also gives
an approximation to the evolution operator at no extra cost, which is
of interest for inversion algorithms based on response functions. The
Hermitian technique can be straightforwardly particularized to the
scalar case, providing a very efficient solution of the RTE in the
absence of magnetic fields. We investigate in detail the consequences
of the oscillations that appear in the evolution operator for large
values of line strength η0. The problems they pose are
shared by all integration schemes, but can be minimized by adopting
nonequally spaced grids.

---------------------------------------------------------
Title: Model Photospheres for Late-Type Stars from the Inversion of
High-Resolution Spectroscopic Observations: The Sun
Authors: Allende Prieto, Carlos; Ruiz Cobo, Basilio; García López, J.
1998ApJ...502..951A Altcode: 1998astro.ph..2353P; 1998astro.ph..2353A
An inversion technique has been developed to recover LTE,
one-dimensional, model photospheres for late-type stars from very high
resolution, high signal-to-noise ratio stellar line profiles. It is
successfully applied to the Sun by using a set of clean Ti I, Ca I,
Cr I, and Fe I normalized line profiles with accurate transition
probabilities, taking advantage of the well-understood collisional
enhancement of the wings of the Ca I line at 6162 Å. Line and
continuum center-to-limb variations, continuum flux, and wings of
strong metal lines are synthesized by means of the model obtained
and are compared with solar observations, as well as with predictions
from other well-known theoretical and empirical solar models, showing
the reliability of the inversion procedure. The prospects for and
limitations of the application of this method to other late-type stars
are discussed.

---------------------------------------------------------
Title: SIR: an Inversion Technique of Spectral Lines
Authors: Ruiz Cobo, B.
1998Ap&SS.263..331R Altcode: 1998Ap&SS.263..331C; 1999Ap&SS.263..331R
During the last years, inversion techniques have become one of
the most powerful tools to obtain, from spectropolarimetric data,
the stratification of physical quantities (temperature, pressure,
velocity fields, magnetic field, etc.) describing different structures
in the solar atmosphere. The SIR code (Stokes Inversion based on
Response functions), developed at the Instituto de Astrofísica
de Canarias (IAC), is internationally acknowledged nowadays at the
most sophisticated and accurate inversion method. The main results
derived from its application to the study of the solar photosphere,
are presented. Two recent extensions are reported as well: inversion
under NLTE conditions, and the MISS code (Multiline Inversion of
Stellar Spectra).

---------------------------------------------------------
Title: Optical Tomography of a Sunspot. I. Comparison between Two
Inversion Techniques
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martínez Pillet, V.; Lites, B. W.; Skumanich, A.
1998ApJ...494..453W Altcode:
A quantitative comparison between the Milne-Eddington (ME) inversion
technique implemented by Skumanich &amp; Lites and the SIR (Stokes
Inversion based on Response Functions) proposed by Ruiz Cobo &amp;
del Toro Iniesta is presented. Numerical experiments are carried
out to explore the capabilities and limitations of both diagnostic
techniques. Such experiments consist of inversions of Stokes profiles
previously synthesized in “realistic” solar atmospheric models. The
results show that the ME inversion provides accurate, line-of-sight
(LOS) averaged values for the input stratification of the vector
magnetic field. Its greater speed compared to SIR makes it useful for
quick analysis of large quantities of data (such as those currently
provided by modern spectropolarimeters) if one is only interested
in LOS-averaged quantities. However, the higher order description
of the atmosphere used by SIR (which acknowledges variation of the
thermal, dynamic, and magnetic parameters through the photosphere)
allows retrieval of the stratification of all these parameters to good
accuracy. This is so even in the presence of discontinuities such
as those foreseen in magnetic canopies of sunspots. The trade-offs
between thermodynamic and magnetic parameters observed in some ME
inversions are reduced considerably in the case of SIR inversions
because of the more realistic treatment of the thermodynamics in this
analysis. Notably, both allow one to extract quantitative inferences of
fairly weak magnetic fields (below 500 G), even when they are applied
to Zeeman-sensitive lines in the visible spectrum; i.e., well below
the commonly accepted limit of 500 G. The thermodynamic parameters
resulting from the ME inversion are understood theoretically in terms
of the generalized response functions introduced by Ruiz Cobo &amp;
del Toro Iniesta and through the concept of height of formation
for inferred values proposed by Sánchez Almeida, Ruiz Cobo, &amp;
del Toro Iniesta. The present comparison and verification of
the reliability of inversion methods is a natural first step toward
the ongoing analysis of the three-dimensional magnetic structure of
a sunspot. By using SIR (with ME results for initialization) on maps
of a whole sunspot observed by the Advanced Stokes Polarimeter, we
obtain maps at different optical layers (i.e., an optical tomography)
of the temperature, vector magnetic field, and LOS velocity. Such a
tomography will appear in subsequent papers of the present series. To
illustrate fits to the observed Stokes profiles, we show here actual
inversion results for three points observed within a sunspot: one
within the umbra, another from the outermost parts of the penumbra,
and a third from the magnetic canopy surrounding the sunspot.

---------------------------------------------------------
Title: An Inversion Technique to Derive Model Photospheres in
Late-Type Stars from High-Resolution Spectroscopy: The Sun
Authors: Allende Prieto, Carlos; Ruiz Cobo, Basilio; Garcia Lopez,
Ramon J.
1998ASPC..154..813A Altcode: 1998csss...10..813A; 1997astro.ph.10067A; 1997astro.ph.10067P
An inversion technique has been developed to recover LTE one-dimensional
model photospheres for late-type stars from very high-resolution high
signal-to-noise stellar line profiles. It is successfully applied
to the Sun using a set of unblended Ti 1, Ca 1, Cr 1 and Fe 1 lines
with accurate transition probabilities. Temperature stratification,
continuum flux, center-to-limb variation and wings of strong metal
lines obtained from the resulting model are compared with those from
other well-known theoretical and empirical solar models and show the
reliability of the procedure.

---------------------------------------------------------
Title: Stratification with Optical Depth of the 5 Minute Oscillation
through the Solar Photosphere
Authors: Ruiz Cobo, Basilio; Rodríguez Hidalgo, Inés; Collados,
Manuel
1997ApJ...488..462R Altcode:
An analysis of the 5 minute oscillation in the solar photosphere
has been carried out using an inversion of the radiative transfer
equation applied to a time-series of K I λ7699 line spectra with
a 5" × 5" window at disk center. A thorough discussion of the
inversion conditions, with special emphasis on the LTE approach
and a careful error treatment, is offered. The inversion has been
performed introducing fixed non-LTE departure coefficients taken from
the literature. A sequence of instantaneous model atmospheres has been
obtained, reliable from log τ ~= 0 to log τ ~= -3.5. From the temporal
power spectra of line-of-sight velocity and temperature fluctuations,
the amplitude and phase stratifications of the oscillations of these
magnitudes have been obtained as functions of the optical depth. The
amplitude accuracies are better than 4 m s-1 and 2 K in
most of the photosphere. The increase of the velocity oscillation
amplitude toward higher photospheric layers has been confirmed and
quantified. Its phase angle remains constant through the photosphere,
and significant power is obtained in temperature.

---------------------------------------------------------
Title: Evidence for a downward mass flux in the penumbral region of
a sunspot
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martinez Pillet, V.; Lites, B. W.; Skumanich, A.
1997Natur.389...47W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Stratification of the 5-min oscillation through the solar
photosphere
Authors: Rodríguez Hidalgo, I.; Ruiz Cobo, B.; Collados, M.
1997SoPh..172...77R Altcode: 1997ESPM....8...77R
The inversion of the radiative transfer equation has been applied to a
time series of low spatial resolution Ki 7699 Å line spectra, allowing
us to obtain a sequence of instantaneous models which are compared
to the mean one to derive the 5-min oscillation stratification. The
resulting models are reliable from log τ ≃ 0 to log τ ≃
-3.5. The increase of the velocity oscillation amplitude towards higher
photospheric layers has been confirmed and quantified. Significant
oscillation is observed in temperature.

---------------------------------------------------------
Title: Flux-Tube Model Atmospheres and Stokes V Zero-crossing
Wavelengths
Authors: Bellot Rubio, L. R.; Ruiz Cobo, B.; Collados, M.
1997ApJ...478L..45B Altcode: 1997astro.ph..1059B; 1997astro.ph..1059R
First results of the inversion of Stokes I and V profiles from plage
regions near disk center are presented. Both low and high spatial
resolution spectra of Fe I 6301.5 and Fe I 6302.5 Å obtained with the
Advanced Stokes Polarimeter (ASP) have been considered for analysis. The
thin flux-tube approximation, implemented in an LTE inversion code
based on response functions, is used to describe unresolved magnetic
elements. The code allows the simultaneous and consistent inference of
all atmospheric quantities determining the radiative transfer with the
sole assumption of hydrostatic equilibrium. By considering velocity
gradients within the tubes, we are able to match the full ASP Stokes
profiles. The magnetic atmospheres derived from the inversion are
characterized by the absence of significant motions in high layers
and strong velocity gradients in deeper layers. These are essential
to reproduce the asymmetries of the observed profiles. Our scenario
predicts a shift of the Stokes V zero-crossing wavelengths, which
is indeed present in observations made with the Fourier Transform
Spectrometer.

---------------------------------------------------------
Title: (Erratum) Response functions for the inversion of data from
unresolved solar magnetic elements.
Authors: Bellot Rubio, L. R.; Ruiz Cobo, B.; Collados, M.
1997A&A...319.1036B Altcode:
Erratum to Astron. Astrophys. 306, 960 (1996).

---------------------------------------------------------
Title: Are NLTE effects important for the inversion of iron lines?
Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B.;
Shchukina, N. G.
1997joso.proc...86S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Inversion of Stokes profiles: what's next?
Authors: Del Toro Iniesta, J. C.; Ruiz Cobo, B.
1997ftst.conf...93D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Inversion Techniques Applied to Sunspot Spectropolarimetric
Data
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martinez Pillet, V.; Lites, B. W.; Skumanich, A.
1997ASPC..118..197W Altcode: 1997fasp.conf..197W
Two inversion techniques are compared: the Unno-Rachkov\-sky fitting
method (UR) and the Stokes Inversion based on Response functions
(SIR). Results with synthetic profiles in sunspot model atmospheres and
real data show that whilst UR is well suited for recovering a constant
vec B, SIR enables us to know the run with depth of vec B and the line
of sight velocity together with the temperature stratification.

---------------------------------------------------------
Title: Optical Tomography of a Sunspot: Preliminary Results
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martinez Pillet, V.; Lites, B. W.; Skumanich, A.
1997ASPC..118..202W Altcode: 1997fasp.conf..202W
Preliminary results of the inversion of spectropolarimetric maps
of a sunspot observed with the Advanced Stokes Polarimeter (ASP) are
presented. The method used, Stokes Inversion based on Response functions
(SIR), does not assume constancy of the different parameters with
depth, thus enabling us to embark on an analysis of the information at
different layers in continuum optical depth (i.e. optical tomography),
of a sunspot's photosphere. Maps of the vector magnetic field and
other physical quantities like temperature or line-of-sight velocity at
several optical depths show a new and promising view of the structure
of a sunspot, casting light on long standing debates as those over
penumbral `corrugated' fields (spines), superpenumbral canopies,
return flux, or the nature of the Evershed effect.

---------------------------------------------------------
Title: Heights of formation for measurements of atmospheric
parameters.
Authors: Sanchez Almeida, J.; Ruiz Cobo, B.; del Toro Iniesta, J. C.
1996A&A...314..295S Altcode:
We argue that heights of formation (HOFs) should not be assigned to
spectral lines since a single line my sample very different layers of
the atmosphere, depending on the physical parameter of interest and
the technique employed to determine it. HOFs should be assigned to
specific measurements. General expressions to compute these HOFs for
measurements are derived. The equations are subsequently used to show,
in representative solar measurements, the uncertainties produced by
assigning HOFs to lines. Only weak lines can probe a single height of
the atmosphere.

---------------------------------------------------------
Title: Stokes Profiles Inversion Techniques
Authors: Del Toro Iniesta, J. C.; Ruiz Cobo, B.
1996SoPh..164..169D Altcode:
Inversion techniques of the radiative transfer equation for polarized
light are presented as one of the best current procedures to infer
the vector magnetic field, as well as other quantities governing the
physical state of the atmospheric layers that photons are coming
from. Several characteristics of the various available inversion
procedures are pointed out. They are mostly based on the diagnostic
contents of the spectral lines as well as on the main hypotheses
assumed in these procedures. In particular, the role of gradients in
the atmospheric quantities is emphasized as of paramount importance in
any diagnostic analysis and, hence, in any interpretation of inversion
results.

---------------------------------------------------------
Title: Two-dimensional, high spatial resolution, solar spectroscopy
using a Correlation Tracker. II. Maps of spectral quantities.
Authors: Collados, M.; Rodriguez Hidalgo, I.; Ballesteros, E.; Ruiz
Cobo, B.; Sanchez Almeida, J.; del Toro Iniesta, J. C.
1996A&AS..115..367C Altcode:
In this paper we illustrate some of the capabilities of the Correlation
Tracker prototype developed at the Instituto de Astrofisica de
Canarias used for two-dimensional, high spatial resolution, solar
spectroscopy. Slit spectra have been taken, using the Correlation
Tracker as a stabilizer (minimizing image motion during exposures) and
as an accurate positioning device (allowing to precisely locate the
entrance slit of the spectrograph at adjacent positions on the solar
disc). Spectral information is obtained from several solar regions
of different sizes. Granules (including some exploding ones) and
intergranules are clearly resolved. Several sub-arcsecond structures
are undoubtedly distinguished as well. The two-dimensional variation
of several spectral quantities in the solar atmosphere is shown,
demonstrating the power of this technique and its future possibilities.

---------------------------------------------------------
Title: Response functions for the inversion of data from unresolved
solar magnetic elements.
Authors: Bellot Rubio, L. R.; Ruiz Cobo, B.; Collados, M.
1996A&A...306..960B Altcode:
By adopting the thin flux tube model as representative of spatially
unresolved magnetic elements in the photosphere of the Sun, we
identify an adequate set of parameters for the model and derive
the corresponding response functions (RFs) which inform about the
variation of the emergent Stokes spectrum when such parameters are
perturbed. We numerically compute these RFs for reasonable values of
the parameters and explore their main properties. It turns out that
the RFs at a given height often depend on the state of layers above as
a result of the constraints imposed by flux tube geometry. As a whole,
RFs can be classified into two well defined groups: one containing RFs
dominated by local effects and another which contains RFs governed by
non-local effects (i.e., contributions coming from layers other than
that where the perturbation takes place). In particular, the RFs to
the temperature, line of sight velocity and microturbulence of both
the internal and the external atmospheres belong to the first group,
while the RFs to the magnetic field strength, external gas pressure and
radius of the tube at the base of the atmosphere must be ascribed to
the second group. The RFs presented in this paper constitute a first
step for the inversion of Stokes spectra from faculae and the network.

---------------------------------------------------------
Title: Amplitude and phase stratification of the solar 5-minute
temperature and velocity oscillations through the photosphere
Authors: Rodriguez Hidalgo, I.; Ruiz Cobo, B.; Collados, M.
1996ASPC..109..151R Altcode: 1996csss....9..151R
No abstract at ADS

---------------------------------------------------------
Title: Empirical granular/intergranular average model atmospheres.
Authors: Rodríguez Hidalgo, I.; Ruiz Cobo, B.; Del Toro Iniesta,
J. C.; Collados, M.; Sánchez Almeida, J.
1996joso.proc..162R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Empirical model of an average solar granule
Authors: Ruiz Cobo, B.; del Toro Iniesta, J. C.; Rodriguez Hidalgo,
I.; Collados, M.; Sanchez Almeida, J.
1996ASPC..109..155R Altcode: 1996csss....9..155R
No abstract at ADS

---------------------------------------------------------
Title: LTE polarized radiative transfer through interlaced
atmospheres.
Authors: del Toro Iniesta, J. C.; Ruiz Cobo, B.; Bellot Rubio, L. R.;
Collados, M.
1995A&A...294..855D Altcode:
We show that the solution of the radiative transfer equation (RTE)
through a line of sight that pierces several times two alternate
atmospheres can be obtained in terms of the solutions of the RTE
through both single atmospheres separately considered. This also
applies to the response functions of the observed Stokes spectrum
to perturbations of the physical quantities. The analytic solution
of the RTE in case that the single atmospheres are Milne-Eddington
is presented. The simplification of the solution in the case of a
longitudinal or transversal (with constant azimuth) magnetic field
is presented as well. Finally, as a numerical example, we synthesize
the Stokes I- and V-spectrum emerging from a thin magnetic flux tube,
achieving a considerable decrease in computation time with respect to
conventional integrations and without loss of accuracy.

---------------------------------------------------------
Title: Observed differences between large and small sunspots.
Authors: Collados, M.; Martinez Pillet, V.; Ruiz Cobo, B.; del Toro
Iniesta, J. C.; Vazquez, M.
1994A&A...291..622C Altcode:
We confirm recent results about the differences in temperature
and magnetic field strength between the umbra of large and small
sunspots. Five Stokes I- and V-spectra from the darkest cores of
three different umbrae have been analysed with the inversion code
of the radiative transfer equation by Ruiz Cobo &amp; del Toro
Iniesta (1992). The run with depth of temperature, magnetic field
(strength and inclination) and velocity along the line of sight are
obtained. The larger sunspots turn out to be cooler and possesing
a larger magnetic field strength, practically throughout the whole
atmosphere. Neither significant gradients of the line-of-sight velocity,
nor of the magnetic field inclination, are detected in any of the
spots analysed. Two model atmospheres are given corresponding to hot
(small) and cool (large) sunspots. The models are, to a large extent,
free from effects of penumbral/photospheric stray-light because it is
nearly absent in the large spots and because in the small one, where
it is important for the Stokes I-profile, only Stokes V is considered
to obtain the model atmosphere. These are the first umbral models in
the literature for which a simultaneous determination of the magnetic
field and thermodynamic stratifications is presented. The implications
of these stratifications for the energy transport in sunspot umbrae
are discussed.

---------------------------------------------------------
Title: On the Temperature and Velocity through the Photosphere of
a Sunspot Penumbra
Authors: del Toro Iniesta, J. C.; Tarbell, T. D.; Ruiz Cobo, B.
1994ApJ...436..400D Altcode:
We investigate the structure in depth of a sunspot penumbra by means
of the inversion code of the radiative transfer equation proposed
by Ruiz Cobo &amp; del Toro Iniesta (1992), applied to a set of
filtergrams of a sunspot, scanning the Fe I line at 5576.1 A, with
a sampling interval of 30 mA, from -120 to 120 mA from line center
(data previously analyzed by Title et al. 1993). The temperature
structure of this penumbra is obtained for each of the 801 pixels
selected (0.32 sec x 0.32 sec). On the average, the temperatures seem
to decrease as we move inward, but the differences are of the order
of the rms values (approximately equal 100-200 K) at a given distance
to sunspot center. The outer parts of the penumbra have also a bigger
curvature in the T versus log tau5 relation than the inner
parts. We realize, however, that these differences might be influenced
by possible stray light effects. Compared to the quiet Sun, penumbral
temperatures are cooler at deep layers and hotter at high layers. A mean
penumbral model atmosphere is presented. The asymmetries observed in the
intensity profile (the line is magnetically insensitive) are deduced
to be produced by strong gradients of the line-of-sight velocity that
sharply vary spatially along slices of almost constant distance to
sunspot center. These variations suggest that such gradients are not
only needed to explain the broadband circular polarization observed
in sunspots (see Sanchez Almeida &amp; Lites 1992) but are a main
characteristic of the fine-scale penumbra. The results are compatible
with an Evershed flow present everywhere, but its gradient with depth
turns out to vary so that the flow seems to be mainly concentrated in
some penumbral fibrils when studied through Dopplergrams. Finally,
as by-products of this study, we put constraints to the practical
usefulness of the Eddington-Barbier relation, and we explain the values
of the Fourier Dopplergrams to be carrying information of layers around
the centroid of the generalized response function of Dopplergrams to
velocity fluctuations.

---------------------------------------------------------
Title: On the sensitivity of Stokes profiles to physical quantities.
Authors: Ruiz Cobo, B.; del Toro Iniesta, J. C.
1994A&A...283..129R Altcode: 1994A&A...283..129C
A thorough analysis of the sensitivities of the four Stokes profiles to
the physical quantities involved in the local thermodynamic equilibrium
(LTE) line formation is presented. We point out the ambiguities
on the definition of a mean depth of formation of a given spectral
line and on the use of Contribution Functions for the ascription of a
measure to a given depth in the atmosphere. Response Functions behave
like partial derivatives of the Stokes spectrum at a given depth
of the atmosphere. They provide the sensitivities of the observed
spectrum to the physical quantities characterizing the state of the
atmosphere. After a theoretical generalization of any measured parameter
over the spectrum, we extend the concept of Response Functions for such
parameters; in detail are discussed the properties and sensitivities
of the equivalent width, the Stokes V peaks distance, the line-ratio
method, and the centre of gravity method. Of particualr interest are
the following results : (1) a constant and longitudinal magnetic field
can desaturate a spectral line with a Zeeman pattern other than a pure
Zeeman triplet; (2) saturation is readily understood as a consequence
of an enhencement of the photon supply; and (3) methods to measure
magnetic field strength are sensitive to temperature variations,
mostly if a field strength gradient is present through the photosphere.

---------------------------------------------------------
Title: Vertical Stratification of a Sunspot Penumbra
Authors: del Toro Iniesta, J. C.; Tarbell, T. D.; Ruiz Cobo, B.
1993BAAS...25Q1221D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Inversion of Stokes Profiles
Authors: Ruiz Cobo, B.; del Toro Iniesta, J. C.
1992ApJ...398..375R Altcode:
An inversion code of Stokes line profiles is presented. It allows
the recovery of the stratification of the temperature, the magnetic
field vector, and the line of sight velocity through the atmosphere,
and the micro- and macroturbulence velocities - which are assumed
to be constant with depth. It is based on the response functions,
which enter a Marquardt nonlinear least-squares algorithm in a natural
way. Response functions are calculated at the same time as the full
radiative transfer equation for polarized light is integrated. This
enables us to obtain values of many free parameters in a reasonable
computation time. Many numerical experiments have been performed in
order to check the behavior of the code. These experiments reveal the
high stability, accuracy, and uniqueness of the results, even when
simulated observations present signal-to-noise ratios of the order of
the lowest acceptable values in real observations.

---------------------------------------------------------
Title: From Filtergrams to Physical Atmospheric Magnitudes: A
Prospective Diagnostic
Authors: del Toro Iniesta, J. C.; Tarbell, T.; Ruiz Cobo, B.
1992AAS...181.8115D Altcode: 1992BAAS...24.1255D
No abstract at ADS

---------------------------------------------------------
Title: Inversión de la ecuación de transporte radiativo Title:
Inversión de la ecuación de transporte radiativo

---------------------------------------------------------
Title: Inversion
of the radiative transfer equation;
Authors: Ruiz Cobo, Basilio
1992PhDT.......246R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Numerical Test of a New V-Profile Inversion Technique
Authors: Ruiz Cobo, B.; del Toro Iniesta, J. C.; Collados, M.;
Sanchez Almeida, J.
1990Ap&SS.170..113R Altcode:
The diagnostic method proposed by Landi Degl'Innocenti and Landolfi
(1982), based on the observation of circular polarization, has been
generalized to derive the thermodynamic properties of unresolved
magnetic elements in the solar atmosphere. The final aim is to derive
the height dependence of several parameters of the flux tube atmosphere
(such as temperature, magnetic field and velocity distributions,
macroturbulence and filling factor). We have used a perturbation
method based on the concept of response functions for the Stokes
profiles introduced by Landi Degl'Innocenti and Landi Degl'Innocenti
(1977). We present here the preliminary results of invertingV-profiles
by an iterative standard least-squares technique, which allows to find
the magnetic 1-D atmosphere consistent with simulated data.

---------------------------------------------------------
Title: Facular points and small-scale magnetic elements
Authors: del Toro Iniesta, J. C.; Collados, M.; Sanchez Almeida, J.;
Martinez Pillet, V.; Ruiz Cobo, B.
1990Ap&SS.170....9D Altcode:
We present spectroscopic observations, with high spatial resolution, of
Ca ii K bright points very near the disc centre. Magnetic concentrations
have been detected in these network (facular) points by only using
intensity profiles of the well-known pair of lines Fe i5250.22 Å and
5247.06 Å. No brightening of these structures with respect to the quiet
photosphere can be ascertained within an accuracy threshold of 1.2%.

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Title: Are small-scale magnetic concentrations spatially coincident
with bright facular points?
Authors: del Toro Iniesta, J. C.; Collados, M.; Sanchez Almeida, J.;
Martinez Pillet, V.; Ruiz Cobo, B.
1990A&A...233..570D Altcode:
The usually assumed identification of small-scale magnetic
concentrations with bright facular or network points on the photosphere
is observationally checked by using high spatial resolution spectra
of Ca II K bright points very near the disk center. The detection of
spatially unresolved magnetic structures is made via a new differential
analysis of the well-known pair of Fe I lines 5247.06 A and 5250.22
A; these concentrations are present in the central part of a line
weakening zone, which is of some 2 arcsec wide. No continuum intensity
enhancement with respect to the quiet photosphere can be ascertained of
these structures, within an accuracy threshold of 1.2 percent. In spite
of this, magnetic concentrations brighter than the quiet photosphere
are compatible with the observations, but if so, they must be narrower
than 0.2 arcsec.

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Title: Stray-light measurements at the Observatorio del Teide
Authors: Martinez Pillet, V.; Ruiz Cobo, B.; Vazquez, M.
1990SoPh..125..211M Altcode:
A new procedure to separate the instrumental and atmospheric components
of stray light is presented. It is based on the dependence of the
aureole's atmospheric component on the air mass and is applied to
measurements taken with the Vacuum Newton Telescope (VNT) at the
Observatorio del Teide (Tenerife). The resulting instrumental part
is independent of the air mass. The variation of both components
with wavelength is also studied. The instrumental component shows no
dependence on wavelength, in contrast to the atmospheric one which is
greater in the blue than in the red. It is concluded that observations
with air masses larger than two will probably be strongly affected by
stray light.