Author name code: ruiz-cobo ADS astronomy entries on 2022-09-14 author:"Ruiz Cobo, Basilio" ------------------------------------------------------------------------ Title: The umbral filament of active region NOAA 12529 Authors: Guglielmino, Salvatore Luigi; Romano, Paolo; Zuccarello, Francesca; Murabito, Mariarita; Ruiz Cobo, Basilio Bibcode: 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. 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. Bibcode: 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. Title: Inference of electric currents in the solar photosphere Authors: Pastor Yabar, A.; Borrero, J. M.; Quintero Noda, C.; Ruiz Cobo, B. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Mapping the Hidden Magnetic Field of the Quiet Sun Authors: Trelles Arjona, J. C.; Martínez González, M. J.; Ruiz Cobo, B. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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'. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Combining magneto-hydrostatic constraints with Stokes profiles inversions Authors: Borrero, J. M.; Pastor Yabar, A.; Rempel, M.; Ruiz Cobo, B. Bibcode: 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. 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. Bibcode: 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. Title: Chromospheric polarimetry through multiline observations of the 850 nm spectral region III: Chromospheric jets driven by twisted magnetic fields Authors: Quintero Noda, C.; Iijima, H.; Katsukawa, Y.; Shimizu, T.; Carlsson, M.; de la Cruz Rodríguez, J.; Ruiz Cobo, B.; Orozco Suárez, D.; Oba, T.; Anan, T.; Kubo, M.; Kawabata, Y.; Ichimoto, K.; Suematsu, Y. Bibcode: 2019MNRAS.486.4203Q Altcode: 2019MNRAS.tmp.1081N; 2019arXiv190409151Q We investigate the diagnostic potential of the spectral lines at 850 nm for understanding the magnetism of the lower atmosphere. For that purpose, we use a newly developed 3D simulation of a chromospheric jet to check the sensitivity of the spectral lines to this phenomenon as well as our ability to infer the atmospheric information through spectropolarimetric inversions of noisy synthetic data. We start comparing the benefits of inverting the entire spectrum at 850 nm versus only the Ca II 8542 Å spectral line. We found a better match of the input atmosphere for the former case, mainly at lower heights. However, the results at higher layers were not accurate. After several tests, we determined that we need to weight more the chromospheric lines than the photospheric ones in the computation of the goodness of the fit. The new inversion configuration allows us to obtain better fits and consequently more accurate physical parameters. Therefore, to extract the most from multiline inversions, a proper set of weights needs to be estimated. Besides that, we conclude again that the lines at 850 nm, or a similar arrangement with Ca II 8542 Å plus Zeeman-sensitive photospheric lines, pose the best-observing configuration for examining the thermal and magnetic properties of the lower solar atmosphere. Title: Properties of the Umbral Filament Observed in Active Region NOAA 12529 Authors: Guglielmino, Salvo L.; Romano, Paolo; Ruiz Cobo, Basilio; Zuccarello, Francesca; Murabito, Mariarita Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2017MNRAS.472..727Q Altcode: 2017arXiv170801333Q In this publication, we continue the work started in Quintero Noda et al., examining this time a numerical simulation of a magnetic flux tube concentration. Our goal is to study if the physical phenomena that take place in it, in particular, the magnetic pumping, leaves a specific imprint on the examined spectral lines. We find that the profiles from the interior of the flux tube are periodically doppler shifted following an oscillation pattern that is also reflected in the amplitude of the circular polarization signals. In addition, we analyse the properties of the Stokes profiles at the edges of the flux tube discovering the presence of linear polarization signals for the Ca II lines, although they are weak with an amplitude around 0.5 per cent of the continuum intensity. Finally, we compute the response functions to perturbations in the longitudinal field, and we estimate the field strength using the weak-field approximation. Our results indicate that the height of formation of the spectral lines changes during the magnetic pumping process, which makes the interpretation of the inferred magnetic field strength and its evolution more difficult. These results complement those from previous works, demonstrating the capabilities and limitations of the 850-nm spectrum for chromospheric Zeeman polarimetry in a very dynamic and complex atmosphere. Title: 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. Bibcode: 2017MNRAS.470.1453Q Altcode: 2017arXiv170510002Q We characterize the K I D1 & 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/579/A3 Title: An open-source, massively parallel code for non-LTE synthesis and inversion of spectral lines and Zeeman-induced Stokes profiles Authors: Socas-Navarro, H.; de la Cruz Rodríguez, J.; Asensio Ramos, A.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2015A&A...577A...7S Altcode: 2014arXiv1408.6101S With the advent of a new generation of solar telescopes and instrumentation, interpreting chromospheric observations (in particular, spectropolarimetry) requires new, suitable diagnostic tools. This paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, for synthesis and inversion of spectral lines and Zeeman-induced polarization profiles, spanning a wide range of atmospheric heights from the photosphere to the chromosphere. The code features a number of unique features and capabilities and has been built from scratch with a powerful parallelization scheme that makes it suitable for application on massive datasets using large supercomputers. The source code is written entirely in Fortran 90/2003 and complies strictly with the ANSI standards to ensure maximum compatibility and portability. It is being publicly released, with the idea of facilitating future branching by other groups to augment its capabilities.

The source code is currently hosted at the following repository: https://github.com/hsocasnavarro/NICOLE Title: VizieR Online Data Catalog: Spatial deconvolution code (Quintero Noda+, 2015) Authors: Quintero Noda, C.; Asensio Ramos, A.; Orozco Suarez, D.; Ruiz Cobo, B. Bibcode: 2015yCat..35790003Q Altcode: This deconvolution method follows the scheme presented in Ruiz Cobo & Asensio Ramos (2013A&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. Bibcode: 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. Bibcode: 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 http://www.aanda.org Title: Returning magnetic flux in sunspot penumbrae Authors: Ruiz Cobo, B.; Asensio Ramos, A. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 >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 & 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 >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. Bibcode: 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, < B >, between 10 and 30 G. The distribution of Stokes V amplitudes depends sensitively on < B >. 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 < B > = 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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, & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2000ApJ...544.1141S Altcode: This paper presents a detailed description, analysis, and interpretation of the spectropolarimetric observations recently reported by Socas-Navarro, Trujillo Bueno, & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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, & 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 Bibcode: 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<=-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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1998ApJ...494..453W Altcode: A quantitative comparison between the Milne-Eddington (ME) inversion technique implemented by Skumanich & Lites and the SIR (Stokes Inversion based on Response Functions) proposed by Ruiz Cobo & 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 & del Toro Iniesta and through the concept of height of formation for inferred values proposed by Sánchez Almeida, Ruiz Cobo, & 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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 & 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 & 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. Bibcode: 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. Bibcode: 1993BAAS...25Q1221D Altcode: No abstract at ADS Title: Inversion of Stokes Profiles Authors: Ruiz Cobo, B.; del Toro Iniesta, J. C. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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%. 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. Bibcode: 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. Title: Stray-light measurements at the Observatorio del Teide Authors: Martinez Pillet, V.; Ruiz Cobo, B.; Vazquez, M. Bibcode: 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.