Author name code: borrero ADS astronomy entries on 2022-09-14 author:"Borrero, Juan Manuel" ------------------------------------------------------------------------ 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: 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: Interaction of Magnetic Fields with a Vortex Tube at Solar Subgranular Scale Authors: Fischer, C. E.; Vigeesh, G.; Lindner, P.; Borrero, J. M.; Calvo, F.; Steiner, O. Bibcode: 2020ApJ...903L..10F Altcode: 2020arXiv201005577F Using high-resolution spectropolarimetric data recorded with the Swedish 1 m Solar Telescope, we have identified several instances of granular lanes traveling into granules. These are believed to be the observational signature of underlying tubes of vortical flow with their axis oriented parallel to the solar surface. Associated with these horizontal vortex tubes, we detect in some cases a significant signal in linear polarization, located at the trailing dark edge of the granular lane. The linear polarization appears at a later stage of the granular lane development, and is flanked by patches of circular polarization. Stokes inversions show that the elongated patch of linear polarization signal arises from the horizontal magnetic field aligned with the granular lane. We analyze snapshots of a magnetohydrodynamic numerical simulation and find cases in which the horizontal vortex tube of the granular lane redistributes and transports the magnetic field to the solar surface causing a polarimetric signature similar to what is observed. We thus witness a mechanism capable of transporting magnetic flux to the solar surface within granules. This mechanism is probably an important component of the small-scale dynamo supposedly acting at the solar surface and generating the quiet-Sun magnetic field. Title: Evolution of Stokes V area asymmetry related to a quiet Sun cancellation observed with GRIS/IFU Authors: Kaithakkal, A. J.; Borrero, J. M.; Fischer, C. E.; Dominguez-Tagle, C.; Collados, M. Bibcode: 2020A&A...634A.131K Altcode: 2020arXiv200105465K A quiet Sun magnetic flux cancellation event at the disk center was recorded using the Integral Field Unit (IFU) mounted on the GREGOR Infrared Spectrograph (GRIS). The GRIS instrument sampled the event in the photospheric Si I 10827 Å spectral line. The cancellation was preceded by a significant rise in line core intensity and excitation temperature, which is inferred from Stokes inversions under local thermodynamic equilibrium (LTE). The opposite polarity features seem to undergo reconnection above the photosphere. We also found that the border pixels neighboring the polarity inversion line of one of the polarities exhibit a systematic variation of area asymmetry. Area asymmetry peaks right after the line core intensity enhancement and gradually declines thereafter. Analyzing Stokes profiles recorded from either side of the polarity inversion line could therefore potentially provide additional information on the reconnection process related to magnetic flux cancellation. Further analysis without assuming LTE will be required to fully characterize this event.

Movie associated to Fig. 2 is available at https://www.aanda.org 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: Observations of solar small-scale magnetic flux-sheet emergence Authors: Fischer, C. E.; Borrero, J. M.; Bello González, N.; Kaithakkal, A. J. Bibcode: 2019A&A...622L..12F Altcode: 2019arXiv190105870F
Aims: Two types of flux emergence were recently discovered in numerical simulations: magnetic loops and magnetic sheet emergence. While magnetic loop emergence has been documented well in recent years using high-resolution full Stokes data from ground-based telescopes as well as satellites, magnetic sheet emergence is still an understudied process. We report here on the first clear observational evidence of a magnetic sheet emergence and characterise its development.
Methods: Full Stokes spectra from the Hinode spectropolarimeter were inverted with the Stokes Inversion based on Response functions (SIR) code to obtain solar atmospheric parameters such as temperature, line-of-sight velocities, and full magnetic field vector information.
Results: We analyse a magnetic flux emergence event observed in the quiet-Sun internetwork. After a large-scale appearance of linear polarisation, a magnetic sheet with horizontal magnetic flux density of up to 194 Mx cm-2 hovers in the low photosphere spanning a region of 2-3 arcsec. The magnetic field azimuth obtained through Stokes inversions clearly shows an organised structure of transversal magnetic flux density emerging. The granule below the magnetic flux sheet tears the structure apart leaving the emerged flux to form several magnetic loops at the edges of the granule.
Conclusions: A large amount of flux with strong horizontal magnetic fields surfaces through the interplay of buried magnetic flux and convective motions. The magnetic flux emerges within 10 minutes and we find a longitudinal magnetic flux at the foot points of the order of ∼1018 Mx. This is one to two orders of magnitude larger than what has been reported for small-scale magnetic loops. The convective flows feed the newly emerged flux into the pre-existing magnetic population on a granular scale.

Movie attached to Fig. 5 is available at https://www.aanda.org Title: Three-lobed near-infrared Stokes V profiles in the quiet Sun Authors: Kiess, Christoph; Borrero, Juan Manuel; Schmidt, Wolgang Bibcode: 2018A&A...616A.109K Altcode: 2018arXiv180309668K Context. The 1.5-m GREGOR solar telescope can resolve structures as small as 0.4'' at near-infrared wavelengths on the Sun. At this spatial resolution the polarized solar spectrum shows complex patterns, such as large horizontal and/or vertical variations of the physical parameters in the solar photosphere.
Aims: We investigate a region of the quiet solar photosphere exhibiting three-lobed Stokes V profiles in the Fe I spectral line at 15 648 Å. The data were acquired with the GRIS spectropolarimeter attached to the GREGOR telescope. We aim at investigating the thermal, kinematic and magnetic properties of the atmosphere responsible for these measured complex signals.
Methods: The SIR inversion code is employed to retrieve the physical parameters of the lower solar photosphere from the observed polarization signals. We follow two different approaches. On the one hand, we consider that the multi-lobe circular polarization signals are only produced by the line-of-sight variation of the physical parameters. We therefore invert the data assuming a single atmospheric component that occupies the entire resolution element in the horizontal plane and where the physical parameters vary with optical depth τ (i.e., line-of-sight). On the other hand, we consider that the multi-lobe circular polarization signals are produced not by the optical depth variations of the physical parameters but instead by their horizontal variations. Here we invert the data assuming that the resolution element is occupied by two different atmospheric components where the kinematic and magnetic properties are constant along the line-of-sight.
Results: Both approaches reveal some common features about the topology responsible for the observed three-lobed Stokes V signals: both a strong (>1000 Gauss) and a very weak (<10 Gauss) magnetic field with opposite polarities and harboring flows directed in opposite directions must co-exist (either vertically or horizontally interlaced) within the resolution element. Conclusions. Title: Solar Magnetoconvection and Small-Scale Dynamo Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2018smf..book..275B Altcode: No abstract at ADS Title: Solar Magnetoconvection and Small-Scale Dynamo. Recent Developments in Observation and Simulation Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2017SSRv..210..275B Altcode: 2015SSRv..tmp..113B; 2015arXiv151104214B A number of observational and theoretical aspects of solar magnetoconvection are considered in this review. We discuss recent developments in our understanding of the small-scale structure of the magnetic field on the solar surface and its interaction with convective flows, which is at the centre of current research. Topics range from plage areas in active regions over the magnetic network shaped by supergranulation to the ubiquituous `turbulent' internetwork fields. On the theoretical side, we focus upon magnetic field generation by small-scale dynamo action. Title: Penumbral thermal structure below the visible surface Authors: Borrero, J. M.; Franz, M.; Schlichenmaier, R.; Collados, M.; Asensio Ramos, A. Bibcode: 2017A&A...601L...8B Altcode: 2017arXiv170502832B Context. The thermal structure of the penumbra below its visible surface (I.e., τ5 ≥ 1) has important implications for our present understanding of sunspots and their penumbrae: their brightness and energy transport, mode conversion of magneto-acoustic waves, sunspot seismology, and so forth.
Aims: We aim at determining the thermal stratification in the layers immediately beneath the visible surface of the penumbra: τ5 ∈ [1,3] (≈70-80 km below the visible continuum-forming layer)
Methods: We analyzed spectropolarimetric data (I.e., Stokes profiles) in three Fe I lines located at 1565 nm observed with the GRIS instrument attached to the 1.5-m solar telescope GREGOR. The data are corrected for the smearing effects of wide-angle scattered light and then subjected to an inversion code for the radiative transfer equation in order to retrieve, among others, the temperature as a function of optical depth T(τ5).
Results: We find that the temperature gradient below the visible surface of the penumbra is smaller than in the quiet Sun. This implies that in the region τ5 ≥ 1 the penumbral temperature diverges from that of the quiet Sun. The same result is obtained when focusing only on the thermal structure below the surface of bright penumbral filaments.
Conclusions: We interpret these results as evidence of a thick penumbra, whereby the magnetopause is not located near its visible surface. In addition, we find that the temperature gradient in bright penumbral filaments is lower than in granules. This can be explained in terms of the limited expansion of a hot upflow inside a penumbral filament relative to a granular upflow, as magnetic pressure and tension forces from the surrounding penumbral magnetic field hinder an expansion like this. Title: Are Internetwork Magnetic Fields in the Solar Photosphere Horizontal or Vertical? Authors: Lites, B. W.; Rempel, M.; Borrero, J. M.; Danilovic, S. Bibcode: 2017ApJ...835...14L Altcode: Using many observations obtained during 2007 with the Spectro-Polarimeter of the Hinode Solar Optical Telescope, we explore the angular distribution of magnetic fields in the quiet internetwork regions of the solar photosphere. Our work follows from the insight of Stenflo, who examined only linear polarization signals in photospheric lines, thereby avoiding complications of the analysis arising from the differing responses to linear and circular polarization. We identify and isolate regions of a strong polarization signal that occupy only a few percent of the observed quiet Sun area yet contribute most to the net linear polarization signal. The center-to-limb variation of the orientation of linear polarization in these strong signal regions indicates that the associated magnetic fields have a dominant vertical orientation. In contrast, the great majority of the solar disk is occupied by much weaker linear polarization signals. The orientation of the linear polarization in these regions demonstrates that the field orientation is dominantly horizontal throughout the photosphere. We also apply our analysis to Stokes profiles synthesized from the numerical MHD simulations of Rempel as viewed at various oblique angles. The analysis of the synthetic data closely follows that of the observations, lending confidence to using the simulations as a guide for understanding the physical origins of the center-to-limb variation of linear polarization in the quiet Sun area. Title: Deep probing of the photospheric sunspot penumbra: no evidence of field-free gaps Authors: Borrero, J. M.; Asensio Ramos, A.; Collados, M.; Schlichenmaier, R.; Balthasar, H.; Franz, M.; Rezaei, R.; Kiess, C.; Orozco Suárez, D.; Pastor Yabar, A.; Berkefeld, T.; von der Lühe, O.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Waldmann, T.; Denker, C.; Hofmann, A.; Staude, J.; Strassmeier, K. G.; Feller, A.; Lagg, A.; Solanki, S. K.; Sobotka, M.; Nicklas, H. Bibcode: 2016A&A...596A...2B Altcode: 2016arXiv160708165B Context. Some models for the topology of the magnetic field in sunspot penumbrae predict regions free of magnetic fields or with only dynamically weak fields in the deep photosphere.
Aims: We aim to confirm or refute the existence of weak-field regions in the deepest photospheric layers of the penumbra.
Methods: We investigated the magnetic field at log τ5 = 0 is by inverting spectropolarimetric data of two different sunspots located very close to disk center with a spatial resolution of approximately 0.4-0.45''. The data have been recorded using the GRIS instrument attached to the 1.5-m solar telescope GREGOR at the El Teide observatory. The data include three Fe I lines around 1565 nm, whose sensitivity to the magnetic field peaks half a pressure scale height deeper than the sensitivity of the widely used Fe I spectral line pair at 630 nm. Before the inversion, the data were corrected for the effects of scattered light using a deconvolution method with several point spread functions.
Results: At log τ5 = 0 we find no evidence of regions with dynamically weak (B< 500 Gauss) magnetic fields in sunspot penumbrae. This result is much more reliable than previous investigations made on Fe I lines at 630 nm. Moreover, the result is independent of the number of nodes employed in the inversion, is independent of the point spread function used to deconvolve the data, and does not depend on the amount of stray light (I.e., wide-angle scattered light) considered. Title: Magnetic fields of opposite polarity in sunspot penumbrae Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.; Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld, T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.; Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar, A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier, K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M. Bibcode: 2016A&A...596A...4F Altcode: 2016arXiv160800513F Context. A significant part of the penumbral magnetic field returns below the surface in the very deep photosphere. For lines in the visible, a large portion of this return field can only be detected indirectly by studying its imprints on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe a narrow layer in the very deep photosphere, providing the possibility of directly measuring the orientation of magnetic fields close to the solar surface.
Aims: We study the topology of the penumbral magnetic field in the lower photosphere, focusing on regions where it returns below the surface.
Methods: We analyzed 71 spectropolarimetric datasets from Hinode and from the GREGOR infrared spectrograph. We inferred the quality and polarimetric accuracy of the infrared data after applying several reduction steps. Techniques of spectral inversion and forward synthesis were used to test the detection algorithm. We compared the morphology and the fractional penumbral area covered by reversed-polarity and three-lobed Stokes V profiles for sunspots at disk center. We determined the amount of reversed-polarity and three-lobed Stokes V profiles in visible and infrared data of sunspots at various heliocentric angles. From the results, we computed center-to-limb variation curves, which were interpreted in the context of existing penumbral models.
Results: Observations in visible and near-infrared spectral lines yield a significant difference in the penumbral area covered by magnetic fields of opposite polarity. In the infrared, the number of reversed-polarity Stokes V profiles is smaller by a factor of two than in the visible. For three-lobed Stokes V profiles the numbers differ by up to an order of magnitude. Title: Probing deep photospheric layers of the quiet Sun with high magnetic sensitivity Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González, M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.; Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.; Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.; Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier, K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.; Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.; Schmidt, D.; Kiess, C.; Nicklas, H. Bibcode: 2016A&A...596A...6L Altcode: 2016arXiv160506324L Context. Investigations of the magnetism of the quiet Sun are hindered by extremely weak polarization signals in Fraunhofer spectral lines. Photon noise, straylight, and the systematically different sensitivity of the Zeeman effect to longitudinal and transversal magnetic fields result in controversial results in terms of the strength and angular distribution of the magnetic field vector.
Aims: The information content of Stokes measurements close to the diffraction limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of spatial straylight and photon noise into account.
Methods: Highly sensitive full Stokes measurements of a quiet-Sun region at disk center in the deep photospheric Fe I lines in the 1.56 μm region were obtained with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise statistics and Stokes V asymmetries were analyzed and compared to a similar data set of the Hinode spectropolarimeter (SOT/SP). Simple diagnostics based directly on the shape and strength of the profiles were applied to the GRIS data. We made use of the magnetic line ratio technique, which was tested against realistic magneto-hydrodynamic simulations (MURaM).
Results: About 80% of the GRIS spectra of a very quiet solar region show polarimetric signals above a 3σ level. Area and amplitude asymmetries agree well with small-scale surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss range with some concentrations of kilo-Gauss fields.
Conclusions: The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4 are so far unique in the combination of high spatial resolution scans and high magnetic field sensitivity. Nevertheless, the unavoidable effect of spatial straylight and the resulting dilution of the weak Stokes profiles means that inversion techniques still bear a high risk of misinterpretating the data. Title: Kinematics and Magnetic Properties of a Light Bridge in a Decaying Sunspot Authors: Falco, M.; Borrero, J. M.; Guglielmino, S. L.; Romano, P.; Zuccarello, F.; Criscuoli, S.; Cristaldi, A.; Ermolli, I.; Jafarzadeh, S.; Rouppe van der Voort, L. Bibcode: 2016SoPh..291.1939F Altcode: 2016arXiv160607229F; 2016SoPh..tmp..107F We present the results obtained by analysing high spatial and spectral resolution data of the solar photosphere acquired by the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope on 6 August 2011 of a large sunspot with a light bridge (LB) observed in NOAA AR 11263. These data are complemented by simultaneous Hinode Spectropolarimeter (SP) observation in the Fe I 630.15 nm and 630.25 nm lines. The continuum intensity map shows a discontinuity in the radial distribution of the penumbral filaments in correspondence with the LB, which shows a dark lane (≈0.3″ wide and ≈8.0″ long) along its main axis. The available data were inverted with the Stokes Inversion based on Response functions (SIR) code and physical parameters maps were obtained. The line-of-sight (LOS) velocity of the plasma along the LB derived from the Doppler effect shows motions towards and away from the observer up to 0.6 kms−1 that are lower in value than the LOS velocities observed in the neighbouring penumbral filaments. The noteworthy result is that we find motions towards the observer of up to 0.6 kms−1 in the dark lane where the LB is located between two umbral cores, while the LOS velocity motion towards the observer is strongly reduced where the LB is located between an umbral core at one side and penumbral filaments on the other side. Statistically, the LOS velocities correspond to upflows or downflows, and comparing these results with Hinode/SP data, we conclude that the surrounding magnetic field configuration (whether more or less inclined) could have a role in maintaining the conditions for the process of plasma pile-up along the dark lane. The results obtained from our study support and confirm outcomes of recent magneto-hydrodynamic simulations showing upflows along the main axis of an LB. Title: Markov Properties of the Magnetic Field in the Quiet Solar Photosphere Authors: Gorobets, A. Y.; Borrero, J. M.; Berdyugina, S. Bibcode: 2016ApJ...825L..18G Altcode: 2016arXiv160500074G The observed magnetic field on the solar surface is characterized by a very complex spatial and temporal behavior. Although feature-tracking algorithms have allowed us to deepen our understanding of this behavior, subjectivity plays an important role in the identification and tracking of such features. In this paper, we study the temporal stochasticity of the magnetic field on the solar surface without relying on either the concept of magnetic feature or on the subjective assumptions about their identification and interaction. The analysis is applied to observations of the magnetic field of the quiet solar photosphere carried out with the Imaging Magnetograph eXperiment (IMaX) instrument on board the stratospheric balloon, Sunrise. We show that the joint probability distribution functions of the longitudinal ({B}\parallel ) and transverse ({B}\perp ) components of the magnetic field, as well as of the magnetic pressure ({B}2={B}\perp 2+{B}\parallel 2), verify the necessary and sufficient condition for the Markov chains. Therefore, we establish that the magnetic field as seen by IMaX with a resolution of 0.″15-0.″18 and 33 s cadence, which can be considered as a memoryless temporal fluctuating quantity. Title: Magneto-static Modeling of the Mixed Plasma Beta Solar Atmosphere Based on Sunrise/IMaX Data Authors: Wiegelmann, T.; Neukirch, T.; Nickeler, D. H.; Solanki, S. K.; Martínez Pillet, V.; Borrero, J. M. Bibcode: 2015ApJ...815...10W Altcode: 2015arXiv151105568W Our aim is to model the three-dimensional magnetic field structure of the upper solar atmosphere, including regions of non-negligible plasma beta. We use high-resolution photospheric magnetic field measurements from SUNRISE/IMaX as the boundary condition for a magneto-static magnetic field model. The high resolution of IMaX allows us to resolve the interface region between the photosphere and corona, but modeling this region is challenging for the following reasons. While the coronal magnetic field is thought to be force-free (the Lorentz force vanishes), this is not the case in the mixed plasma β environment in the photosphere and lower chromosphere. In our model, pressure gradients and gravity forces are self-consistently taken into account and compensate for the non-vanishing Lorentz force. Above a certain height (about 2 Mm) the non-magnetic forces become very weak and consequently the magnetic field becomes almost force-free. Here, we apply a linear approach where the electric current density consists of a superposition of a field-line parallel current and a current perpendicular to the Sun's gravity field. We illustrate the prospects and limitations of this approach and give an outlook for an extension toward a nonlinear model. Title: Variation in sunspot properties between 1999 and 2014 Authors: Rezaei, R.; Beck, C.; Lagg, A.; Borrero, J. M.; Schmidt, W.; Collados, M. Bibcode: 2015A&A...578A..43R Altcode:
Aims: We study the variation in the magnetic field strength, area, and continuum intensity of umbrae in solar cycles 23 and 24.
Methods: We analyzed a sample of 374 sunspots observed from 1999 until 2014 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter at the Dunn Solar Telescope. The sample of field strength, area, and intensities was used to trace any long-term or cyclic trend of umbral properties in the last 15 years.
Results: Sunspots are systematically weaker, that is, have a weaker field strength and stronger continuum intensity, toward the end of cycle 23 than they had at the maximum of cycle 23. The linear trend reverses with the onset of cycle 24. We find that the field strength decreases in the declining phase of cycle 23 by about 112 (± 16) G yr-1, while it increases in the rising phase of cycle 24 by about 138 (± 72) G yr-1. The umbral intensity shows the opposite trend: the intensity increases with a rate of 0.7 (± 0.3)% of Ic yr-1 toward the end of cycle 23 and decreases with a rate of 3.8 (± 1.5)% of Ic yr-1 toward the maximum of cycle 24. The distribution of the umbral maximum field strength in cycle 24 is similar to that of cycle 23, but is slightly shifted toward lower values by about 80 G, corresponding to a possible long-term gradient in umbral field strength of about 7 ± 4 G yr-1. If instead of the maximum umbral field we consider the average value over the entire umbra, the distribution shifts by about 44 Gauss.
Conclusions: The umbral brightness decreases in the rising stage of a solar cycle, but increases from maximum toward the end of the cycle. Our results do not indicate a drastic change of the solar cycle toward a grand minimum in the near future. Title: Comparison of inversion codes for polarized line formation in MHD simulations. I. Milne-Eddington codes Authors: Borrero, J. M.; Lites, B. W.; Lagg, A.; Rezaei, R.; Rempel, M. Bibcode: 2014A&A...572A..54B Altcode: 2014arXiv1409.3376B Milne-Eddington (M-E) inversion codes for the radiative transfer equation are the most widely used tools to infer the magnetic field from observations of the polarization signals in photospheric and chromospheric spectral lines. Unfortunately, a comprehensive comparison between the different M-E codes available to the solar physics community is still missing, and so is a physical interpretation of their inferences. In this contribution we offer a comparison between three of those codes (VFISV, ASP/HAO, and HeLIx+). These codes are used to invert synthetic Stokes profiles that were previously obtained from realistic non-grey three-dimensional magnetohydrodynamical (3D MHD) simulations. The results of the inversion are compared with each other and with those from the MHD simulations. In the first case, the M-E codes retrieve values for the magnetic field strength, inclination and line-of-sight velocity that agree with each other within σB ≤ 35 (Gauss), σγ ≤ 1.2°, and σv ≤ 10 m s-1, respectively. Additionally, M-E inversion codes agree with the numerical simulations, when compared at a fixed optical depth, within σB ≤ 130 (Gauss), σγ ≤ 5°, and σv ≤ 320 m s-1. Finally, we show that employing generalized response functions to determine the height at which M-E codes measure physical parameters is more meaningful than comparing at a fixed geometrical height or optical depth. In this case the differences between M-E inferences and the 3D MHD simulations decrease to σB ≤ 90 (Gauss), σγ ≤ 3°, and σv ≤ 90 m s-1. 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: Temporal relation between quiet-Sun transverse fields and the strong flows detected by IMaX/SUNRISE Authors: Quintero Noda, C.; Martínez Pillet, V.; Borrero, J. M.; Solanki, S. K. Bibcode: 2013A&A...558A..30Q Altcode: 2013arXiv1309.0627Q Context. Localized strongly Doppler-shifted Stokes V signals were detected by IMaX/SUNRISE. These signals are related to newly emerged magnetic loops that are observed as linear polarization features.
Aims: We aim to set constraints on the physical nature and causes of these highly Doppler-shifted signals. In particular, the temporal relation between the appearance of transverse fields and the strong Doppler shifts is analyzed in some detail.
Methods: We calculated the time difference between the appearance of the strong flows and the linear polarization. We also obtained the distances from the center of various features to the nearest neutral lines and whether they overlap or not. These distances were compared with those obtained from randomly distributed points on observed magnetograms. Various cases of strong flows are described in some detail.
Results: The linear polarization signals precede the appearance of the strong flows by on average 84 ± 11 s. The strongly Doppler-shifted signals are closer (0.″19) to magnetic neutral lines than randomly distributed points (0.″5). Eighty percent of the strongly Doppler-shifted signals are close to a neutral line that is located between the emerging field and pre-existing fields. That the remaining 20% do not show a close-by pre-existing field could be explained by a lack of sensitivity or an unfavorable geometry of the pre-existing field, for instance, a canopy-like structure.
Conclusions: Transverse fields occurred before the observation of the strong Doppler shifts. The process is most naturally explained as the emergence of a granular-scale loop that first gives rise to the linear polarization signals, interacts with pre-existing fields (generating new neutral line configurations), and produces the observed strong flows. This explanation is indicative of frequent small-scale reconnection events in the quiet Sun. Title: Is Magnetic Reconnection the Cause of Supersonic Upflows in Granular Cells? Authors: Borrero, J. M.; Martínez Pillet, V.; Schmidt, W.; Quintero Noda, C.; Bonet, J. A.; del Toro Iniesta, J. C.; Bellot Rubio, L. R. Bibcode: 2013ApJ...768...69B Altcode: 2013arXiv1303.2557B In a previous work, we reported on the discovery of supersonic magnetic upflows on granular cells in data from the SUNRISE/IMaX instrument. In the present work, we investigate the physical origin of these events employing data from the same instrument but with higher spectral sampling. By means of the inversion of Stokes profiles we are able to recover the physical parameters (temperature, magnetic field, line-of-sight velocity, etc.) present in the solar photosphere at the time of these events. The inversion is performed in a Monte-Carlo-like fashion, that is, repeating it many times with different initializations and retaining only the best result. We find that many of the events are characterized by a reversal in the polarity of the magnetic field along the vertical direction in the photosphere, accompanied by an enhancement in the temperature and by supersonic line-of-sight velocities. In about half of the studied events, large blueshifted and redshifted line-of-sight velocities coexist above/below each other. These features can be explained in terms of magnetic reconnection, where the energy stored in the magnetic field is released in the form of kinetic and thermal energy when magnetic field lines of opposite polarities coalesce. However, the agreement with magnetic reconnection is not perfect and, therefore, other possible physical mechanisms might also play a role. Title: Evolution of the Fine Structure of Magnetic Fields in the Quiet Sun: Observations from Sunrise/IMaX and Extrapolations Authors: Wiegelmann, Thomas; Solanki, Sami; Borrero, Juan; Peter, Hardi; Sunrise Team Bibcode: 2013EGUGA..15.5251W Altcode: Observations with the balloon-borne Sunrise/ Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyse a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun's atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop footpoints is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3 ± 1 minutes in the upper solar atmosphere and 12 ± 4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun. Title: Evolution of the Fine Structure of Magnetic Fields in the Quiet Sun: Observations from Sunrise/IMaX and Extrapolations Authors: Wiegelmann, T.; Solanki, S. K.; Borrero, J. M.; Peter, H.; Barthol, P.; Gandorfer, A.; Martínez Pillet, V.; Schmidt, W.; Knölker, M. Bibcode: 2013SoPh..283..253W Altcode: Observations with the balloon-borne Sunrise/Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyze a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun's atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop foot points is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3±1 minutes in the upper solar atmosphere and 12±4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun. Title: Inferring the magnetic field vector in the quiet Sun. III. Disk variation of the Stokes profiles and isotropism of the magnetic field Authors: Borrero, J. M.; Kobel, P. Bibcode: 2013A&A...550A..98B Altcode: 2012arXiv1212.0788B Recent investigations of the magnetic field vector properties in the solar internetwork have provided diverging results. While some works found that the internetwork is mostly pervaded by horizontal magnetic fields, other works argued in favor of an isotropic distribution of the magnetic field vector. Motivated by these seemingly contradictory results and by the fact that most of these works have employed spectropolarimetric data at disk center only, we have revisited this problem employing high-quality data (noise level σ ≈ 3 × 10-4 in units of the quiet-Sun intensity) at different latitudes recorded with the Hinode/SP instrument. Instead of applying traditional inversion codes of the radiative transfer equation to retrieve the magnetic field vector at each spatial point on the solar surface and studying the resulting distribution of the magnetic field vector, we surmised a theoretical distribution function of the magnetic field vector and used it to obtain the theoretical histograms of the Stokes profiles. These histograms were then compared to the observed ones. Any mismatch between them was ascribed to the theoretical distribution of the magnetic field vector, which was subsequently modified to produce a better fit to the observed histograms. With this method we find that Stokes profiles with signals above 2 × 10-3 (in units of the continuum intensity) cannot be explained by an isotropic distribution of the magnetic field vector. We also find that the differences between the histograms of the Stokes profiles observed at different latitudes cannot be explained in terms of line-of-sight effects. However, they can be explained by a distribution of the magnetic field vector that inherently varies with latitude. We note that these results are based on a series of assumptions that, although briefly discussed in this paper, need to be considered in more detail in the future. Title: Inferring the magnetic field vector in the quiet Sun. II. Interpreting results from the inversion of Stokes profiles Authors: Borrero, J. M.; Kobel, P. Bibcode: 2012A&A...547A..89B Altcode: 2012arXiv1209.4830B In a previous paper, we argued that the inversion of Stokes profiles applied to spectropolarimetric observations of the solar internetwork yield unrealistically large values of the inclination of the magnetic field vector (γ). This is because photon noise in Stokes Q and U are interpreted by the inversion code as valid signals, that leads to an overestimation of the transverse component B⊥, thus the inclination γ. However, our study was based on the analysis of linear polarization signals that featured only uncorrelated noise. In this paper, we develop this idea further and study this effect in Stokes Q and U profiles that also show correlated noise. In addition, we extend our study to the three components of the magnetic field vector, as well as the magnetic filling factor α. With this, we confirm the tendency to overestimate γ when inverting linear polarization profiles that, although non-zero, are still below the noise level. We also establish that the overestimation occurs mainly for magnetic fields that are nearly vertical γ ≲ 20°. This indicates that a reliable inference of the inclination of the magnetic field vector cannot be achieved by analyzing only Stokes I and V. In addition, when inverting Stokes Q and U profiles below the noise, the inversion code retrieves a randomly uniform distribution of the azimuth of the magnetic field vector φ. To avoid these problems, we propose only inverting Stokes profiles for which the linear polarization signals are sufficiently above the noise level. However, this approach is also biased because, in spite of allowing for a very accurate retrieval of the magnetic field vector from the selected Stokes profiles, it selects only profiles arising from highly inclined magnetic fields. Title: The continuum intensity as a function of magnetic field. II. Local magnetic flux and convective flows Authors: Kobel, P.; Solanki, S. K.; Borrero, J. M. Bibcode: 2012A&A...542A..96K Altcode: 2014arXiv1402.3474K Context. To deepen our understanding of the role of small-scale magnetic fields in active regions (ARs) and in the quiet Sun (QS) on the solar irradiance, it is fundamental to investigate the physical processes underlying their continuum brightness. Previous results showed that magnetic elements in the QS reach larger continuum intensities than in ARs at disk center, but left this difference unexplained.
Aims: We use Hinode/SP disk center data to study the influence of the local amount of magnetic flux on the vigour of the convective flows and the continuum intensity contrasts.
Methods: The apparent (i.e. averaged over a pixel) longitudinal field strength and line-of-sight (LOS) plasma velocity were retrieved by means of Milne-Eddington inversions (VFISV code). We analyzed a series of boxes taken over AR plages and the QS, to determine how the continuum intensity contrast of magnetic elements, the amplitude of the vertical flows and the box-averaged contrast were affected by the mean longitudinal field strength in the box (which scales with the total unsigned flux in the box).
Results: Both the continuum brightness of the magnetic elements and the dispersion of the LOS velocities anti-correlate with the mean longitudinal field strength. This can be attributed to the "magnetic patches" (here defined as areas where the longitudinal field strength is above 100 G) carrying most of the flux in the boxes. There the velocity amplitude and the spatial scale of convection are reduced. Due to this hampered convective transport, these patches appear darker than their surroundings. Consequently, the average brightness of a box decreases as the the patches occupy a larger fraction of it and the amount of embedded flux thereby increases.
Conclusions: Our results suggest that as the magnetic flux increases locally (e.g. from weak network to strong plage), the heating of the magnetic elements is reduced by the intermediate of a more suppressed convective energy transport within the larger and stronger magnetic patches. This, together with the known presence of larger magnetic features, could explain the previously found lower contrasts of the brightest magnetic elements in ARs compared to the QS. The inhibition of convection also affects the average continuum brightness of a photospheric region, so that at disk center, an area of photosphere in strong network or plage appears darker than a purely quiet one. This is qualitatively consistent with the predictions of 3D MHD simulations. Title: First Results from the SUNRISE Mission Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller, T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González, M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta, J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González, N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M. Bibcode: 2012ASPC..455..143S Altcode: The SUNRISE balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system, and further infrastructure. The first science flight of SUNRISE yielded high-quality data that reveal the structure, dynamics, and evolution of solar convection, oscillations, and magnetic fields at a resolution of around 100 km in the quiet Sun. Here we describe very briefly the mission and the first results obtained from the SUNRISE data, which include a number of discoveries. Title: Supersonic Magnetic Flows in the Quiet Sun Observed with SUNRISE/IMaX Authors: Borrero, J. M.; Pillet, V. M.; Schlichenmaier, R.; Schmidt, W.; Berkefeld, T.; Solanki, S. K.; Bonet, J. A.; Iniesta, J. C. d. T.; Domingo, V.; Barthol, P.; Gandorfer, A. Bibcode: 2012ASPC..455..155B Altcode: 2012arXiv1202.4354B In this contribution we describe some recent observations of high-speed magnetized flows in the quiet Sun granulation. These observations were carried out with the Imaging Magnetograph eXperiment (IMaX) onboard the stratospheric balloon SUNRISE, and possess an unprecedented spatial resolution and temporal cadence. These flows were identified as highly shifted circular polarization (Stokes V) signals. We estimate the LOS velocity responsible for these shifts to be larger than 6 km s-1, and therefore we refer to them as supersonic magnetic flows. The average lifetime of the detected events is 81.3 s and they occupy an average area of about 23 000 km2. Most of the events occur within granular cells and correspond therefore to upflows. However some others occur in intergranular lanes or bear no clear relation to the convective velocity pattern. We analyze a number of representative examples and discuss them in terms of magnetic loops, reconnection events, and convective collapse. Title: A First Look at Magnetic Field Data Products from SDO/HMI Authors: Liu, Y.; Scherrer, P. H.; Hoeksema, J. T.; Schou, J.; Bai, T.; Beck, J. G.; Bobra, M.; Bogart, R. S.; Bush, R. I.; Couvidat, S.; Hayashi, K.; Kosovichev, A. G.; Larson, T. P.; Rabello-Soares, C.; Sun, X.; Wachter, R.; Zhao, J.; Zhao, X. P.; Duvall, T. L., Jr.; DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Centeno, R.; Tomczyk, S.; Borrero, J. M.; Norton, A. A.; Barnes, G.; Crouch, A. D.; Leka, K. D.; Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Muglach, K.; Schuck, P. W.; Wiegelmann, T.; Turmon, M.; Linker, J. A.; Mikić, Z.; Riley, P.; Wu, S. T. Bibcode: 2012ASPC..455..337L Altcode: The Helioseismic and Magnetic Imager (HMI; Scherrer & Schou 2011) is one of the three instruments aboard the Solar Dynamics Observatory (SDO) that was launched on February 11, 2010 from Cape Canaveral, Florida. The instrument began to acquire science data on March 24. The regular operations started on May 1. HMI measures the Doppler velocity and line-of-sight magnetic field in the photosphere at a cadence of 45 seconds, and the vector magnetic field at a 135-second cadence, with a 4096× 4096 pixels full disk coverage. The vector magnetic field data is usually averaged over 720 seconds to suppress the p-modes and increase the signal-to-noise ratio. The spatial sampling is about 0".5 per pixel. HMI observes the Fe i 6173 Å absorption line, which has a Landé factor of 2.5. These data are further used to produce higher level data products through the pipeline at the HMI-AIA Joint Science Operations Center (JSOC) - Science Data Processing (Scherrer et al. 2011) at Stanford University. In this paper, we briefly describe the data products, and demonstrate the performance of the HMI instrument. We conclude that the HMI is working extremely well. Title: Polarization Calibration of the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) Authors: Schou, J.; Borrero, J. M.; Norton, A. A.; Tomczyk, S.; Elmore, D.; Card, G. L. Bibcode: 2012SoPh..275..327S Altcode: As part of the overall ground-based calibration of the Helioseismic and Magnetic Imager (HMI) instrument an extensive set of polarimetric calibrations were performed. This paper describes the polarimetric design of the instrument, the test setup, the polarimetric model, the tests performed, and some results. It is demonstrated that HMI achieves an accuracy of 1% or better on the crosstalks between Q, U, and V and that our model can reproduce the intensities in our calibration sequences to about 0.4%. The amount of depolarization is negligible when the instrument is operated as intended which, combined with the flexibility of the polarimeter design, means that the polarimetric efficiency is excellent. Title: Evolution of the fine structure of magnetic fields in the quiet Sun: Combining Sunrise observations and modelling Authors: Wiegelmann, T.; Solanki, S.; Borrero, J.; Martinez Pillet, V.; Sunrise Team Bibcode: 2011AGUFMSH41B..06W Altcode: Observations with the balloon borne SUNRISE/IMAX instrument provide us with unprecedented high spatial resolution (pixel size 40 km) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona we extrapolate these photospheric measurements into the upper solar atmosphere and analyse a timeseries with a cadence of 33s. We find that the majority of closed loops which reach into the chromosphere or corona have one foot point in strong photospheric magnetic field regions (B>300 G). Most loops are asymmetric and the weaker foot point is often located in the internetwork. We find that the magnetic connectivity of the loops changes rapidly with a typical recycling time of about 2 min in the upper solar atmosphere and 14 min in the photosphere. We discuss, to which extend the observed topological changes can be interpreted as evidence for magnetic reconnection and the relevance of these processes for coronal heating. Title: HMI vector magnetic field products: the long-awaited release has come! Now what? Authors: Centeno, R.; Barnes, G.; Borrero, J.; Couvidat, S. P.; Hayashi, K.; Hoeksema, J. T.; Leka, K. D.; Liu, Y.; Schou, J.; Schuck, P. W.; Sun, X.; Tomczyk, S. Bibcode: 2011AGUFMSH31A1985C Altcode: HMI vector magnetic field test products will be released, alongside with the corresponding documentation, soon after the submission of this abstract. These data represent a stage of the project at which the HMI vector team has a large degree of confidence in the results. However, longer-term research topics on how to improve certain aspects of the data pipeline in general -and the spectral line inversion code in particular- are being pursued as we get valuable input from the user community. I will give a brief summary of the characteristics of the released inversion data products and an update of where we stand now. Title: VFISV: Very Fast Inversion of the Stokes Vector for the Helioseismic and Magnetic Imager Authors: Borrero, J. M.; Tomczyk, S.; Kubo, M.; Socas-Navarro, H.; Schou, J.; Couvidat, S.; Bogart, R. Bibcode: 2011SoPh..273..267B Altcode: 2009arXiv0901.2702B In this paper we describe in detail the implementation and main properties of a new inversion code for the polarized radiative transfer equation (VFISV: Very Fast Inversion of the Stokes Vector). VFISV will routinely analyze pipeline data from the Helioseismic and Magnetic Imager (HMI) on-board of the Solar Dynamics Observatory (SDO). It will provide full-disk maps (4096×4096 pixels) of the magnetic field vector on the Solar Photosphere every ten minutes. For this reason VFISV is optimized to achieve an inversion speed that will allow it to invert sixteen million pixels every ten minutes with a modest number (approx. 50) of CPUs. Here we focus on describing a number of important details, simplifications and tweaks that have allowed us to significantly speed up the inversion process. We also give details on tests performed with data from the spectropolarimeter on-board of the Hinode spacecraft. Title: Magnetic Structure of Sunspots Authors: Borrero, Juan M.; Ichimoto, Kiyoshi Bibcode: 2011LRSP....8....4B Altcode: 2011arXiv1109.4412B In this review we give an overview about the current state-of-knowledge of the magnetic field in sunspots from an observational point of view. We start by offering a brief description of tools that are most commonly employed to infer the magnetic field in the solar atmosphere with emphasis in the photosphere of sunspots. We then address separately the global and local magnetic structure of sunspots, focusing on the implications of the current observations for the different sunspots models, energy transport mechanisms, extrapolations of the magnetic field towards the corona, and other issues. Title: The Sun at high resolution: first results from the Sunrise mission Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez; Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.; Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M. Bibcode: 2011IAUS..273..226S Altcode: The Sunrise balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system and further infrastructure. The first science flight of Sunrise yielded high-quality data that reveal the structure, dynamics and evolution of solar convection, oscillations and magnetic fields at a resolution of around 100 km in the quiet Sun. Here we describe very briefly the mission and the first results obtained from the Sunrise data, which include a number of discoveries. Title: The continuum intensity as a function of magnetic field. I. Active region and quiet Sun magnetic elements Authors: Kobel, P.; Solanki, S. K.; Borrero, J. M. Bibcode: 2011A&A...531A.112K Altcode: 2011arXiv1105.1958K Context. Small-scale magnetic fields are major contributors to the solar irradiance variations. Hence, the continuum intensity contrast of magnetic elements in the quiet Sun (QS) network and in active region (AR) plage is an essential quantity that needs to be measured reliably.
Aims: By using Hinode/SP disk center data at a constant, high spatial resolution, we aim at updating results of earlier ground-based studies of contrast vs. magnetogram signal, and to look for systematic differences between AR plages and QS network.
Methods: The field strength, filling factor and inclination of the field was retrieved by means of a Milne-Eddington inversion (VFISV code). As in earlier studies, we then performed a pixel-by-pixel study of 630.2 nm continuum contrast vs. apparent (i.e. averaged over a pixel) longitudinal magnetic field over large fields of view in ARs and in the QS.
Results: The continuum contrast of magnetic elements reaches larger values in the QS (on average 3.7%) than in ARs (on average 1.3%). This could not be attributed to any systematic difference in the chosen contrast references, so that it mainly reflects an intrinsic brightness difference. The larger contrasts in the QS are in agreement with earlier, lower resolution results, although our values are larger due to our better spatial resolution. At Hinode's spatial resolution, moreover, the relationship between contrast and apparent longitudinal field strength exhibits a peak at around 700 G in both the QS and ARs, whereas earlier lower resolution studies only found a peak in the QS and a monotonic decrease in ARs. We attribute this discrepancy both to our careful removal of the pores and their close surroundings affected by the telescope diffraction, as well as to the enhanced spatial resolution and very low scattered light of the Hinode Solar Optical Telescope. We verified that the magnetic elements producing the peak in the contrast curve are rather vertical in the AR and in the QS, so that the larger contrasts in the QS cannot be explained by larger inclinations, as had been proposed earlier. The opposite polarities in ARs do not exhibit any noticeable difference in inclination either, although they reach different contrasts when the amount of flux is significantly unbalanced between the polarities.
Conclusions: According to our inversions, the magnetic elements producing the peak of the contrast curves have similar properties (field strength, inclination, filling factor) in ARs and in the QS, so that the larger brightness of magnetic elements in the QS remains unexplained. Indirect evidence suggests that the contrast difference is not primarily due to any difference in average size of the magnetic elements. A possible explanation lies in the different efficiencies of convective energy transport in the QS and in ARs, which will be the topic of a second paper. Title: The Continuum Contrast of Magnetic Elements as a Function of Magnetic Field (Disk Center): Early Studies and Hinode/SP Results Authors: Kobel, P.; Solanki, S. K.; Borrero, J. M. Bibcode: 2011ASPC..437..297K Altcode: To deepen our understanding of the role of small-scale magnetic fields on the solar irradiance, it is essential to quantify the continuum contrast of magnetic elements in the quiet Sun (QS) network and in active region (AR) plage. By using Hinode/SP disk center data at constant spatial resolution, we aimed at updating results of earlier ground-based studies of contrast vs. magnetogram signal, and to look for systematic differences between AR plages and QS network. We performed a pixel-per-pixel study of continuum contrast vs. longitudinal flux density over large fields of view in AR and in QS (as in earlier studies). Even at Hinode's resolution, the contrast of magnetic elements reaches larger values in the QS than in ARs. We show that this difference cannot be explained by different inclinations of magnetic elements in ARs and QS. We compared our contrast vs. magnetic flux density with earlier studies and attributed the differences both to our proper removal of the pores and their surrounding diffraction-spread radiation, as well as to our enhanced spatial resolution and quasi-absence of scattered light. At Hinode's resolution, the contrast of magnetic elements peaks on average at similar magnetic flux densities in ARs and in the QS, which indicates that the brightest flux tubes have similar sizes in ARs and QS. Title: HMI: First Results Authors: Centeno, R.; Tomczyk, S.; Borrero, J. M.; Couvidat, S. Hayashi, K.; Hoeksema, T.; Liu, Y.; Schou, J. Bibcode: 2011ASPC..437..147C Altcode: 2010arXiv1012.3796C The Helioseismic and Magnetic Imager (HMI) has just started producing data that will help determine what the sources and mechanisms of variability in the Sun's interior are. The instrument measures the Doppler shift and the polarization of the Fe I 6173 Å line, on the entire solar disk at a relatively-high cadence, in order to study the oscillations and the evolution of the full vector magnetic field of the solar Photosphere. After the data are properly calibrated, they are given to a Milne-Eddington inversion code (VFISV, Borrero et al. 2010) whose purpose is to infer certain aspects of the physical conditions in the Sun's Photosphere, such as the full 3-D topology of the magnetic field and the line-of-sight velocity at the solar surface. We will briefly describe the characteristics of the inversion code, its advantages and limitations -both in the context of the model atmosphere and the actual nature of the data-, and other aspects of its performance on such a remarkable data load. Also, a cross-comparison with near-simultaneous maps from the Spectro-Polarimeter (SP) onboard Hinode will be made. Title: Inferring the magnetic field vector in the quiet Sun. I. Photon noise and selection criteria Authors: Borrero, J. M.; Kobel, P. Bibcode: 2011A&A...527A..29B Altcode: 2010arXiv1011.4380B In the past, spectropolarimetric data from Hinode/SP were employed to infer the distribution of the magnetic field vector in the quiet Sun. While some authors found predominantly horizontal magnetic fields, others favor an isotropic distribution. We investigate whether it is actually possible to accurately retrieve the magnetic field vector in regions with very low polarization signals (e.g. internetwork), employing the FeI line pair at 6300 Å. We first perform inversions of the Stokes vector observed with Hinode/SP in the quiet Sun at disk center in order to confirm the distributions retrieved by other authors. We then carry out several Monte-Carlo simulations with synthetic data, with which we show that the observed distribution of the magnetic field vector can be explained in terms of purely vertical (γ = 0°) and weak fields (bar{B<20} G), which are misinterpreted by the analysis technique (Stokes inversion code) as being horizontal (γ ≈ 90°) and stronger (bar{B ≈ 100} G), owing to the effect of the photon noise. This challenges the correctness of previous results, which presented the distributions for the magnetic field vector peaking at γ = 90° and bar{B=100} G. We propose that an accurate determination of the magnetic field vector can be achieved by decreasing the photon noise to a point where most of the observed profiles posses Stokes Q or U profiles that are above the noise level. Unfortunately, for noise levels as low as 2.8 × 10-4, only 30 % of the observed region with Hinode/SP have sufficiently strong Q or U signals, implying that the magnetic field vector remains unknown in the rest of the internetwork. Title: Supersonic Magnetic Upflows in Granular Cells Observed with SUNRISE/IMAX Authors: Borrero, J. M.; Martínez-Pillet, V.; Schlichenmaier, R.; Solanki, S. K.; Bonet, J. A.; del Toro Iniesta, J. C.; Schmidt, W.; Barthol, P.; Gandorfer, A.; Domingo, V.; Knölker, M. Bibcode: 2010ApJ...723L.144B Altcode: 2010arXiv1009.1227B Using the IMaX instrument on board the SUNRISE stratospheric balloon telescope, we have detected extremely shifted polarization signals around the Fe I 5250.217 Å spectral line within granules in the solar photosphere. We interpret the velocities associated with these events as corresponding to supersonic and magnetic upflows. In addition, they are also related to the appearance of opposite polarities and highly inclined magnetic fields. This suggests that they are produced by the reconnection of emerging magnetic loops through granular upflows. The events occupy an average area of 0.046 arcsec2 and last for about 80 s, with larger events having longer lifetimes. These supersonic events occur at a rate of 1.3 × 10-5 occurrences per second per arcsec2. Title: Magnetic Loops in the Quiet Sun Authors: Wiegelmann, T.; Solanki, S. K.; Borrero, J. M.; Martínez Pillet, V.; del Toro Iniesta, J. C.; Domingo, V.; Bonet, J. A.; Barthol, P.; Gandorfer, A.; Knölker, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.185W Altcode: 2010arXiv1009.4715W We investigate the fine structure of magnetic fields in the atmosphere of the quiet Sun. We use photospheric magnetic field measurements from SUNRISE/IMaX with unprecedented spatial resolution to extrapolate the photospheric magnetic field into higher layers of the solar atmosphere with the help of potential and force-free extrapolation techniques. We find that most magnetic loops that reach into the chromosphere or higher have one footpoint in relatively strong magnetic field regions in the photosphere. Ninety-one percent of the magnetic energy in the mid-chromosphere (at a height of 1 Mm) is in field lines, whose stronger footpoint has a strength of more than 300 G, i.e., above the equipartition field strength with convection. The loops reaching into the chromosphere and corona are also found to be asymmetric in the sense that the weaker footpoint has a strength B < 300 G and is located in the internetwork (IN). Such loops are expected to be strongly dynamic and have short lifetimes, as dictated by the properties of the IN fields. Title: Fully Resolved Quiet-Sun Magnetic flux Tube Observed with the SUNRISE/IMAX Instrument Authors: Lagg, A.; Solanki, S. K.; Riethmüller, T. L.; Martínez Pillet, V.; Schüssler, M.; Hirzberger, J.; Feller, A.; Borrero, J. M.; Schmidt, W.; del Toro Iniesta, J. C.; Bonet, J. A.; Barthol, P.; Berkefeld, T.; Domingo, V.; Gandorfer, A.; Knölker, M.; Title, A. M. Bibcode: 2010ApJ...723L.164L Altcode: 2010arXiv1009.0996L Until today, the small size of magnetic elements in quiet-Sun areas has required the application of indirect methods, such as the line-ratio technique or multi-component inversions, to infer their physical properties. A consistent match to the observed Stokes profiles could only be obtained by introducing a magnetic filling factor that specifies the fraction of the observed pixel filled with magnetic field. Here, we investigate the properties of a small magnetic patch in the quiet Sun observed with the IMaX magnetograph on board the balloon-borne telescope SUNRISE with unprecedented spatial resolution and low instrumental stray light. We apply an inversion technique based on the numerical solution of the radiative transfer equation to retrieve the temperature stratification and the field strength in the magnetic patch. The observations can be well reproduced with a one-component, fully magnetized atmosphere with a field strength exceeding 1 kG and a significantly enhanced temperature in the mid to upper photosphere with respect to its surroundings, consistent with semi-empirical flux tube models for plage regions. We therefore conclude that, within the framework of a simple atmospheric model, the IMaX measurements resolve the observed quiet-Sun flux tube. Title: Quiet-sun Intensity Contrasts in the Near-ultraviolet as Measured from SUNRISE Authors: Hirzberger, J.; Feller, A.; Riethmüller, T. L.; Schüssler, M.; Borrero, J. M.; Afram, N.; Unruh, Y. C.; Berdyugina, S. V.; Gandorfer, A.; Solanki, S. K.; Barthol, P.; Bonet, J. A.; Martínez Pillet, V.; Berkefeld, T.; Knölker, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.154H Altcode: We present high-resolution images of the Sun in the near-ultraviolet spectral range between 214 nm and 397 nm as obtained from the first science flight of the 1 m SUNRISE balloon-borne solar telescope. The quiet-Sun rms intensity contrasts found in this wavelength range are among the highest values ever obtained for quiet-Sun solar surface structures—up to 32.8% at a wavelength of 214 nm. We compare the rms contrasts obtained from the observational data with theoretical intensity contrasts obtained from numerical magnetohydrodynamic simulations. For 388 nm and 312 nm the observations agree well with the numerical simulations whereas at shorter wavelengths discrepancies between observed and simulated contrasts remain. Title: Quiet-Sun intensity contrasts in the near ultraviolet Authors: Hirzberger, Johann; Feller, Alex; Riethmüller, Tino L.; Schüssler, Manfred; Borrero, Juan M.; Afram, Nadine; Unruh, Yvonne C.; Berdyugina, Svetlana V.; Gandorfer, Achim; Solanki, Sami K.; Barthol, Peter; Bonet, Jose A.; Martínez Pillet, Valentin; Berkefeld, Thomas; Knölker, Michael; Schmidt, Wolfgang; Title, Alan M. Bibcode: 2010arXiv1009.1050H Altcode: We present high-resolution images of the Sun in the near ultraviolet spectral range between 214 nm and 397 nm as obtained from the first science flight of the 1-m Sunrise balloon-borne solar telescope. The quiet-Sun rms intensity contrasts found in this wavelength range are among the highest values ever obtained for quiet-Sun solar surface structures - up to 32.8% at a wavelength of 214 nm. We compare with theoretical intensity contrasts obtained from numerical magneto-hydrodynamic simulations. For 388 nm and 312 nm the observations agree well with the numerical simulations whereas at shorter wavelengths discrepancies between observed and simulated contrasts remain. Title: Spectropolarimetric analysis of 3D MHD sunspot simulations Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K. Bibcode: 2010AN....331..567B Altcode: We have employed 3D non-grey MHD simulations of sunspots to compute theoretical Stokes profiles and compare the levels of circular and linear polarization in the simulations with those observed in a real sunspot. We find that the spatial distribution and average values of these quantities agree very well with the observations, although the polarization levels in the simulations are slightly larger. This can be explained by a slightly larger magnetic field strength or a larger temperature gradient in the simulated penumbra as compared to the observations. Title: Convective Motions and Net Circular Polarization in Sunspot Penumbrae Authors: Borrero, J. M.; Solanki, S. K. Bibcode: 2010ApJ...709..349B Altcode: 2009arXiv0911.2570B We have employed a penumbral model, which includes the Evershed flow and convective motions inside penumbral filaments, to reproduce the azimuthal variation of the net circular polarization (NCP) in sunspot penumbrae at different heliocentric angles for two different spectral lines. The theoretical NCP fits the observations as satisfactorily as penumbral models based on flux tubes. The reason for this is that the effect of convective motions on the NCP is very small compared to the effect of the Evershed flow. In addition, the NCP generated by convective upflows cancels out the NCP generated by the downflows. We have also found that, in order to fit the observed NCP, the strength of the magnetic field inside penumbral filaments must be very close to 1000 G. In particular, field-free or weak-field filaments fail to reproduce both the correct sign of the NCP and its dependence on the azimuthal and heliocentric angles. Title: UV intensity distributions of the quiet Sun observed with Sunrise Authors: Hirzberger, Johann; Feller, A.; Riethmueller, T.; Borrero, J. M.; Schüssler, M.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knoelker, M.; Martínez Pillet, V.; Schmidt, W.; Solanki, S.; Title, A. Bibcode: 2010cosp...38.1735H Altcode: 2010cosp.meet.1735H High resolution solar images in the near UV have been obtained with the Solar UV Filtergraph (SUFI) onboard the Sunrise balloon borne observatory, amongst others in wavelength regions not accessible from the ground. We present intensity distributions of the quiet Sun at different heliocentric angles, from disk center to the solar limb. These results, obtained in spectral windows at 214 nm, 313 nm (OH band), 388 nm (CN band) and 396.7 nm (CaIIH), represent an important validation of numerical models of the solar photosphere and are, thus, fundamental ingredients for our understanding of the thermal processes in the solar surface region. Title: Models and observations of sunspot penumbrae Authors: Borrero, Juan Manuel Bibcode: 2009ScChG..52.1670B Altcode: 2008arXiv0810.0080B The mysteries of sunspot penumbrae have been under an intense scrutiny for the past 10 years. During this time, some models have been proposed and refuted, while the surviving ones had to be modified, adapted and evolved to explain the ever-increasing array of observational constraints. In this contribution I will review two of the present models, emphasizing their contributions to this field, but also pinpointing some of their inadequacies to explain a number of recent observations at very high spatial resolution (0.32″). To help explaining these new observations I propose some modifications to each of those models. These modifications bring those two seemingly opposite models closer together into a general picture that agrees well with recent 3D magneto-hydrodynamic simulations. Title: Dipolar Evolution in a Coronal Hole Region Authors: Yang, Shuhong; Zhang, Jun; Borrero, Juan Manuel Bibcode: 2009ApJ...703.1012Y Altcode: 2009arXiv0908.0578Y Using observations from the Solar and Heliospheric Observatory, the Solar Terrestrial Relations Observatory, and Hinode, we investigate magnetic field evolution in an equatorial coronal hole region. Two dipoles emerge one by one. The negative element of the first dipole disappears due to the interaction with the positive element of the second dipole. During this process, a jet and a plasma eruption are observed. The opposite polarities of the second dipole separate at first, and then cancel with each other, which is first reported in a coronal hole. With the reduction of unsigned magnetic flux of the second dipole from 9.8 × 1020 Mx to 3.0 × 1020 Mx in two days, 171 Å brightness decreases by 75% and coronal loops shrink obviously. At the cancellation sites, the transverse fields are strong and point directly from the positive elements to the negative ones, meanwhile Doppler redshifts with an average velocity of 0.9 km s-1 are observed, comparable to the horizontal velocity (1.0 km s-1) derived from the canceling island motion. Several days later, the northeastern part of the coronal hole, where the dipoles are located, appears as a quiet region. These observations support the idea that the interaction between the two dipoles is caused by flux reconnection, while the cancellation between the opposite polarities of the second dipole is due to the submergence of original loops. These results will help us to understand coronal hole evolution. Title: Are There Field-Free Gaps near τ = 1 in Sunspot Penumbrae? Authors: Borrero, J. M.; Solanki, S. K. Bibcode: 2008ApJ...687..668B Altcode: 2008arXiv0806.4454B The vertical stratification of the magnetic field strength in sunspot penumbrae is investigated by means of spectropolarimetric observations at high spatial resolution from the Hinode spacecraft. Assuming that the magnetic field changes linearly with optical depth we find that, in those regions where the magnetic field is more inclined and the Evershed flow is strongest (penumbral intraspines), the magnetic field can either increase or decrease with depth. Allowing more degrees of freedom to the magnetic field stratification reveals that the magnetic field initially decreases from log τ5 = - 3 until log τ5simeq - 1.0, but increases again below that. The presence of strong magnetic fields near the continuum is at odds with the existence of regions void of magnetic fields at, or right below, the τ5 = 1 level in the penumbra. However, they are compatible with the presence of a horizontal flux-tube-like field embedded in a magnetic atmosphere. Title: Solar spectropolarimetry at high spatial resolution: Quiet-Sun magnetic fields Authors: Bellot Rubio, L. R.; Borrero, J. M. Bibcode: 2008ESPM...12..2.4B Altcode: Spectropolarimetry permits quantitative inferences of solar magnetic fields to be carried out. It is in fact the only means at our disposal to characterize the physical properties of small magnetic structures from low spatial-resolution observations. The accuracy of the results derived from spectropolarimetric measurements has improved dramatically with the advent of simultaneous observations in different spectral regions and high angular resolution measurements. The main advantage brought about by high spatial resolution is that there is less need to model complex scenarios involving different fields and/or flows coexisting in the pixel. Currently it is possible to achieve resolutions of 0.3 arcsec from space (with the Hinode satellite) and 0.15 arcsec from the ground (with the 1-m Swedish Solar Telescope). In this talk we will show examples of high spatial resolution spectropolarimetric observations and how they are challenging our understanding of quiet Sun magnetic fields and sunspot penumbrae. In addition, we will describe previously unknown issues that affect diffraction-limited observations. Proper interpretations of the measurements require these effects to be taken into account. Title: Evidence of magnetic field wrapping around penumbral filaments Authors: Borrero, J. M.; Lites, B. W.; Solanki, S. K. Bibcode: 2008A&A...481L..13B Altcode: 2007arXiv0712.2548B We employ high-spatial resolution spectropolarimetric observations from the Solar Optical Telescope on-board the Hinode spacecraft to investigate the fine structure of the penumbral magnetic fields. The Stokes vector of two neutral iron lines at 630 nm is inverted at every spatial pixel to retrieve the depth-dependence of the magnetic field vector, line-of-sight velocity and thermodynamic parameters. We show that the azimuthal angle of the magnetic field vector has opposite sign on both sides above the penumbral filaments. This is consistent with the wrapping of an inclined field around the horizontal filaments. The wrapping effect is stronger for filaments with larger horizontal extensions. In addition, we find that the external magnetic field can penetrate into the intraspines, leading to non-radial magnetic fields inside them. These findings shed some light on the controversial small-scale structure of the sunspot penumbra. 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: Temporal evolution of the Evershed flow in sunspots. II. Physical properties and nature of Evershed clouds Authors: Cabrera Solana, D.; Bellot Rubio, L. R.; Borrero, J. M.; Del Toro Iniesta, J. C. Bibcode: 2008A&A...477..273C Altcode: 2007arXiv0709.1601C Context: Evershed clouds (ECs) represent the most conspicuous variation of the Evershed flow in sunspot penumbrae.
Aims: We determine the physical properties of ECs from high spatial and temporal resolution spectropolarimetric measurements. This information is used to investigate the nature of the EC phenomenon.
Methods: The Stokes profiles of four visible and three infrared spectral lines are subject to inversions based on simple one-component models as well as more sophisticated realizations of penumbral flux tubes embedded in a static ambient field (uncombed models).
Results: According to the one-component inversions, the EC phenomenon can be understood as a perturbation of the magnetic and dynamic configuration of the penumbral filaments along which the ECs move. The uncombed inversions, on the other hand, suggest that ECs are the result of enhancements in the visibility of penumbral flux tubes. We conjecture that these enhancements are caused by a perturbation of the thermodynamic properties of the tubes, rather than by changes in the vector magnetic field. This mechanism is investigated performing numerical experiments of thick penumbral tubes in mechanical equilibrium with a background field.
Conclusions: While the one-component inversions confirm many of the properties indicated by a simple line parameter analysis (Paper I of this series), we give more credit to the results of the uncombed inversions because they take into account, at least in an approximate manner, the fine structure of the penumbra.

Appendix A is only available in electronic form at http://www.aanda.org Title: On the Role of Magnetic Fields in Abundance Determinations Authors: Borrero, J. M. Bibcode: 2008ApJ...673..470B Altcode: 2007arXiv0709.3809B Although there is considerable evidence supporting an ubiquitous magnetic field in solar/stellar photospheres, its impact in the determination of abundances has never been quantified. In this work we investigate whether the magnetic field plays a measurable role for this kind of study. To that end, we carry out simulations of spectral line formation in the presence of a magnetic field and then use those profiles to derive the abundance of several atomic species (Fe, Si, C, and O), neglecting the magnetic field. In this way, we find that the derived iron abundance can be significantly biased, with systematic errors of up to 0.1 dex. In the case of silicon, carbon, and oxygen, their role is very marginal (errors smaller than 0.02 dex). We also find that the effect of the magnetic field strongly depends on its inclination with respect to the observer. We show that fields that are aligned with the observer lead to an underestimation of the real abundance, whereas more inclined ones overestimate it. In the case of a mixture of fields with different inclinations, these effects are likely to partly cancel each other out, making the role of the magnetic field even less important. Finally, we derive a simple model that can be used to determine the suitability of a spectral line when we wish to avoid the bias introduced by the neglect of the magnetic field. 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: Flux Tubes as the Origin of Net Circular Polarization in Sunspot Penumbrae Authors: Borrero, J. M.; Bellot Rubio, L. R.; Müller, D. A. N. Bibcode: 2007ApJ...666L.133B Altcode: 2007arXiv0707.4145B We employ a three-dimensional magnetohydrostatic model of a horizontal flux tube, embedded in a magnetic surrounding atmosphere, to successfully reproduce the azimuthal and center-to-limb variations of the net circular polarization observed in sunspot penumbrae. This success is partly due to the realistic modeling of the interaction between the flux tube and the surrounding magnetic field. Title: The structure of sunspot penumbrae. IV. MHS equilibrium for penumbral flux tubes and the origin of dark core penumbral filaments and penumbral grains Authors: Borrero, J. M. Bibcode: 2007A&A...471..967B Altcode: 2007arXiv0704.3219B Aims:We study the magnetohydrostatic equilibrium of magnetic flux tubes with circular cross sections embedded in a magnetic surrounding atmosphere.
Methods: We solve the static momentum equation in 2.5D to obtain the thermodynamics that are consistent with a prescribed velocity and magnetic fields.
Results: We show that force balance is roughly satisfied if the flux tube's magnetic field is aligned with its axis. Equilibrium is guaranteed if this magnetic field possesses a transverse component. Several forms of this transverse field are investigated. The resulting magnetic field configurations are critically reviewed in terms of the results from spectropolarimetric observations. The thermodynamic structure that allows the flux tube to be in mechanical equilibrium is also calculated. We show that the inferred pressure, density, and temperature stratification reproduce intensity features similar to dark core penumbral filaments and penumbral grains. Title: Modified p-modes in penumbral filaments? Authors: Bloomfield, D. S.; Solanki, S. K.; Lagg, A.; Borrero, J. M.; Cally, P. S. Bibcode: 2007A&A...469.1155B Altcode: 2007arXiv0705.0481B Aims:The primary objective of this study is to search for and identify wave modes within a sunspot penumbra.
Methods: Infrared spectropolarimetric time series data are inverted using a model comprising two atmospheric components in each spatial pixel. Fourier phase difference analysis is performed on the line-of-sight velocities retrieved from both components to determine time delays between the velocity signals. In addition, the vertical separation between the signals in the two components is calculated from the Stokes velocity response functions.
Results: The inversion yields two atmospheric components, one permeated by a nearly horizontal magnetic field, the other with a less-inclined magnetic field. Time delays between the oscillations in the two components in the frequency range 2.5-4.5 mHz are combined with speeds of atmospheric wave modes to determine wave travel distances. These are compared to expected path lengths obtained from response functions of the observed spectral lines in the different atmospheric components. Fast-mode (i.e., modified p-mode) waves exhibit the best agreement with the observations when propagating toward the sunspot at an angle ~50° to the vertical. Title: Magnetic Field Vector Retrieval With the Helioseismic and Magnetic Imager Authors: Borrero, J. M.; Tomczyk, S.; Norton, A.; Darnell, T.; Schou, J.; Scherrer, P.; Bush, R.; Liu, Y. Bibcode: 2007SoPh..240..177B Altcode: 2006astro.ph.11565B We investigate the accuracy to which we can retrieve the solar photospheric magnetic field vector using the Helioseismic and Magnetic Imager (HMI) that will fly onboard of the Solar Dynamics Observatory by inverting simulated HMI profiles. The simulated profiles realistically take into account the effects of the photon noise, limited spectral resolution, instrumental polarization modulation, solar p modes, and temporal averaging. The accuracy of the determination of the magnetic field vector is studied by considering the different operational modes of the instrument. Title: Modified p-modes in penumbral filaments Authors: Bloomfield, D. S.; Lagg, A.; Solanki, S. K.; Borrero, J. M. Bibcode: 2007msfa.conf..241B Altcode: A time series analysis was performed on velocity signals in a sunspot penumbra to search for possible wave modes. The spectropolarimetric photospheric data obtained by the Tenerife Infrared Polarimeter were inverted using the SPINOR code. An atmospheric model comprising two magnetic components and one stray-light component gave an optimal fit to the data. Fourier phase difference analysis between line-of-sight velocities of both magnetic components provided time delays between the two atmospheres. These delays were combined with the speeds of atmospheric wave modes and compared to height separations derived from velocity response functions to determine the wave mode. Title: Magnetic Field Vector Retrieval with HMI Authors: Borrero, J. M.; Tomczyk, S.; Norton, A. A.; Darnell, T.; Schou, J.; Scherrer, P.; Bush, R. I.; Lui, Y. Bibcode: 2006ASPC..358..144B Altcode: The Helioseismic and Magnetic Imager (HMI), on board the Solar Dynamics Observatory (SDO), will begin data acquisition in 2008. It will provide the first full-disk, high temporal cadence observations of the full Stokes vector with a 0.5 arcsec pixel size. This will allow for a continuous monitoring of the Solar magnetic-field vector. HMI data will advance our understanding of the small- and large-scale magnetic field evolution, its relation to the solar and global dynamic processes, coronal field extrapolations, flux emergence, magnetic helicity, and the nature of the polar magnetic fields. We summarize HMI's expected operation modes, focusing on the polarization cross-talk induced by the solar oscillations, and how this affects the magnetic-field vector determination. Title: The Uncombed Penumbra Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K. Bibcode: 2006ASPC..358...19B Altcode: 2006astro.ph..2130B The uncombed penumbral model explains the structure of the sunspot penumbra in terms of thick magnetic fibrils embedded in a surrounding, magnetic atmosphere. This model has been successfully applied to explain the polarization signals emerging from the sunspot penumbra. Thick penumbral fibrils face some physical problems, however. In this contribution we will offer possible solutions to these shortcomings. Title: On the fine structure of sunspot penumbrae. III. The vertical extension of penumbral filaments Authors: Borrero, J. M.; Solanki, S. K.; Lagg, A.; Socas-Navarro, H.; Lites, B. Bibcode: 2006A&A...450..383B Altcode: 2005astro.ph.10586B In this paper we study the fine structure of the penumbra as inferred from the uncombed model (flux tube embedded in a magnetic surrounding) when applied to penumbral spectropolarimetric data from the neutral iron lines at 6300 Å. The inversion infers very similar radial dependences in the physical quantities (LOS velocity, magnetic field strength etc.) as those obtained from the inversion of the Fe I 1.56 μm lines. In addition, the large Stokes V area asymmetry exhibited by the visible lines helps to constrain the size of the penumbral flux tubes. As we demonstrate here, the uncombed model is able to reproduce the area asymmetry with striking accuracy, returning flux tubes as thick as 100-300 kilometers in the vertical direction, in good agreement with previous investigations. Title: The uncombed penumbra Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K. Bibcode: 2006astro.ph..2129B Altcode: The uncombed penumbral model explains the structure of the sunspot penumbra in terms of thick magnetic fibrils embedded in a magnetic surrounding atmosphere. This model has been successfully applied to explain the polarization signals emerging from the sunspot penumbra. Thick penumbral fibrils face some physical problems, however. In this contribution we will offer possible solutions to these shortcomings. Title: On the fine structure of sunspot penumbrae. II. The nature of the Evershed flow Authors: Borrero, J. M.; Lagg, A.; Solanki, S. K.; Collados, M. Bibcode: 2005A&A...436..333B Altcode: 2005astro.ph..3677B We investigate the fine structure of the sunspot penumbra by means of a model that allows for a flux tube in horizontal pressure balance with the magnetic background atmosphere in which it is embedded. We apply this model to spectropolarimetric observations of two neutral iron lines at 1.56 μm and invert several radial cuts in the penumbra of the same sunspot at two different heliocentric angles. In the inner part of the penumbra we find hot flux tubes that are somewhat inclined to the horizontal. They become gradually more horizontal and cooler with increasing radial distance. This is accompanied by an increase in the velocity of the plasma and a decrease of the gas pressure difference between flux tube and the background component. At large radial distances the flow speed exceeds the critical speed and evidence is found for the formation of a shock front. These results are in good agreement with simulations of the penumbral fine structure and provide strong support for the siphon flow as the physical mechanism driving the Evershed flow. Title: Thermal-magnetic relation in a sunspot and a map of its Wilson depression Authors: Mathew, S. K.; Solanki, S. K.; Lagg, A.; Collados, M.; Borrero, J. M.; Berdyugina, S. Bibcode: 2004A&A...422..693M Altcode: We present relations between thermal and magnetic quantities in a simple, isolated sunspot, as deduced from the inversion of 1.56 μm spectropolarimetric data. We used a combination of two infrared Fe I lines at 15 648.5 Å and 15 652.8 Å/ in the inversions. Due to the high Zeeman sensitivity of these lines, we can study this relationship in the entire sunspot. The relevant parameters were derived both as a function of location within the sunspot and of height in the atmosphere using an inversion technique based on response functions. In this paper we relate the magnetic vector with temperature. We find a non-linear relationship between the various components of the magnetic vector and temperature, which confirm the results from earlier investigations. We also computed the Wilson depression and the plasma β for the observed sunspot and compare our results with earlier findings. Title: On the fine structure of sunspot penumbrae. I. A quantitative comparison of two semiempirical models with implications for the Evershed effect Authors: Borrero, J. M.; Solanki, S. K.; Bellot Rubio, L. R.; Lagg, A.; Mathew, S. K. Bibcode: 2004A&A...422.1093B Altcode: Sunspot penumbrae exhibit prominent fine structure. Different interpretations of spectropolarimetric observations suggest different, sometimes contradictory, properties of this fine structure. In this paper we show that the results of inversions of penumbral infrared profiles based on one-component models with gradients of the atmospheric parameters and two-component models without gradients are compatible with each other. Our analysis reconciles the results of previous investigations and provides further support for the picture that sunspot penumbrae are composed of penumbral flux tubes embedded in a magnetic background. The magnetic field in the tubes is more horizontal and weaker than that of the background atmosphere. While the tubes carry most of the Evershed flow, the background is essentially at rest. We notice also that the magnetic field strength in the flux tubes drops much more slowly with radial distance than the background field. This finding is discussed as a possible driver for the Evershed flow. Title: The fine structure of the sunspot penumbra Authors: Borrero, Juan Manuel Bibcode: 2004PhDT.......307B Altcode: No abstract at ADS Title: Three dimensional structure of a regular sunspot from the inversion of IR Stokes profiles Authors: Mathew, S. K.; Lagg, A.; Solanki, S. K.; Collados, M.; Borrero, J. M.; Berdyugina, S.; Krupp, N.; Woch, J.; Frutiger, C. Bibcode: 2003A&A...410..695M Altcode: The magnetic, thermal and velocity structure of a regular sunspot, observed close to solar disk center is presented. Spectropolarimetric data obtained with the Tenerife Infrared Polarimeter (TIP) in two infrared FeI lines at 15 648.5 Å and 15 652.8 Å are inverted employing a technique based on response functions to retrieve the atmospheric stratification at every point in the sunspot. In order to improve the results for the umbra, profiles of Zeeman split OH lines blending the FeI 15 652.8 Å are also consistently fit. Thus we obtain maps of temperature, line-of-sight velocity, magnetic field strength, inclination, and azimuth, as a function of both location within the sunspot and height in the atmosphere. We present these maps for an optical depth range between log tau5 = 0 and log tau5 = -1.5, where these lines provide accurate results. We find decreasing magnetic field strength with increasing height all over the sunspot, with a particularly large vertical field gradient of ~ -4 G km-1 in the umbra. We also observe the so called ``spine'' structures in the penumbra, i.e. extended radial features with a stronger and more vertical magnetic field than the surroundings. Also we found that the magnetic field zenith angle increases with height. From the velocity map it is clear that the Evershed flow avoids the spines and mostly concentrates in the more inclined intervening field. The field inclination at a few locations in the outer penumbra in lower layers goes beyond 90o. These locations coincide with the strongest flows in the velocity map. Title: Infrared Polarimetry at the MPAe: The Solar Atmosphere from the Photosphere to the Upper Chromosphere Authors: Lagg, Andreas; Woch, Joachim; Solanki, Sami K.; Mathew, Shibu; Borrero, Juan M.; Krupp, N.; Raouafi, N. E. Bibcode: 2003ANS...324...29L Altcode: 2003ANS...324..D04L No abstract at ADS Title: Accurate atomic parameters for near-infrared spectral lines Authors: Borrero, J. M.; Bellot Rubio, L. R.; Barklem, P. S.; del Toro Iniesta, J. C. Bibcode: 2003A&A...404..749B Altcode: A realistic two-component model of the quiet solar photosphere is used to fit the intensity spectrum of the Sun in the wavelength range 0.98-1.57 mu m. Our approach differs from earlier attempts in many respects: proper account of convective inhomogeneities is made, accurate collisional broadening parameters from quantum mechanical computations are used, and the effects of possible blends in the local continuum are corrected empirically. This allows us to derive oscillator strengths and central wavelengths for virtually any unblended line of the solar spectrum. The accuracy of the inferred atomic parameters, about 0.06 dex for oscillator strengths and 5 mÅ at 1 mu m for central wavelengths, is similar to that of the best laboratory measurements. We apply our method to 83 near-infrared lines belonging to 6 different atomic species. The availability of accurate oscillator strengths and central wavelengths for lines of different species is essential for the interpretation of high resolution spectroscopic observations. The method is especially useful in the infrared, a wavelength domain where laboratory measurements are scarce. Title: Two-Component Modeling of Convective Motions in the Solar Photosphere and Determination of Atomic Parameters Authors: Borrero, J. M.; Bellot Rubio, L. R. Bibcode: 2003IAUS..210P..C9B Altcode: No abstract at ADS Title: Accurate Atomic Parameters from the Solar Spectrum Authors: Bellot Rubio, Luis Ramon; Borrero, Juan Manuel; Barklem, Paul; del Toro Iniesta, Jose Carlos Bibcode: 2003IAUJD..20E..16B Altcode: A realistic two-component model of the quiet solar photosphere is used to fit the full shape of the intensity profiles of unblended lines in the solar spectrum. Our approach differs from previous attempts in many respects: proper account of granulation inhomogeneities is made accurate collisional broadening parameters from quantum mechanical computations are used and possible absorptions in the local continuum due to blends are corrected empirically. This allows us to derive oscillator strengths and central wavelengths for any clean line with an accuracy comparable with that of the best laboratory measurements. The availability of very precise atomic parameters for lines of different species is essential for the interpretation of high resolution spectroscopic observations. Abundance determinations and investigations of granular motions in stellar atmospheres are among the applications that would benefit from such accurate atomic data. As an example we determine the oscillator strengths and central wavelengths of 100 unblended lines in the near-infrared (0.99-1.56 microns) a wavelength domain where laboratory measurements are particularly scarce. Title: Modeling the Fine Structure of a Sunspot Penumbra through the Inversion of Stokes Profiles Authors: Borrero, J. M.; Lagg, A.; Solanki, S. K.; Frutiger, C.; Collados, M.; Bellot Rubio, L. R. Bibcode: 2003ASPC..286..235B Altcode: 2003ctmf.conf..235B No abstract at ADS Title: Iron abundance in the solar photosphere. Application of a two-component model atmosphere Authors: Bellot Rubio, L. R.; Borrero, J. M. Bibcode: 2002A&A...391..331B Altcode: A realistic two-component model of the quiet Sun is used to determine the solar abundance of iron from the inversion of a number of Fe I and Fe II spectral lines for which accurate atomic parameters (oscillator strengths, central wavelengths, and collisional broadening cross sections) exist. From 33 Fe I lines we infer an abundance of A_Fe = 7.43 +/- 0.06, whereas we estimate A_Fe = 7.45 +/- 0.08 from 10 Fe II lines. These values are in excellent agreement with the results of analyses based on realistic 3D hydrodynamical simulations of the solar granulation, and imply a low photospheric iron abundance. We investigate the effects of convective motions and granular temperatures and conclude that both are important for reliable abundance determinations. For Fe I lines, the effects of convective motions can be simulated by using a microturbulent velocity of about 1 km s-1, whereas it is possible to account for temperature inhomogeneities by adopting an average temperature stratification which is cooler than the Holweger & Müller model in the upper layers. Title: A two-component model of the solar photosphere from the inversion of spectral lines Authors: Borrero, J. M.; Bellot Rubio, L. R. Bibcode: 2002A&A...385.1056B Altcode: A two-component model of the solar photosphere is obtained from the inversion of the intensity profiles of 22 Fe I spectral lines for which very accurate atomic data (oscillator strengths, central wavelengths, and collisional broadening parameters) exist. The model is meant to describe the effects of convective motions in the solar photosphere. It has been subject to various tests to confront its predictions with observations of the solar spectrum. The model is able to reproduce the observed line shifts and equivalent widths of about 800 spectral lines of iron and other species. It is also capable of matching the observed center-to-limb variation of the continuum intensity with unprecedented accuracy. This allows us to determine line-transition parameters from the fitting of the solar spectrum. Exploratory calculations demonstrate that the model can be used to derive transition probabilities and central wavelengths of Fe I and Fe II lines, as well as other elements, within the uncertainties of the best laboratory measurements.