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Author name code: borrero
ADS astronomy entries on 2022-09-14
author:"Borrero, Juan Manuel" 

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Title: Inference of electric currents in the solar photosphere
Authors: Pastor Yabar, A.; Borrero, J. M.; Quintero Noda, C.; Ruiz
   Cobo, B.
2021A&A...656L..20P    Altcode: 2021arXiv211204356P
  Context. Despite their importance, routine and direct measurements
  of electric currents, j, in the solar atmosphere have generally not
  been possible. <BR /> Aims: We aim at demonstrating the capabilities
  of a newly developed method for determining electric currents in
  the solar photosphere. <BR /> 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. <BR /> 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 τ<SUB>5</SUB>∈[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. <BR />
  Conclusions: We present a method that allows the inference of the three
  components of the electric current vector at deep atmospheric layers
  (photospheric layers) from spectropolarimetric observations.

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Title: Combining magneto-hydrostatic constraints with Stokes profiles
    inversions. II. Application to Hinode/SP observations
Authors: Borrero, J. M.; Pastor Yabar, A.; Ruiz Cobo, B.
2021A&A...647A.190B    Altcode: 2021arXiv210104394B
  Context. Inversion techniques applied to the radiative transfer equation
  for polarized light are capable of inferring the physical parameters in
  the solar atmosphere (temperature T, magnetic field B, and line-of-sight
  velocity v<SUB>los</SUB>) from observations of the Stokes vector
  (i.e., spectropolarimetric observations) in spectral lines. Inferences
  are usually performed in the (x, y, τ<SUB>c</SUB>) domain, where
  τ<SUB>c</SUB> 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. <BR /> Aims:
  We aim to develop a new inversion code capable of reliably inferring
  the physical parameters in the (x, y, z) domain. <BR /> 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. <BR />
  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. <BR /> Conclusions: Our results open the door
  for the possibility of calculating reliable electric currents in three
  dimensions, j(x, y, z), in the solar photosphere. Further consistency
  checks would include a comparison with other methods that have recently
  been proposed and which achieve similar goals.

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Title: 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.
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.

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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.
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. <P />Movie associated to Fig. 2 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201936941/olm">https://www.aanda.org</A>

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Title: Combining magnetohydrostatic constraints with Stokes profiles
    inversions. I. Role of boundary conditions
Authors: Borrero, J. M.; Pastor Yabar, A.; Rempel, M.; Ruiz Cobo, B.
2019A&A...632A.111B    Altcode:
  Context. Inversion codes for the polarized radiative transfer
  equation, when applied to spectropolarimetric observations (i.e.,
  Stokes vector) in spectral lines, can be used to infer the temperature
  T, line-of-sight velocity v<SUB>los</SUB>, and magnetic field B as
  a function of the continuum optical-depth τ<SUB>c</SUB>. However,
  they do not directly provide the gas pressure P<SUB>g</SUB> 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 P<SUB>g</SUB> 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 τ<SUB>c</SUB> to geometrical height z. <BR /> Aims:
  We aim at improving the determination of the gas pressure and density
  via the application of magnetohydrostatic (MHS) equilibrium instead of
  HE. <BR /> 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 P<SUB>g</SUB>
  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 - τ<SUB>c</SUB> conversion (i.e., Wilson depression). On the
  other hand, the application of MHS equilibrium with similar boundary
  conditions allows determination of P<SUB>g</SUB> 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 - τ<SUB>c</SUB> 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. <BR
  /> 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 τ<SUB>c</SUB>. This method could be used in
  conjunction with the inversion of the radiative transfer equation for
  polarized light in order to determine the thermodynamic, kinematic,
  and magnetic parameters of the solar atmosphere.

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

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

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Title: Observations of solar small-scale magnetic flux-sheet emergence
Authors: Fischer, C. E.; Borrero, J. M.; Bello González, N.;
   Kaithakkal, A. J.
2019A&A...622L..12F    Altcode: 2019arXiv190105870F
  <BR /> 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. <BR /> 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. <BR /> 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<SUP>-2</SUP>
  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. <BR /> 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 ∼10<SUP>18</SUP>
  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. <P />Movie attached to Fig. 5 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201834628/olm">https://www.aanda.org</A>

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Title: Three-lobed near-infrared Stokes V profiles in the quiet Sun
Authors: Kiess, Christoph; Borrero, Juan Manuel; Schmidt, Wolgang
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. <BR /> 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. <BR />
  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. <BR
  /> Results: Both approaches reveal some common features about the
  topology responsible for the observed three-lobed Stokes V signals:
  both a strong (&gt;1000 Gauss) and a very weak (&lt;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.

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Title: Solar Magnetoconvection and Small-Scale Dynamo
Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K.
2018smf..book..275B    Altcode:
  No abstract at ADS

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Title: Solar Magnetoconvection and Small-Scale Dynamo. Recent
    Developments in Observation and Simulation
Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K.
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.

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Title: Penumbral thermal structure below the visible surface
Authors: Borrero, J. M.; Franz, M.; Schlichenmaier, R.; Collados,
   M.; Asensio Ramos, A.
2017A&A...601L...8B    Altcode: 2017arXiv170502832B
  Context. The thermal structure of the penumbra below its visible
  surface (I.e., τ<SUB>5</SUB> ≥ 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. <BR /> Aims: We aim at
  determining the thermal stratification in the layers immediately
  beneath the visible surface of the penumbra: τ<SUB>5</SUB> ∈ [1,3]
  (≈70-80 km below the visible continuum-forming layer) <BR /> 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(τ<SUB>5</SUB>). <BR /> 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 τ<SUB>5</SUB> ≥
  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. <BR /> 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.

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Title: Are Internetwork Magnetic Fields in the Solar Photosphere
    Horizontal or Vertical?
Authors: Lites, B. W.; Rempel, M.; Borrero, J. M.; Danilovic, S.
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.

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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.
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. <BR /> Aims:
  We aim to confirm or refute the existence of weak-field regions in
  the deepest photospheric layers of the penumbra. <BR /> Methods:
  We investigated the magnetic field at log τ<SUB>5</SUB> = 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. <BR /> Results: At log τ<SUB>5</SUB>
  = 0 we find no evidence of regions with dynamically weak (B&lt;
  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.

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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.
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. <BR /> Aims: We study the topology of the penumbral magnetic
  field in the lower photosphere, focusing on regions where it returns
  below the surface. <BR /> 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. <BR /> 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.

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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.
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. <BR /> 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. <BR /> 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). <BR /> 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. <BR /> 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.
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<SUP>″</SUP> wide and ≈8.0<SUP>″</SUP> 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−<SUP>1</SUP> 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−<SUP>1</SUP> 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.
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}<SUB>\parallel </SUB>)
  and transverse ({B}<SUB>\perp </SUB>) components of the magnetic field,
  as well as of the magnetic pressure ({B}<SUP>2</SUP>={B}<SUB>\perp
  </SUB><SUP>2</SUP>+{B}<SUB>\parallel </SUB><SUP>2</SUP>), 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.
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.
2015A&A...578A..43R    Altcode:
  <BR /> Aims: We study the variation in the magnetic field strength,
  area, and continuum intensity of umbrae in solar cycles 23 and 24. <BR
  /> 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. <BR /> 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<SUP>-1</SUP>,
  while it increases in the rising phase of cycle 24 by about 138 (±
  72) G yr<SUP>-1</SUP>. The umbral intensity shows the opposite trend:
  the intensity increases with a rate of 0.7 (± 0.3)% of I<SUB>c</SUB>
  yr<SUP>-1</SUP> toward the end of cycle 23 and decreases with a rate
  of 3.8 (± 1.5)% of I<SUB>c</SUB> yr<SUP>-1</SUP> 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<SUP>-1</SUP>. If instead of
  the maximum umbral field we consider the average value over the entire
  umbra, the distribution shifts by about 44 Gauss. <BR /> 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.
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<SUP>+</SUP>). 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 σ<SUB>B</SUB>
  ≤ 35 (Gauss), σ<SUB>γ</SUB> ≤ 1.2°, and σ<SUB>v</SUB> ≤
  10 m s<SUP>-1</SUP>, respectively. Additionally, M-E inversion codes
  agree with the numerical simulations, when compared at a fixed optical
  depth, within σ<SUB>B</SUB> ≤ 130 (Gauss), σ<SUB>γ</SUB> ≤ 5°,
  and σ<SUB>v</SUB> ≤ 320 m s<SUP>-1</SUP>. 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 σ<SUB>B</SUB> ≤ 90 (Gauss), σ<SUB>γ</SUB> ≤ 3°,
  and σ<SUB>v</SUB> ≤ 90 m s<SUP>-1</SUP>.

---------------------------------------------------------
Title: High speed magnetized flows in the quiet Sun
Authors: Quintero Noda, C.; Borrero, J. M.; Orozco Suárez, D.;
   Ruiz Cobo, B.
2014A&A...569A..73Q    Altcode: 2014arXiv1407.7477Q
  Context. We analyzed spectropolarimetric data recorded with
  Hinode/SP in quiet-Sun regions located at the disk center. We found
  single-lobed Stokes V profiles showing highly blue- and red-shifted
  signals. Oftentimes both types of events appear to be related to
  each other. <BR /> 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. <BR />
  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. <BR /> 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
  τ<SUB>500</SUB> ∈ (-1, -3) and a strong magnetic field, B ≥ 1 kG,
  at the bottom of the atmosphere that quickly decreases upward until
  vanishing at log τ<SUB>500</SUB> ≈ -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. <BR
  /> 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.
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. <BR
  /> 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. <BR /> 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. <BR /> 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. <BR /> 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.
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
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.
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.
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<SUP>-4</SUP> 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<SUP>-3</SUP> (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.
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<SUB>⊥</SUB>, 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.
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. <BR />
  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. <BR /> 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). <BR /> 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. <BR />
  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.
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.
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<SUP>-1</SUP>, 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 km<SUP>2</SUP>. 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.
2012ASPC..455..337L    Altcode:
  The Helioseismic and Magnetic Imager (HMI; Scherrer &amp; 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.
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
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&gt;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.
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.
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
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.
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.
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. <BR /> 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. <BR
  /> 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. <BR /> 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. <BR /> 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.
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.
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.
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&lt;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<SUP>-4</SUP>, 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.
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 arcsec<SUP>2</SUP> and last for
  about 80 s, with larger events having longer lifetimes. These supersonic
  events occur at a rate of 1.3 × 10<SUP>-5</SUP> occurrences per second
  per arcsec<SUP>2</SUP>.

---------------------------------------------------------
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.
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 &lt;
  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.
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.
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.
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.
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.
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.
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
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
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 × 10<SUP>20</SUP> Mx to 3.0 × 10<SUP>20</SUP>
  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<SUP>-1</SUP> are observed, comparable to the horizontal velocity (1.0
  km s<SUP>-1</SUP>) 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.
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 τ<SUB>5</SUB> = - 3 until log τ<SUB>5</SUB>simeq -
  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 τ<SUB>5</SUB> = 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.
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.
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.
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.
2008A&A...477..273C    Altcode: 2007arXiv0709.1601C
  Context: Evershed clouds (ECs) represent the most conspicuous
  variation of the Evershed flow in sunspot penumbrae. <BR />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. <BR />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). <BR />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. <BR />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. <P />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.
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.
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.
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.
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. <BR />Methods: We solve the static momentum equation
  in 2.5D to obtain the thermodynamics that are consistent with a
  prescribed velocity and magnetic fields. <BR />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.
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. <BR />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. <BR />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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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 tau<SUB>5</SUB> =
  0 and log tau<SUB>5</SUB> = -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<SUP>-1</SUP> 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 90<SUP>o</SUP>. 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.
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.
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.
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
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.
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.
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<SUP>-1</SUP>, whereas it
  is possible to account for temperature inhomogeneities by adopting an
  average temperature stratification which is cooler than the Holweger
  &amp; 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.
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.