Author name code: lopez-ariste
ADS astronomy entries on 2022-09-14
author:"Lopez Ariste, Arturo"
------------------------------------------------------------------------  

Title: Three-dimensional imaging of convective cells in the
    photosphere of Betelgeuse
Authors: López Ariste, A.; Georgiev, S.; Mathias, Ph.; Lèbre, A.;
   Wavasseur, M.; Josselin, E.; Konstantinova-Antova, R.; Roudier, Th.
Bibcode: 2022A&A...661A..91L
Altcode: 2022arXiv220212011L
  <BR /> Aims: Understanding convection in red supergiants and the
  mechanisms that trigger the mass loss from these evolved stars are
  the general goals of most observations of Betelgeuse and its inner
  circumstellar environment. <BR /> Methods: Linear spectropolarimetry
  of the atomic lines of the spectrum of Betelgeuse reveals information
  about the three-dimensional (3D) distribution of brightness in its
  atmosphere. We model the distribution of plasma and its velocities and
  use inversion algorithms to fit the observed linear polarization. <BR
  /> Results: We obtain the first 3D images of the photosphere of
  Betelgeuse. Within the limits of the used approximations, we recover
  vertical convective flows and measure the velocity of the rising plasma
  at different heights in the photosphere. In several cases, we find
  this velocity to be constant with height, indicating the presence of
  forces other than gravity acting on the plasma and counteracting it. In
  some cases, these forces are sufficient to maintain plasma rising at
  60 km s<SUP>−1</SUP> to heights where this velocity is comparable
  to the escape velocity. <BR /> Conclusions: Forces are present in the
  photosphere of Betelgeuse that allow plasma to reach velocities close
  to the escape velocity. These mechanisms may suffice to trigger mass
  loss and sustain the observed large stellar winds of these evolved
  stars. <P />Based on observations obtained at the Télescope Bernard
  Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and Université
  de Toulouse, France.

Title: Precision requirements for the POLLUX-LUVOIR spectropolarimeter
Authors: Muslimov, Eduard; Bouret, Jean-Claude; Neiner, Coralie;
   Ferrari, Marc; Lombardo, Simona; Lopez Ariste, Arturo; Le Gal, Maëlle
Bibcode: 2020SPIE11444E..6GM
Altcode:
  POLLUX is a project of high-resolution UV spectropolarimeter for
  the prospective LUVOIR space observatory. It will work in the range
  of 90-400 nm with R &gt;= 120000. The baseline design consists of 3
  channels representing echelle spectrographs with dedicated polarimeters,
  customized echelles and freeform-based concave holographic gratings. We
  study the requirements for manufacturing and assembly precision. We
  consider three performance criteria related to the spectral
  resolving power, calibration stability, and the gratings diffraction
  efficiency. We perform the tolerance analysis in corresponding modes
  using raytracing and numerical methods of diffraction modelling in a
  Monte-Carlo loop. We emphasize the most influential design parameters,
  which may require high precision of manufacturing and alignment and
  should be the subjects of further studies. We also briefly discuss
  possible compensators for maintenance of the imaging performance.

Title: Solar surveillance with CLIMSO: instrumentation, database
    and on-going developments
Authors: Pitout, Frédéric; Koechlin, Laurent; López Ariste, Arturo;
   Dettwiller, Luc; Glorian, Jean-Michel
Bibcode: 2020JSWSC..10...47P
Altcode:
  CLIMSO is a suite of solar telescopes installed at Pic du Midi
  observatory in the southwest of France. It consists of two refractors
  that image the full solar disk in Hα and CaII K, and two coronagraphs
  that capture the prominences and ejections of chromospheric matter
  in Hα and HeI. Synoptic observations are carried out since 2007
  and they follow those of previous instruments. CLIMSO, together
  with its predecessors, offer a temporal coverage of several solar
  cycles. With a direct access to its images, CLIMSO contributes
  to real time monitoring of the Sun. For that matter, the national
  research council for astrophysics (CNRS/INSU) has labelled CLIMSO as
  a national observation service for "surveillance of the Sun and the
  terrestrial space environment". Products, under the form of images,
  movies or data files, are available via the CLIMSO DataBase. In this
  paper, we present the current instrumental configuration; we detail
  the available products and show how to access them; we mention some
  possible applications for solar and space weather; and finally, we
  evoke developments underway, both numerical to valorise our data,
  and instrumental to offer more and better capabilities.

Title: Modelling and observations: Comparison of the magnetic field
    properties in a prominence
Authors: Mackay, D. H.; Schmieder, B.; López Ariste, A.; Su, Y.
Bibcode: 2020A&A...637A...3M
Altcode:
  Context. Direct magnetic field measurements in solar prominences occur
  infrequently and are difficult to make and interpret. As a consequence,
  alternative methods are needed to derive the main properties of the
  magnetic field that supports the prominence mass. This is important for
  our understanding of solar prominences, but also for understanding how
  eruptive prominences may affect space weather. <BR /> Aims: We present
  the first direct comparison of the magnetic field strength derived
  from spectro-polarimetric observations of a solar prominence, with
  corresponding results from a theoretical flux rope model constructed
  from on-disc normal component magnetograms. <BR /> Methods: We first
  used spectro-polarimetric observations of a prominence obtained with
  the magnetograph THEMIS operating in the Canary Islands to derive the
  magnetic field of the observed prominence by inverting the Stokes
  parameters measured in the He D3 line. Next, we constructed two
  data-constrained non-linear force-free field (NLFFF) models of the
  same prominence. In one model we assumed a strongly twisted flux rope
  solution, and in the other a weakly twisted flux rope solution. <BR />
  Results: The physical extent of the prominence at the limb (height
  and length) is best reproduced with the strongly twisted flux rope
  solution. The line-of-sight average of the magnetic field for the
  strongly twisted solution results in a magnetic field that has a
  magnitude of within a factor of 1-2 of the observed magnetic field
  strength. For the peak field strength along the line of sight,
  an agreement to within 20% of the observations is obtained for
  the strongly twisted solution. The weakly twisted solution produces
  significantly lower magnetic field strengths and gives a poor agreement
  with the observations. <BR /> Conclusions: The results of this first
  comparison are promising. We found that the flux rope insertion method
  of producing a NLFFF is able to deduce the overall properties of the
  magnetic field in an observed prominence.

Title: Asymmetric shocks in χ Cygni observed with linear
    spectropolarimetry
Authors: López Ariste, A.; Tessore, B.; Carlín, E. S.; Mathias, Ph.;
   Lèbre, A.; Morin, J.; Petit, P.; Aurière, M.; Gillet, D.; Herpin, F.
Bibcode: 2019A&A...632A..30L
Altcode: 2019arXiv190903720L
  <BR /> Aims: We derive information about the dynamics of the stellar
  photosphere, including pulsation, from a coherent interpretation of the
  linear polarisation detected in the spectral lines of the Mira star χ
  Cyg. <BR /> Methods: From spectropolarimetric observations of χ Cyg,
  we performed a careful analysis of the polarisation signals observed
  in atomic and molecular lines, both in absorption and emission, using
  radiative transfer in the context of polarisation produced through two
  mechanisms: intrinsic polarisation and continuum depolarisation. We also
  explain the observed line doubling phenomenon in terms of an expanding
  shell in spherical geometry, which allows us to pinpoint the coordinates
  over the stellar disc with enhanced polarisation. <BR /> Results: We
  find that the polarised spectrum of χ Cyg is dominated by intrinsic
  polarisation and has a negligible continuum depolarisation. The
  observed polarised signals can only be explained by assuming that
  this polarisation is locally enhanced by velocity fields. During the
  pulsation, radial velocities are not homogeneous over the disc. We map
  these regions of enhanced velocities. <BR /> Conclusions: We set an
  algorithm to distinguish the origin of this polarisation in any stellar
  spectra of linear polarisation and to find a way to increase the signal
  by coherently adding many lines with an appropriated weight. Applied
  to the Mira star χ Cyg, we reached the unexpected result that during
  the pulsation, velocities are radial but not homogeneous over the
  disc. The reason for these local velocity enhancements are probably
  related to the interplay between the atmospheric pulsation dynamics and
  the underlying stellar convection. <P />Based on observations obtained
  at the Télescope Bernard Lyot (TBL) at Observatoire du Pic du Midi,
  CNRS/INSU and Université de Toulouse, France.

Title: Solar survey at Pic du Midi: Calibrated data and improved
    images
Authors: Koechlin, Laurent; Dettwiller, Luc; Audejean, Maurice;
   Valais, Maël; López Ariste, Arturo
Bibcode: 2019A&A...631A..55K
Altcode: 2019arXiv190206980K
  Context. We carry out a solar survey with images of the photosphere,
  prominences, and corona at Pic du Midi observatory. This survey, named
  CLIMSO (for CLIchés Multiples du SOleil), is in the following spectral
  lines: Fe XIII corona (1.075 μm), Hα (656.3 nm), and He I (1.083 μm)
  prominences, and Hα and Ca II (393.4 nm) photosphere. All frames
  cover 1.3 times the diameter of the Sun with an angular resolution
  approaching one arcsecond. The frame rate is one per minute per channel
  (weather permitting) for the prominences and chromosphere, and one per
  hour for the Fe XIII corona. This survey started in 2007 for the disk
  and prominences and in 2015 for the corona. We have almost completed one
  solar cycle and hope to cover several more, keeping the same wavelengths
  or adding others. <BR /> Aims: We seek to make the CLIMSO images
  easier to use and more profitable for the scientific community. <BR />
  Methods: At the beginning of the survey, the images that we sent to the
  CLIMSO database were not calibrated. We have implemented a photometric
  calibration for the present and future images, in order to provide
  "science-ready" data. The old images have been calibrated. We have
  also improved the contrast capabilities of our coronagraphs, which
  now provide images of the Fe XIII corona, in addition to previous
  spectral channels. We also implemented an autoguiding system based on
  a diffractive Fresnel array for precise positioning of the Sun behind
  coronagraphic masks. <BR /> Results: The data, including the images
  and films, are publicly available and downloadable through virtual
  observatories and dedicated websites (use "CLIMSO" and "IRAP" keywords
  to find them). For the Hα and Ca II channels we calibrate the data into
  physical units, independent of atmospheric or instrumental conditions;
  we provide solar maps of spectral radiances in W m<SUP>-2</SUP>
  sr<SUP>-1</SUP> nm<SUP>-1</SUP>. The instrumental improvements and
  calibration process are presented in this paper.

Title: Atmospheric circulation of Venus measured with visible imaging
    spectroscopy at the THEMIS observatory
Authors: Gaulme, Patrick; Schmider, François-Xavier; Widemann,
   Thomas; Gonçalves, Ivan; López Ariste, Arturo; Gelly, Bernard
Bibcode: 2019A&A...627A..82G
Altcode: 2019arXiv190511078G
  Measuring the atmospheric circulation of Venus at different altitudes
  is important for understanding its complex dynamics, in particular the
  mechanisms driving super-rotation. Observationally, Doppler imaging
  spectroscopy is in principle the most reliable way to measure wind
  speeds of planetary atmospheres because it directly provides the
  projected speed of atmospheric particles. However, high-resolution
  imaging spectroscopy is challenging, especially in the visible domain,
  and most knowledge about atmospheric dynamics has been obtained with
  the cloud tracking technique. The objective of the present work is to
  measure the global properties of the atmospheric dynamics of Venus at
  the altitude of the uppermost clouds, which is probed by reflected solar
  lines in the visible domain. Our results are based on high-resolution
  spectroscopic observations with the long-slit spectrometer of the solar
  telescope THEMIS. We present the first instantaneous "radial-velocity
  snapshot" of any planet of the solar system in the visible domain, i.e.,
  a complete radial-velocity map of the planet obtained by stacking data
  on less than 10% of its rotation period. From this, we measured the
  properties of the zonal and meridional winds, which we unambiguously
  detect. We identify a wind circulation pattern that significantly
  differs from previous knowledge about Venus. The zonal wind reveals a
  "hot spot" structure, featuring about 200 m s<SUP>-1</SUP> at sunrise
  and 70 m s<SUP>-1</SUP> at noon in the equatorial region. Regarding
  meridional winds, we detect an equator-to-pole meridional flow
  peaking at 45 m s<SUP>-1</SUP> at mid-latitudes, i.e., about
  twice as large as what has been reported so far. <P />Tables
  A.1-A.3 are only available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A82">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A82</A>

Title: VUV test of a new polarimeter for spectropolarimetric
    measurements on board space missions
Authors: Le Gal, Maelle; Pertenais, Martin; Lopez Ariste, Arturo;
   Neiner, Coralie; Champion, Norbert; Younes, Youssef; Reess, Jean-Michel
Bibcode: 2019arXiv190711549L
Altcode:
  High-resolution spectropolarimetry is a useful astronomical technique,
  in particular to study stellar magnetic fields. It has been extensively
  used in the past but mostly in the visible range. Space missions
  equipped with high-resolution spectropolarimeters working in the
  ultra-violet (UV) are now being studied. We propose a concept of a
  polarimeter working with temporal modulation and allowing to perform
  Stokes IQUV measurements over the full UV + Visible range. The purpose
  of this article is to describe the polarimeter concept, two prototypes
  and the bench developed to perform on ground testing to establish the
  performances of this new polarimeter.

Title: Far ultra-violet polarimeter by reflection for Pollux (LUVOIR)
Authors: Le Gal, Maëlle; López Ariste, Arturo; Neiner, Coralie
Bibcode: 2019SPIE11180E..4VL
Altcode: 2019arXiv190712281L
  The ultra-violet (UV) high-resolution spectropolarimeter Pollux is
  being studied in Europe under CNES leadership for the LUVOIR space
  mission. LUVOIR is a projected 15-m telescope equipped with a suite of
  instruments proposed to NASA. Pollux will perform spectropolarimetric
  measurements from 90 to 400 nm with a resolution of 120000. The
  spectrograph will be divided in three channels, each with its own
  polarimeter: far UV (FUV, 90-124.5 nm), mid UV (MUV, 118.5-195 nm),
  and near UV (NUV, 190-390 nm). We present here our FUV prototype and
  our investigation to optimize this polarimeter (angle, materials,
  coating…).

Title: VizieR Online Data Catalog: Venus photometric flux and los
    velocities (Gaulme+, 2019)
Authors: Gaulme, P.; Schmider, F. -X.; Widemann, T.; Goncalves, I.;
   Lopez Ariste, A.; Gelly, B.
Bibcode: 2019yCat..36270082G
Altcode:
  Two-dimension maps of Venus from data taken on September 14th, 16th and
  17th, 2009. <P />Velocity data are provided for the data points that
  are reliable, i.e., for all the point visible on Fig. 13 (middle panel),
  whereas photometry is provided for out-of-mask regions, in case a user
  would like to fit the complete photometric profile. <P />(3 data files).

Title: Detection of the Linearly Polarised Spectrum of the Red
    Supergiant Star α Ori
Authors: Tessore, B.; Tessore, B.; López Ariste, A.; Mathias, P.;
   Josselin, E.; Lèbre, A.; Morin, J.; Josselin, E.
Bibcode: 2019ASPC..526..249T
Altcode:
  At the solar limb, the observed linear polarisation is due to the
  anisotropy of the radiation field induced by limb darkening. It is
  maximal when it is seen parallel to the limb and it vanishes when it
  is integrated over the spherically-symmetric solar disk. Therefore for
  distant stars, that present spherical symmetry, linear polarisation
  signatures are very difficult to observe. However, strong linear
  polarisation features have been reported in the prototypical red
  supergiant star α Ori (Betelgeuse). We propose to explain them with
  an analytical model.

Title: POLLUX, an innovative instrument providing a unique UV
    spectropolarimetric capability to LUVOIR
Authors: Ferrari, Marc; Bouret, Jean-Claude; Neiner, Coralie; Muslimov,
   Eduard; Le Gal, Maelle; Lopez Ariste, Arturo
Bibcode: 2019AAS...23314809F
Altcode:
  The Large Ultraviolet/Optical/Infrared Surveyor (LUVOIR) is one of
  four large mission concept studies led by NASA for the 2020 Decadal
  Survey. Under the leadership of French Institutes and French Space
  Agency, European institutes have come together to propose an instrument,
  POLLUX, that would be onboard the 15-meter primary mirror option
  of LUVOIR. POLLUX will operate over a broad spectral range (90 to
  400 nm), at high spectral resolution (R &gt;=120,000), with a unique
  spectropolarimetric capability. It is designed to address a range of
  questions at the core of the LUVOIR Science portfolio. The working range
  is split into 3 channels: Far (90-125 nm), Medium (119-200 nm), and Near
  (200-400 nm) ultraviolet. MUV and NUV channel, separated by a dichroic
  splitter, can be recorded simultaneously. The FUV channel is recorded
  separately (temporal separation), using a dedicated flip-mirror. The
  coatings on the optical elements of POLLUX are optimized for each
  channel, to maximize the throughput. Each channel will include an
  optimized echelle spectrograph integrating advanced technologies,
  i.e. high groove densities, holographic recording on a freeform surface
  for the cross-dispersors, etc. The polarimeters design were optimized
  for each channel accounting for the technological feasibility. They
  are retractable in the MUV and NUV to allow the pure spectroscopic
  mode. The FUV modulator is retractable while the analyzer is kept in
  the optical path to direct the beam towards the collimator. Detectors
  will be delta-doped EMCCDs, combining the linearity of CCDs with
  photon-counting ability, which is a key capability enabling detection
  of faint UV signals. Furthermore, these detectors deliver high
  quantum efficiency thus offering the possibility to reach very high
  signal-to-noise ratios. CMOS are also considered as a viable option
  in the development time-frame of POLLUX. The complete study will be
  included as a dedicated POLLUX chapter in the document presenting
  the final study of LUVOIR to the NASA decadal 2020 committee. In this
  poster, we present the instrument concept, as well as the challenges
  offered by the development of POLLUX.

Title: Characterizing the photospheric convection of red supergiant
    stars at high angular resolution
Authors: Montargès, M.; Norris, R.; Tessore, B.; López Ariste, A.;
   Chiavassa, A.; Lèbre, A.
Bibcode: 2018sf2a.conf..333M
Altcode:
  Over the past few years, our knowledge of red supergiant stars
  has changed dramatically thanks to the development of high angular
  resolution techniques (interferometry in both the optical and mm
  domains, adaptive optics) and of numerical modeling. We present here our
  last results on the observation of the photosphere of red supergiants
  using near infrared interferometry and visible spectropolarimetry.

Title: POLLUX, a high-resolution UV spectropolarimeter for LUVOIR
Authors: Bouret, J. -C.; Muslimov, E.; Neiner, C.; Le Gal, M.; Lopez
   Ariste, A.; Ferrari, M.
Bibcode: 2018sf2a.conf...61B
Altcode:
  POLLUX is a high-resolution, UV spectropolarimeter proposed for the
  15-meter primary mirror option of LUVOIR. The instrument Phase 0 study
  is supported by the French Space Agency (CNES) and performed by a
  consortium of European scientists. POLLUX has been designed to deliver
  high-resolution spectroscopy (R ≥ 120,000) over a broad spectral
  range (90-400 nm). Its unique spectropolarimetric capabilities will
  open-up a vast new parameter space, in particular in the unexplored UV
  domain and in a regime where high-resolution observations with current
  facilities in the visible domain are severely photon starved. In this
  paper, we introduce the general context of LUVOIR, the design of POLLUX,
  and the required technology development needed to achieve the desired
  performances of the instrument.

Title: Convective cells in Betelgeuse: imaging through
    spectropolarimetry
Authors: López Ariste, A.; Mathias, P.; Tessore, B.; Lèbre, A.;
   Aurière, M.; Petit, P.; Ikhenache, N.; Josselin, E.; Morin, J.;
   Montargès, M.
Bibcode: 2018A&A...620A.199L
Altcode: 2018arXiv181110362L
  <BR /> Aims: We assess the ability to image the photosphere of red
  supergiants and, in particular Betelgeuse, through the modelling
  of the observed linear polarization in atomic spectral lines. We
  also aim to analyse the resulting images over time, to measure the
  size and dynamics of the convective structures in these stars. <BR />
  Methods: Rayleigh scattering polarizes the continuum and spectral lines
  depolarize it. This depolarization is seen as a linear polarization
  signal parallel to the radial direction on the stellar disk. Integrated
  over the disk, it would result in a null signal, except if brightness
  asymmetries/inhomogeneities are present. This is the basic concept
  behind our imaging technique. Through several tests and comparisons, we
  have tried to assess and extend its validity, and to determine what can
  be learnt unambiguously through it. <BR /> Results: The several tests
  and comparisons performed prove that our technique reliably retrieves
  the salient brightness structures in the photosphere of Betelgeuse,
  and should be relevant to other red supergiants. For Betelgeuse,
  we demonstrate that these structures we infer are convective cells,
  with a characteristic size of more than 60% of the stellar radius. We
  also derive the characteristic upflow and downflow speeds, 22 and 10 km
  s<SUP>-1</SUP>, respectively. We find weak magnetic fields concentrated
  in the downflow lanes in between granules, similar to the quiet sun
  magnetism. We follow those convective structures in time. Changes
  happen on timescales of 1 week, but individual structures can be
  tracked over 4 yr of observations. <BR /> Conclusions: The measured
  characteristics of the convection in Betelgeuse confirm the predictions
  of numerical simulations in both the strong, supersonic upflows and
  the size of the convective cells. They also concur in the presence of
  weak magnetic fields that are completely dominated by the convective
  flows and constrained to the dark lanes of down-flowing plasma. <P
  />Based on observations obtained at the Télescope Bernard Lyot (TBL)
  at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse,
  France.

Title: UV Polarimeters for Pollux onboard LUVOIR
Authors: Le Gal, M.; López Ariste, A.; Pertenais, M.; Neiner, C.
Bibcode: 2018sf2a.conf...65L
Altcode:
  Pollux, the European high-resolution spectropolarimeter designed
  for LUVOIR, will work from 90 to 400 nm. In order to optimize its
  efficiency, the range is divided in 3 channels: far ultra-violet (FUV)
  from 90 to 124.5 nm, mid-UV (MUV) from 118.5 to 195 nm and near-UV
  (NUV) from 190 to 400 nm. Optical materials' properties being different
  between channels, each one will benefit from its own polarimeter
  adapted to its wavelength range. All polarimeters will use temporal
  modulation and will be composed by a modulator and an analyzer. The
  NUV polarimeter is similar to the one often used in in visible range:
  it uses waveplates and a polarizer and works thus in transmission. The
  FUV polarimeter has to be innovative because no birefringent material
  transmits light at these wavelengths. It will use mirrors and work by
  reflexion. The MUV polarimeter will benefit from the design of the two
  others so that it will be the most efficient possible. This proceeding
  presents these three polarimeters designed for Pollux.

Title: Venus' winds measured with visible imaging-spectroscopy at
    the THEMIS observatory
Authors: Gaulme, Patrick; Schmider, François Xavier; Widemann,
   Thomas; Gonçalves, Ivan; Lopez Ariste, Arturo; Gelly, Bernard
Bibcode: 2018EPSC...12..275G
Altcode:
  The objective of the present work is to measure the global properties
  of Venus' atmospheric dynamics, through the obtention of a complete
  radial-velocity map of Venus at the altitude of the uppermost
  cloud layer. Our results are based on high-resolution spectroscopic
  observations of Venus performed in the visible domain with the long
  slit spectrometer of the solar telescope THEMIS (Spain). We present
  the first instantaneous ``radial-velocity snapshot'' of any planet
  of the solar system in the visible domain, i.e., a complete RV map
  of the planet obtained by stacking data on less than 10 % of its
  rotation period. From this, we measure the properties of the zonal
  and meridional winds, which we unambiguously detect. We identify
  a wind circulation pattern that significantly differs from what we
  know about Venus. The zonal wind displays a ``hot spot'' structure,
  featuring about 200 m s<SUP>-1</SUP> at sunrise and 70 m s<SUP>-1</SUP>
  at noon in the equatorial region. Regarding meridional winds, we detect
  an equator-to-pole meridional flow peaking at 45 m s<SUP>-1</SUP>
  at mid latitudes, i.e., which is about twice as large as what was
  reported so far.

Title: VUV test of a new polarimeter for spectropolarimetric
    measurements on board space missions
Authors: Le Gal, Maëlle; Pertenais, Martin; López Ariste, Arturo;
   Neiner, Coralie; Champion, Norbert; Younes, Youssef; Reess, Jean-Michel
Bibcode: 2018SPIE10706E..1ML
Altcode:
  High-resolution spectropolarimetry is a useful astronomical technique,
  in particular to study stellar magnetic fields. It has been extensively
  used in the past but mostly in the visible range. Space missions
  equipped with high-resolution spectropolarimeters working in the
  ultra-violet (UV) are now being studied. We propose a concept of a
  polarimeter working with temporal modulation and allowing to perform
  Stokes IQUV measurements over the full UV + Visible range. The purpose
  of this article is to describe the polarimeter concept, two prototypes
  and the bench developed to perform on ground testing to establish the
  performances of this new polarimeter.

Title: Superoscillations in solar MHD waves and their possible role
    in heating coronal loops
Authors: López Ariste, A.; Facchin, M.
Bibcode: 2018A&A...614A.145L
Altcode: 2018arXiv180108330L
  <BR /> Aims: We aim to study the presence of superoscillations in
  coronal magnetoacoustic (MHD) waves and their possible role in heating
  coronal loops through the strong and localised gradients that they
  generate on the wave. <BR /> Methods: An analytic model is built
  for the transition between sausage and kink wave modes propagating
  along field lines in the corona. We compute in this model the local
  frequencies, the wave gradients, and the associated heating rates due
  to compressive viscosity. <BR /> Results: We find superoscillations
  associated with the transition between wave modes accompanying the wave
  dislocation that shifts through the wave domain. Frequencies ten times
  higher than the normal frequency are found. This means that a typical
  three-minute coronal wave will oscillate locally in 10 to 20 s. Such
  high frequencies bring up strong gradients that efficiently dissipate
  the wave through compressive viscosity. We compute the associated
  heating rates; locally, they are very strong, largely compensating
  typical radiative losses. <BR /> Conclusions: We find a new heating
  mechanism associated to magnetoacoustic waves in the corona. Heating
  due to superoscillations only happens along particular field lines with
  small cross sections, comparable in size to coronal loops, inside the
  much larger magnetic flux tubes and wave propagation domain.

Title: Prominence/Tornado plasma parameters
Authors: Schmieder, Brigitte; Mein, Pierre; Zapior, Maciej; Labrosse,
   Nicolas; Lopez Ariste, Arturo
Bibcode: 2018cosp...42E3025S
Altcode:
  We present a comparison of the plasma physical parameters in prominences
  and tornadoes using IRIS data and ground based polarimetry measurements
  obtained with THEMIS. Mg II lines give a good diagnostics of the
  temperature and optical thickness of the structures. The Stokes
  parameters from the He D3 line allow to distinguish the behaviour
  of the magnetic field in typical prominences and atypical prominences
  (e.g. bubbles, eruptive prominence). We concentrate on the Dopplershifts
  in a tornado observed in transition region lines and in Halpha. Our
  results support the existence of oscillations in tornadoes but not
  rotation.A reconstruction of the 3D geometry of a helical prominence
  obtained by following the trajectory of kernels yields surprising
  results. The loops are shown to be quasi-horizontal structures with
  no curvature.We conclude that it is important to take into account the
  3D structure of the prominence to study the dynamics of the prominence
  plasma.

Title: Evolution of the magnetic field of Betelgeuse from 2009-2017
Authors: Mathias, P.; Aurière, M.; López Ariste, A.; Petit, P.;
   Tessore, B.; Josselin, E.; Lèbre, A.; Morin, J.; Wade, G.; Herpin,
   F.; Chiavassa, A.; Montargès, M.; Konstantinova-Antova, R.; Kervella,
   P.; Perrin, G.; Donati, J. -F.; Grunhut, J.
Bibcode: 2018A&A...615A.116M
Altcode: 2018arXiv180401831M
  Context. Betelgeuse is an M-type supergiant that presents a circularly
  polarized (Stokes V) signal in its line profiles, interpreted in
  terms of a surface magnetic field. <BR /> Aims: The weak circular
  polarization signal has been monitored over 7.5 years in order to
  follow its evolution on different timescales, and eventually to
  determine its physical origin. Linear polarization measurements have
  also been obtained regularly in the last few years. <BR /> Methods:
  We used both the ESPaDOnS and Narval spectropolarimeters to obtain
  high signal-to-noise ratio spectra, which were processed by means
  of the least-squares deconvolution method. In order to ensure the
  reality of the very weak circular polarization, special care has been
  taken to limit instrumental effects. In addition, several tests were
  performed on the Stokes V signal to establish its stellar and Zeeman
  origin. <BR /> Results: We confirm the magnetic nature of the circular
  polarization, pointing to a surface magnetic field of the order of 1
  G. The Stokes V profiles present variations over different timescales,
  the most prominent one being close to the long secondary period (LSP;
  around 2000 d for Betelgeuse) often invoked in red evolved stars. This
  long period is also dominant for all the other Stokes parameters. The
  circular polarization is tentatively modeled by means of magnetic
  field concentrations mimicking spots, showing in particular that the
  velocity associated with each "spot" also follows the long timescale,
  and that this signal is nearly always slightly redshifted. <BR />
  Conclusions: From the coupled variations of both linear and circular
  polarization signatures in amplitude, velocity and timescale, we favour
  giant convection cells as the main engine at the origin of polarization
  signatures and variations in all the Stokes parameters. This strengthens
  support for the hypothesis that large convective cells are at the origin
  of the LSP. <P />Based on observations obtained at the Télescope
  Bernard Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and
  Université de Toulouse, France, and at the Canada-France-Hawaii
  Telescope (CFHT) which is operated by the National Research Council
  of Canada, CNRS/INSU and the University of Hawaii.

Title: POLLUX: a UV spectropolarimeter for the LUVOIR space telescope
    project
Authors: Muslimov, Eduard; Bouret, Jean-Claude; Neiner, Coralie;
   López Ariste, Arturo; Ferrari, Marc; Vivès, Sébastien; Hugot,
   Emmanuel; Grange, Robert; Lombardo, Simona; Lopes, Louise; Costeraste,
   Josiane; Brachet, Frank
Bibcode: 2018SPIE10699E..06M
Altcode: 2018arXiv180509067M
  The present paper describes the current baseline optical design
  of POLLUX, a high-resolution spectropolarimeter for the future
  LUVOIR mission. The instrument will operate in the ultraviolet
  (UV) domain from 90 to 390 nm in both spectropolarimetric and pure
  spectroscopic modes. The working range is split between 3 channels -
  far (90-124.5 nm), medium (118.5-195 nm) and near (195-390 nm) UV. Each
  of the channels is composed of a polarimeter followed by an echelle
  spectrograph consisting of a classical off-axis paraboloid collimator,
  echelle grating with a high grooves frequency and a cross-disperser
  grating operating also as a camera. The latter component integrates
  some advanced technologies: it is a blazed grating with a complex
  grooves pattern formed by holographic recording, which is manufactured
  on a freeform surface. One of the key features underlying the current
  design is the large spectral length of each order 6 nm, which allows to
  record wide spectral lines without any discontinuities. The modelling
  results show that the optical design will provide the required spectral
  resolving power higher than R 120,000 over the entire working range
  for a point source object with angular size of 30 mas. It is also shown
  that with the 15-m primary mirror of the LUVOIR telescope the instrument
  will provide an effective collecting area up to 38 569 cm<SUP>2</SUP>
  . Such a performance will allow to perform a number of groundbreaking
  scientific observations. Finally, the future work and the technological
  risks of the design are discussed in details.

Title: POLLUX: A UV High-Resolution Spectropolatimeter for LUVOIR
Authors: Bouret, Jean-Claude; Neiner, Coralie; Lopez Ariste, Arturo;
   Vivès, Sébastien; Muslimov, Eduard; Lopes, Louise; Costeraste,
   Josiane; Brachet, Frank; POLLUX Consortium
Bibcode: 2018AAS...23141901B
Altcode:
  The Large Ultraviolet/Optical/Infrared Surveyor (LUVOIR) is one of
  four large mission concept studies led by NASA for the 2020 Decadal
  Survey. A versatile suite of instruments is envisioned for LUVOIR,
  to advance our understanding of the origin and evolution of galaxies,
  stars and planets that make up our Universe, and the life within it. We
  present POLLUX, a high-resolution spectropolarimeter, operating at
  UV wavelengths, designed for the 15-meter primary mirror option of
  LUVOIR. POLLUX study is supported by the French Space Agency (CNES)
  and developed by a European consortium of scientists.POLLUX will
  operate over a broad spectral range (98 to 390 nm), at high spectral
  resolution (R = 120,000). This will permit to resolve narrow UV
  emission and absorption lines, thus to follow the baryon cycle over
  cosmic time, from galaxies forming stars out of interstellar gas and
  grains, and stars forming planets, to the various forms of feedback
  into the interstellar and intergalactic medium (ISM and IGM), and
  active galactic nuclei (AGN).The most innovative characteristic
  of POLLUX is its unique spectropolarimetric capability, that will
  enable detection of the polarized light reflected from Earth-like
  exoplanets or from their circumplanetary material, and moons, and
  characterization of the magnetospheres of stars and planets, and their
  interactions. The magnetospheric properties of planets in the solar
  system will be accessible to exquisite level of details, while the
  influence of magnetic fields at the galactic scale and in the IGM will
  be measured. UV circular and linear polarisation will provide a full
  picture of magnetic field properties and impact for a variety of media
  and objects, from AGN outflows to all types of stars. It will probe
  the physics of accretion disks around young stars and white dwarfs,
  or supermassive black holes in AGNs, and constrain the properties,
  especially sphericity, of stellar ejecta and explosions. Since the
  parameter space opened by POLLUX is essentially uncharted territory,
  its potential for ground-breaking discoveries is high.We introduce
  the science case and the instrument concept, as well as the challenges
  offered by the development of this instrument.

Title: Reconstruction of a helical prominence in 3D from IRIS spectra
    and images
Authors: Schmieder, B.; Zapiór, M.; López Ariste, A.; Levens, P.;
   Labrosse, N.; Gravet, R.
Bibcode: 2017A&A...606A..30S
Altcode: 2017arXiv170608078S
  Context. Movies of prominences obtained by space instruments e.g. the
  Solar Optical Telescope (SOT) aboard the Hinode satellite and the
  Interface Region Imaging Spectrograph (IRIS) with high temporal
  and spatial resolution revealed the tremendous dynamical nature
  of prominences. Knots of plasma belonging to prominences appear
  to travel along both vertical and horizontal thread-like loops,
  with highly dynamical nature. <BR /> Aims: The aim of the paper
  is to reconstruct the 3D shape of a helical prominence observed
  over two and a half hours by IRIS. <BR /> Methods: From the IRIS
  Mg II k spectra we compute Doppler shifts of the plasma inside
  the prominence and from the slit-jaw images (SJI) we derive the
  transverse field in the plane of the sky. Finally we obtain the
  velocity vector field of the knots in 3D. Results.We reconstruct the
  real trajectories of nine knots travelling along ellipses. <BR />
  Conclusions: The spiral-like structure of the prominence observed
  in the plane of the sky is mainly due to the projection effect of
  long arches of threads (up to 8 × 10<SUP>4</SUP> km). Knots run
  along more or less horizontal threads with velocities reaching 65 km
  s<SUP>-1</SUP>. The dominant driving force is the gas pressure. <P
  />Movies associated to Figs. 1, 9, 10, and 13 are available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201730839/olm">http://www.aanda.org</A>

Title: Comparing UV/EUV line parameters and magnetic field in a
    quiescent prominence with tornadoes
Authors: Levens, P. J.; Labrosse, N.; Schmieder, B.; López Ariste,
   A.; Fletcher, L.
Bibcode: 2017A&A...607A..16L
Altcode: 2017arXiv170804606L
  Context. Understanding the relationship between plasma and the
  magnetic field is important for describing and explaining the
  observed dynamics of solar prominences. <BR /> Aims: We determine
  if a close relationship can be found between plasma and magnetic
  field parameters, measured at high resolution in a well-observed
  prominence. <BR /> Methods: A prominence observed on 15 July 2014 by
  the Interface Region Imaging Spectrograph (IRIS), Hinode, the Solar
  Dynamics Observatory (SDO), and the Télescope Héliographique pour
  l'Étude du Magnétisme et des Instabilités Solaires (THEMIS) is
  selected. We perform a robust co-alignment of data sets using a 2D
  cross-correlation technique. Magnetic field parameters are derived
  from spectropolarimetric measurements of the He I D<SUB>3</SUB> line
  from THEMIS. Line ratios and line-of-sight velocities from the Mg II h
  and k lines observed by IRIS are compared with magnetic field strength,
  inclination, and azimuth. Electron densities are calculated using Fe xii
  line ratios from the Hinode Extreme-ultraviolet Imaging Spectrometer,
  which are compared to THEMIS and IRIS data. <BR /> Results: We find
  Mg II k/h ratios of around 1.4 everywhere, similar to values found
  previously in prominences. Also, the magnetic field is strongest (
  30 G) and predominantly horizontal in the tornado-like legs of the
  prominence. The k<SUB>3</SUB> Doppler shift is found to be between
  ±10 km s<SUP>-1</SUP> everywhere. Electron densities at a temperature
  of 1.5 × 10<SUP>6</SUP> K are found to be around 10<SUP>9</SUP>
  cm<SUP>-3</SUP>. No significant correlations are found between the
  magnetic field parameters and any of the other plasma parameters
  inferred from spectroscopy, which may be explained by the large
  differences in the temperatures of the lines used in this study. <BR />
  Conclusions: This is the first time that a detailed statistical study of
  plasma and magnetic field parameters has been performed at high spatial
  resolution in a prominence. Our results provide important constraints
  on future models of the plasma and magnetic field in these structures.

Title: Prominence and tornado dynamics observed with IRIS and THEMIS
Authors: Schmieder, Brigitte; Levens, Peter; Labrosse, Nicolas; Mein,
   Pierre; Lopez Ariste, Arturo; Zapior, Maciek
Bibcode: 2017SPD....4820104S
Altcode:
  Several prominences were observed during campaigns in September 2013 and
  July 2014 with the IRIS spectrometer and the vector magnetograph THEMIS
  (Tenerife). SDO/AIA and IRIS provided images and spectra of prominences
  and tornadoes corresponding to different physical conditions of the
  transition region between the cool plasma and the corona. The vector
  magnetic field was derived from THEMIS observations by using the He
  D3 depolarisation due to the magnetic field. The inversion code (PCA)
  takes into account the Hanle and Zeeman effects and allows us to compute
  the strength and the inclination of the magnetic field which is shown
  to be mostly horizontal in prominences as well as in tornadoes. Movies
  from SDO/AIA in 304 A and Hinode/SOT in Ca II show the highly dynamic
  nature of the fine structures. From spectra in Mg II and Si IV lines
  provided by IRIS and H-alpha observed by the Multi-channel Subtractive
  Double Pass (MSDP) spectrograph in the Meudon Solar Tower we derived
  the Doppler shifts of the fine structures and reconstructed the 3D
  structure of tornadoes. We conclude that the apparent rotation of AIA
  tornadoes is due to large-scale quasi-periodic oscillations of the
  plasma along more or less horizontal magnetic structures.

Title: Spectral Characteristics of the He I D<SUB>3</SUB> Line in
    a Quiescent Prominence Observed by THEMIS
Authors: Koza, Július; Rybák, Ján; Gömöry, Peter; Kozák, Matúš;
   López Ariste, Arturo
Bibcode: 2017SoPh..292...98K
Altcode: 2017arXiv171209255K
  We analyze the observations of a quiescent prominence acquired by
  the Téléscope Heliographique pour l'Étude du Magnetisme et des
  Instabilités Solaires (THEMIS) in the He I 5876 Å (He I D<SUB>3</SUB>)
  multiplet aiming to measure the spectral characteristics of the He
  I D<SUB>3</SUB> profiles and to find for them an adequate fitting
  model. The component characteristics of the He I D<SUB>3</SUB> Stokes I
  profiles are measured by the fitting system by approximating them with
  a double Gaussian. This model yields an He I D<SUB>3</SUB> component
  peak intensity ratio of 5.5 ±0.4 , which differs from the value of
  8 expected in the optically thin limit. Most of the measured Doppler
  velocities lie in the interval ± 5 km s<SUP>−1</SUP>, with a standard
  deviation of ± 1.7 km s<SUP>−1</SUP> around the peak value of 0.4
  km s<SUP>−1</SUP>. The wide distribution of the full-width at half
  maximum has two maxima at 0.25 Å and 0.30 Å for the He I D<SUB>3</SUB>
  blue component and two maxima at 0.22 Å and 0.31 Å for the red
  component. The width ratio of the components is 1.04 ±0.18 . We show
  that the double-Gaussian model systematically underestimates the blue
  wing intensities. To solve this problem, we invoke a two-temperature
  multi-Gaussian model, consisting of two double-Gaussians, which
  provides a better representation of He I D<SUB>3</SUB> that is free
  of the wing intensity deficit. This model suggests temperatures of
  11.5 kK and 91 kK, respectively, for the cool and the hot component
  of the target prominence. The cool and hot components of a typical He
  I D<SUB>3</SUB> profile have component peak intensity ratios of 6.6
  and 8, implying a prominence geometrical width of 17 Mm and an optical
  thickness of 0.3 for the cool component, while the optical thickness of
  the hot component is negligible. These prominence parameters seem to
  be realistic, suggesting the physical adequacy of the multi-Gaussian
  model with important implications for interpreting He I D<SUB>3</SUB>
  spectropolarimetry by current inversion codes.

Title: Detection of the linearly polarised spectrum of the red
    supergiant star alpha Ori
Authors: Tessore, Benjamin; Lòpez-Ariste, Arturo; Mathias, Philippe;
   Lèbre, Agnès; Morin, Julien; Josselin, Eric
Bibcode: 2017arXiv170202002T
Altcode:
  In the solar limb, linear polarisation is due to anisotropy of the
  radiation field induced by limb darkening. It is maximal when it is
  seen parallel to the limb and it vanishes when it is integrated over
  the spherically-symmetric solar disk. Therefore for distant stars,
  that present spherical symmetry, linear polarisation signatures are
  very difficult to observe. However strong linear polarisation features
  have been reported in the prototypical red supergiant star alpha Ori
  (Betelgeuse). With an analytical model we propose to explain them.

Title: Space-weather assets developed by the French space-physics
    community
Authors: Rouillard, A. P.; Pinto, R. F.; Brun, A. S.; Briand, C.;
   Bourdarie, S.; Dudok De Wit, T.; Amari, T.; Blelly, P. -L.; Buchlin,
   E.; Chambodut, A.; Claret, A.; Corbard, T.; Génot, V.; Guennou, C.;
   Klein, K. L.; Koechlin, L.; Lavarra, M.; Lavraud, B.; Leblanc, F.;
   Lemorton, J.; Lilensten, J.; Lopez-Ariste, A.; Marchaudon, A.; Masson,
   S.; Pariat, E.; Reville, V.; Turc, L.; Vilmer, N.; Zucarello, F. P.
Bibcode: 2016sf2a.conf..297R
Altcode:
  We present a short review of space-weather tools and services developed
  and maintained by the French space-physics community. They include
  unique data from ground-based observatories, advanced numerical
  models, automated identification and tracking tools, a range of space
  instrumentation and interconnected virtual observatories. The aim of
  the article is to highlight some advances achieved in this field of
  research at the national level over the last decade and how certain
  assets could be combined to produce better space-weather tools
  exploitable by space-weather centres and customers worldwide. This
  review illustrates the wide range of expertise developed nationally
  but is not a systematic review of all assets developed in France.

Title: Magnetic Field in Atypical Prominence Structures: Bubble,
    Tornado, and Eruption
Authors: Levens, P. J.; Schmieder, B.; López Ariste, A.; Labrosse,
   N.; Dalmasse, K.; Gelly, B.
Bibcode: 2016ApJ...826..164L
Altcode: 2016arXiv160505964L
  Spectropolarimetric observations of prominences have been obtained with
  the THEMIS telescope during four years of coordinated campaigns. Our aim
  is now to understand the conditions of the cool plasma and magnetism
  in “atypical” prominences, namely when the measured inclination
  of the magnetic field departs, to some extent, from the predominantly
  horizontal field found in “typical” prominences. What is the role
  of the magnetic field in these prominence types? Are plasma dynamics
  more important in these cases than the magnetic support? We focus our
  study on three types of “atypical” prominences (tornadoes, bubbles,
  and jet-like prominence eruptions) that have all been observed by THEMIS
  in the He I D<SUB>3</SUB> line, from which the Stokes parameters can
  be derived. The magnetic field strength, inclination, and azimuth in
  each pixel are obtained by using the inversion method of principal
  component analysis on a model of single scattering in the presence of
  the Hanle effect. The magnetic field in tornadoes is found to be more
  or less horizontal, whereas for the eruptive prominence it is mostly
  vertical. We estimate a tendency toward higher values of magnetic
  field strength inside the bubbles than outside in the surrounding
  prominence. In all of the models in our database, only one magnetic
  field orientation is considered for each pixel. While sufficient
  for most of the main prominence body, this assumption appears to be
  oversimplified in atypical prominence structures. We should consider
  these observations as the result of superposition of multiple magnetic
  fields, possibly even with a turbulent field component.

Title: Daytime sky polarization calibration limitations
Authors: Harrington, David M.; Kuhn, Jeffrey R.; López Ariste, Arturo
Bibcode: 2016SPIE.9912E..6SH
Altcode:
  The daytime sky has been recently demonstrated as a useful
  calibration tool for deriving polarization cross-talk properties of
  large astronomical telescopes. The Daniel K Inouye Solar Telescope
  (DKIST) and other large telescopes under construction can benefit from
  precise polarimetric calibration of large off-axis mirrors. Several
  atmospheric phenomena and instrumental errors potentially limit the
  techniques accuracy. At the 3.67m AEOS telescope on Haleakala, we have
  performed a large observing campaign with the HiVIS spectropolarimeter
  to identify limitations and develop algorithms for extracting consistent
  calibrations. Effective sampling of the telescope optical configurations
  and filtering of data for several derived parameters provide robustness
  to the derivedMueller matrix calibrations. Second-order scattering
  models of the sky show that this method is relatively insensitive to
  assumptions about telescope induced polarization provided the mirror
  coatings are highly reflective. Zemax-derived polarization models show
  agreement between predictions and on-sky calibrations.

Title: New life for the THEMIS solar telescope
Authors: Gelly, Bernard; Langlois, Maud; Moretto, Gil; Douet, Richard;
   Lopez Ariste, Arturo; Tallon, Michel; Thiébaut, Eric; Geyskens,
   Nicolas; Lorgeoux, Guillaume; Léger, Johnathan; Le Men, Claude
Bibcode: 2016SPIE.9906E..5AG
Altcode:
  The THEMIS solar telescope is building a classical adaptive optics
  (AO) system to be operating on the Sun in 2017. To make compatible
  its excellent dual beam spectropolarimetric features with the AO
  also requires a major refurbishment of the relay optics starting at
  the M2 and down to the spectrograph entrance. This paper presents
  the design parameters and expected performances of our AO system,
  and explains why and how we intend to control to a few percent the
  Mueller matrix of the whole optical path from the prime focus to the
  spectropolarimetric cameras. This project is co-funded by the European
  Union SOLARNET Project Ref.:312495, and the Centre National de la
  Recherche Scientifique.

Title: Prominence plasma and magnetic field structure - A coordinated
    observation with IRIS, Hinode and THEMIS
Authors: Schmieder, Brigitte; Labrosse, Nicolas; Levens, Peter;
   Lopez Ariste, Arturo
Bibcode: 2016cosp...41E1749S
Altcode:
  During an international campaign in 2014, utilising both space-based
  (IRIS and Hinode) and ground-based (THEMIS) instruments, we focused
  on observing prominences. We compare IRIS observations with those of
  Hinode (EIS and SOT) in order to build a more complete picture of
  the prominence structure for a quiescent prominence observed on 15
  July 2014, identified to have tornado-like structure. THEMIS provides
  valuable information on the orientation and strength of the internal
  magnetic field. Here we find there is almost ubiquitously horizontal
  field with respect to the local limb, with possibly a turbulent
  component. The Mg II lines form the majority of our IRIS analysis,
  with a mixture of reversed and non-reversed profiles present in the
  prominence spectra. Comparing the differences between the Mg II data
  from IRIS and the Ca II images from Hinode/SOT provides an intriguing
  insight into the prominence legs in these channels. We present plasma
  diagnostics from IRIS, with line of sight velocities of around 10
  km/s in either direction along the magnetic loops of material in the
  front of the prominence, and line widths comparable to those found
  for prominences by previous authors (e.g. Schmieder et al. 2014). We
  also take a look into the lines formed at higher, coronal plasma
  temperatures, as seen by Hinode/EIS, to compare plasma structures at
  a full range of temperatures.

Title: Discovery of a complex linearly polarized spectrum of
    Betelgeuse dominated by depolarization of the continuum
Authors: Aurière, M.; López Ariste, A.; Mathias, P.; Lèbre, A.;
   Josselin, E.; Montargès, M.; Petit, P.; Chiavassa, A.; Paletou, F.;
   Fabas, N.; Konstantinova-Antova, R.; Donati, J. -F.; Grunhut, J. H.;
   Wade, G. A.; Herpin, F.; Kervella, P.; Perrin, G.; Tessore, B.
Bibcode: 2016A&A...591A.119A
Altcode: 2016arXiv160504702A
  Context. <ASTROBJ>Betelgeuse</ASTROBJ> is an M supergiant that
  harbors spots and giant granules at its surface and presents linear
  polarization of its continuum. <BR /> Aims: We have previously
  discovered linear polarization signatures associated with
  individual lines in the spectra of cool and evolved stars. Here,
  we investigate whether a similar linearly polarized spectrum exists
  for <ASTROBJ>Betelgeuse</ASTROBJ>. <BR /> Methods: We used the
  spectropolarimeter Narval, combining multiple polarimetric sequences
  to obtain high signal-to-noise ratio spectra of individual lines, as
  well as the least-squares deconvolution (LSD) approach, to investigate
  the presence of an averaged linearly polarized profile for the
  photospheric lines. <BR /> Results: We have discovered the existence
  of a linearly polarized spectrum for <ASTROBJ>Betelgeuse</ASTROBJ>,
  detecting a rather strong signal (at a few times 10<SUP>-4</SUP>
  of the continuum intensity level), both in individual lines and in
  the LSD profiles. Studying its properties and the signal observed for
  the resonant Na I D lines, we conclude that we are mainly observing
  depolarization of the continuum by the absorption lines. The linear
  polarization of the Betelgeuse continuum is due to the anisotropy of
  the radiation field induced by brightness spots at the surface and
  Rayleigh scattering in the atmosphere. We have developed a geometrical
  model to interpret the observed polarization, from which we infer the
  presence of two brightness spots and their positions on the surface
  of <ASTROBJ>Betelgeuse</ASTROBJ>. We show that applying the model
  to each velocity bin along the Stokes Q and U profiles allows the
  derivation of a map of the bright spots. We use the Narval linear
  polarization observations of <ASTROBJ>Betelgeuse</ASTROBJ> obtained
  over a period of 1.4 yr to study the evolution of the spots and of the
  atmosphere. <BR /> Conclusions: Our study of the linearly polarized
  spectrum of <ASTROBJ>Betelgeuse</ASTROBJ> provides a novel method
  for studying the evolution of brightness spots at its surface and
  complements quasi-simultaneous observations obtained with PIONIER at
  the VLTI. <P />Based on observations obtained at the Télescope Bernard
  Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and Université
  de Toulouse, France.

Title: Vortex waves in sunspots
Authors: López Ariste, A.; Centeno, R.; Khomenko, E.
Bibcode: 2016A&A...591A..63L
Altcode:
  Context. Waves in the magnetized solar atmosphere are one of the
  favourite means of transferring and depositing energy into the solar
  corona. The study of waves brings information not just on the dynamics
  of the magnetized plasma, but also on the possible ways in which the
  corona is heated. <BR /> Aims: The identification and analysis of the
  phase singularities or dislocations provide us with a complementary
  approach to the magnetoacoustic and Aflvén waves propagating in the
  solar atmosphere. They allow us to identify individual wave modes,
  shedding light on the probability of excitation or the nature of the
  triggering mechanism. <BR /> Methods: We use a time series of Doppler
  shifts measured in two spectral lines, filtered around the three-minute
  period region. The data show a propagating magnetoacoustic slow
  mode with several dislocations and, in particular, a vortex line. We
  study under what conditions the different wave modes propagating in
  the umbra can generate the observed dislocations. <BR /> Results:
  The observed dislocations can be fully interpreted as a sequence
  of sausage and kink modes excited sequentially on average during
  15 min. Kink and sausage modes appear to be excited independently
  and sequentially. The transition from one to the other lasts less
  than three minutes. During the transition we observe and model the
  appearance of superoscillations inducing large phase gradients and
  phase mixing. <BR /> Conclusions: The analysis of the observed wave
  dislocations leads us to the identification of the propagating wave
  modes in umbrae. The identification in the data of superoscillatory
  regions during the transition from one mode to the other may be an
  important indicator of the location of wave dissipation.

Title: Magnetic Field and Plasma Diagnostics from Coordinated
    Prominence Observations
Authors: Schmieder, B.; Levens, P.; Dalmasse, K.; Mein, N.; Mein,
   P.; Lopez-Ariste, A.; Labrosse, N.; Heinzel, P.
Bibcode: 2016ASPC..504..119S
Altcode:
  We study the magnetic field in prominences from a statistical point of
  view, by using THEMIS in the MTR mode, performing spectropolarimetry
  of the He I D<SUB>3</SUB> line. Combining these measurements with
  spectroscopic data from IRIS, Hinode/EIS as well as ground-based
  telescopes, such as the Meudon Solar Tower, we infer the temperature,
  density, and flow velocities of the plasma. There are a number of
  open questions that we aim to answer: - What is the general direction
  of the magnetic field in prominences? Is the model using a single
  orientation of magnetic field always valid for atypical prominences? %-
  Does this depend on the location of the filament on the disk (visible
  in Hα, in He II 304 Å) over an inversion line between weak or strong
  network ? - Are prominences in a weak environment field dominated by
  gas pressure? - Measuring the Doppler shifts in Mg II lines (with IRIS)
  and in Hα can tell us if there are substantial velocities to maintain
  vertical rotating structures, as has been suggested for tornado-like
  prominences. We present here some results obtained with different
  ground-based and space-based instruments in this framework.

Title: The close circumstellar environment of
    Betelgeuse. IV. VLTI/PIONIER interferometric monitoring of the
    photosphere
Authors: Montargès, M.; Kervella, P.; Perrin, G.; Chiavassa, A.;
   Le Bouquin, J. -B.; Aurière, M.; López Ariste, A.; Mathias, P.;
   Ridgway, S. T.; Lacour, S.; Haubois, X.; Berger, J. -P.
Bibcode: 2016A&A...588A.130M
Altcode: 2016arXiv160205108M
  Context. The mass-loss mechanism of cool massive evolved stars is
  poorly understood. The proximity of Betelgeuse makes it an appealing
  target to study its atmosphere, map the shape of its envelope,
  and follow the structure of its wind from the photosphere out to
  the interstellar medium. <BR /> Aims: A link is suspected between
  the powerful convective motions in Betelgeuse and its mass loss. We
  aim to constrain the spatial structure and temporal evolution of the
  convective pattern on the photosphere and to search for evidence of
  this link. <BR /> Methods: We report new interferometric observations
  in the infrared H-band using the VLTI/PIONIER instrument. We monitored
  the photosphere of Betelgeuse between 2012 January and 2014 November
  to look for evolutions that may trigger the outflow. <BR /> Results:
  Our interferometric observations at low spatial frequencies are
  compatible with the presence of a hot spot on the photosphere that has a
  characteristic width of one stellar radius. It appears to be superposed
  on the smaller scale convective pattern. In the higher spatial frequency
  domain, we observe a significant difference between the observations and
  the predictions of 3D hydrodynamical simulations. <BR /> Conclusions:
  We bring new evidence for the presence of a convective pattern in
  the photosphere of red supergiants. The inferred hot spot is probably
  the top of a giant convection cell although an asymmetric extension
  of the star cannot be excluded by these interferometric observations
  alone. The properties of the observed surface features show a stronger
  contrast and inhomogeneity as predicted by 3D radiative hydrodynamical
  simulations. We propose that the large observed feature is modifying the
  signature of the convective pattern at the surface of the star in a way
  that simulations cannot reproduce. <P />Based on observations made with
  ESO telescopes at Paranal Observatory, under ESO programs 288.D-5035(A),
  090.D-0548(A), 092.D-0366(A), 092.D-0366(B) and 094.D-0869 (A).

Title: Detection of ultra-weak magnetic fields in Am stars: β Ursae
    Majoris and θ Leonis
Authors: Blazère, A.; Petit, P.; Lignières, F.; Aurière, M.; Ballot,
   J.; Böhm, T.; Folsom, C. P.; Gaurat, M.; Jouve, L.; Lopez Ariste,
   A.; Neiner, C.; Wade, G. A.
Bibcode: 2016A&A...586A..97B
Altcode: 2016arXiv160101829B
  Context. An extremely weak circularly polarized signature was recently
  discovered in spectral lines of the chemically peculiar Am star
  Sirius A. A weak surface magnetic field was proposed to account for
  the observed polarized signal, but the shape of the phase-averaged
  signature, dominated by a prominent positive lobe, is not expected
  in the standard theory of the Zeeman effect. <BR /> Aims: We aim
  at verifying the presence of weak circularly polarized signatures
  in two other bright Am stars, β UMa and θ Leo, and investigating
  the physical origin of Sirius-like polarized signals further. <BR />
  Methods: We present here a set of deep spectropolarimetric observations
  of β UMa and θ Leo, observed with the NARVAL spectropolarimeter. We
  analyzed all spectra with the least squares deconvolution multiline
  procedure. To improve the signal-to-noise ratio and detect extremely
  weak signatures in Stokes V profiles, we co-added all available
  spectra of each star (around 150 observations each time). Finally,
  we ran several tests to evaluate whether the detected signatures are
  consistent with the behavior expected from the Zeeman effect. <BR />
  Results: The line profiles of the two stars display circularly polarized
  signatures similar in shape and amplitude to the observations previously
  gathered for Sirius A. Our series of tests brings further evidence of a
  magnetic origin of the recorded signal. <BR /> Conclusions: These new
  detections suggest that very weak magnetic fields may well be present
  in the photospheres of a significant fraction of intermediate-mass
  stars. The strongly asymmetric Zeeman signatures measured so far in Am
  stars (featuring a dominant single-sign lobe) are not expected in the
  standard theory of the Zeeman effect and may be linked to sharp vertical
  gradients in photospheric velocities and magnetic field strengths.

Title: Structure of Prominence Legs: Plasma and Magnetic Field
Authors: Levens, P. J.; Schmieder, B.; Labrosse, N.; López Ariste, A.
Bibcode: 2016ApJ...818...31L
Altcode: 2015arXiv151204727L
  We investigate the properties of a “solar tornado” observed
  on 2014 July 15, and aim to link the behavior of the plasma to the
  internal magnetic field structure of the associated prominence. We
  made multi-wavelength observations with high spatial resolution and
  high cadence using SDO/AIA, the Interface Region Imaging Spectrograph
  (IRIS) spectrograph, and the Hinode/Solar Optical Telescope (SOT)
  instrument. Along with spectropolarimetry provided by the Télescope
  Héliographique pour l’Etude du Magnétisme et des Instabilités
  Solaires telescope we have coverage of both optically thick emission
  lines and magnetic field information. AIA reveals that the two legs
  of the prominence are strongly absorbing structures which look like
  they are rotating, or oscillating in the plane of the sky. The two
  prominence legs, which are both very bright in Ca II (SOT), are not
  visible in the IRIS Mg II slit-jaw images. This is explained by the
  large optical thickness of the structures in Mg II, which leads to
  reversed profiles, and hence to lower integrated intensities at these
  locations than in the surroundings. Using lines formed at temperatures
  lower than 1 MK, we measure relatively low Doppler shifts on the order
  of ±10 km s<SUP>-1</SUP> in the tornado-like structure. Between the two
  legs we see loops in Mg II, with material flowing from one leg to the
  other, as well as counterstreaming. It is difficult to interpret our
  data as showing two rotating, vertical structures that are unrelated
  to the loops. This kind of “tornado” scenario does not fit with
  our observations. The magnetic field in the two legs of the prominence
  is found to be preferentially horizontal.

Title: Polarimetric calibration of large mirrors
Authors: Lopez Ariste, A.
Bibcode: 2015arXiv151103177L
Altcode:
  Aims: To propose a method for the polarimetric calibration of large
  astronomical mirrors that does not require use of special optical
  devices nor knowledge of the exact polarization properties of the
  calibration target. Methods: We study the symmetries of the Mueller
  matrix of mirrors to exploit them for polarimetric calibration under
  the assumptions that only the orientation of the linear polarization
  plane of the calibration target is known with certainty. Results:
  A method is proposed to calibrate the polarization effects of single
  astronomical mirrors by the observation of calibration targets with
  known orientation of the linear polarization. We study the uncertainties
  of the method and the signal-to-noise ratios required for an acceptable
  calibration. We list astronomical targets ready for the method. We
  finally extend the method to the calibration of two or more mirrors,
  in particular to the case when they share the same incidence plane.

Title: Polarimetric measurements in prominences and "tornadoe"
    observed by THEMIS
Authors: Schmieder, Brigitte; López Ariste, Arturo; Levens, Peter;
   Labrosse, Nicolas; Dalmasse, Kévin
Bibcode: 2015IAUS..305..275S
Altcode:
  Since 2013, coordinated campaigns with the THEMIS spectropolarimeter in
  Tenerife and other instruments (space based: Hinode/SOT, IRIS or ground
  based: Sac Peak, Meudon) are organized to observe prominences. THEMIS
  records spectropolarimetry at the He I D<SUB>3</SUB> and we use the
  PCA inversion technique to derive their field strength, inclination
  and azimuth.

Title: On the nature of transverse coronal waves revealed by wavefront
    dislocations
Authors: López Ariste, A.; Luna, M.; Arregui, I.; Khomenko, E.;
   Collados, M.
Bibcode: 2015A&A...579A.127L
Altcode: 2015arXiv150503348L
  Context. Coronal waves are an important aspect of the dynamics of the
  plasma in the corona. Wavefront dislocations are topological features
  of most waves in nature and also of magnetohydrodynamic waves. Are there
  dislocations in coronal waves? <BR /> Aims: The finding and explanation
  of dislocations may shed light on the nature and characteristics of the
  propagating waves, their interaction in the corona, and in general on
  the plasma dynamics. <BR /> Methods: We positively identify dislocations
  in coronal waves observed by the Coronal Multi-channel Polarimeter
  (CoMP) as singularities in the Doppler shifts of emission coronal
  lines. We study the possible singularities that can be expected in
  coronal waves and try to reproduce the observed dislocations in terms of
  localization and frequency of appearance. <BR /> Results: The observed
  dislocations can only be explained by the interference of a kink and
  sausage wave modes propagating with different frequencies along the
  coronal magnetic field. In the plane transverse to the propagation,
  the cross-section of the oscillating plasma must be smaller than the
  spatial resolution, and the two waves result in net longitudinal and
  transverse velocity components that are mixed through projection onto
  the line of sight. Alfvén waves can be responsible for the kink mode,
  but a magnetoacoustic sausage mode is necessary in all cases. Higher
  (flute) modes are excluded. The kink mode has a pressure amplitude
  that is less than the pressure amplitude of the sausage mode, though
  its observed velocity is higher. This concentrates dislocations on
  the top of the loop. <BR /> Conclusions: To explain dislocations,
  any model of coronal waves must include the simultaneous propagation
  and interference of kink and sausage wave modes of comparable but
  different frequencies with a sausage wave amplitude much smaller than
  the kink one. <P />Appendix A is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201424340/olm">http://www.aanda.org</A>

Title: Magnetometry of Prominences
Authors: López Ariste, Arturo
Bibcode: 2015ASSL..415..179L
Altcode: 2015ASSL..415..179A
  We describe the measurement of magnetic fields in prominences. Using
  the He D<SUB>3</SUB> line as example we describe and illustrate the
  computation of the polarization emitted by He atoms in the presence
  of magnetic fields. The relatively weak magnetic fields expected in
  prominences require taking into consideration a long series of quantum
  coherences between the atomic levels of the He atom, coherences that
  are critical for the sensitivity of the emitted radiation and its
  polarization to the magnetic fields. But solving that quantum problem
  is only half the task: the observed polarized profiles need to be
  compared to the computed ones until a match is found. This inference
  or inversion can only work when the appropriate numerical technicals
  are put to work, techniques that help identify what magnetic field is
  the best solution for an observed profile, that can make use of all
  the available observables while being robust in front of noise and the
  low brightness of prominences and filaments respect to the entouring
  photosphere. These difficulties can be tackled but at the prixe of
  some approximations that have to be kept in mind in the analysis of
  prominence magnetic fields at the risk of serious mistakes on the
  inferred magnetic fields. Improving upon those approximations marks
  also the path for the future, with which description we will conclude
  this chapter.

Title: Open questions on prominences from coordinated observations
    by IRIS, Hinode, SDO/AIA, THEMIS, and the Meudon/MSDP
Authors: Schmieder, B.; Tian, H.; Kucera, T.; López Ariste, A.;
   Mein, N.; Mein, P.; Dalmasse, K.; Golub, L.
Bibcode: 2014A&A...569A..85S
Altcode: 2014arXiv1407.3171S
  Context. A large prominence was observed by multiple instruments on the
  ground and in space during an international campaign on September 24,
  2013, for three hours (12:12 UT -15:12 UT). Instruments used in the
  campaign included the newly launched (June 2013) Interface Region
  Imaging Spectrograph (IRIS), THEMIS (Tenerife), the Hinode Solar
  Optical Telescope (SOT), the Solar Dynamic Observatory's Atmospheric
  Imaging Assembly (SDO/AIA), and the Multichannel Subtractive Double
  Pass spectrograph (MSDP) in the Meudon Solar Tower. The movies obtained
  in 304 Å with the EUV imager SDO/AIA, and in Ca II line by SOT show
  the dynamic nature of the prominence. <BR /> Aims: The aim of this
  work is to study the dynamics of the prominence fine structures in
  multiple wavelengths to understand their formation. <BR /> Methods:
  The spectrographs IRIS and MSDP provided line profiles with a high
  cadence in Mg II h (2803.5 Å) and k (2796.4 Å) lines along four
  slit positions (IRIS), and in Hα in a 2D field of view (MSDP). The
  spectropolarimetry of THEMIS (Tenerife) allowed us to derive the
  magnetic field of the prominence using the He D<SUB>3</SUB> line
  depolarization (Hanle effect combined with the Zeeman effect). <BR />
  Results: The magnetic field is found to be globally horizontal with
  a relatively weak field strength (8-15 Gauss). On the other hand,
  the Ca II movie reveals turbulent-like motion that is not organized in
  specific parts of the prominence. We tested the addition of a turbulent
  magnetic component. This model is compatible with the polarimetric
  observations at those places where the plasma turbulence peaks. On the
  other hand, the Mg II line profiles show multiple peaks well separated
  in wavelength. This is interpreted by the existence of small threads
  along the line of sight with a large dispersion of discrete values of
  Doppler shifts, from 5 km s<SUP>-1</SUP> (a quasi-steady component) to
  60-80 km s<SUP>-1</SUP>. Each peak corresponds to a Gaussian profile,
  and not to a reversed profile as was expected by the present non-LTE
  radiative transfer modeling. This is a very surprising behavior for
  the Mg II line observed in prominences. <BR /> Conclusions: Turbulent
  fields on top of the macroscopic horizontal component of the magnetic
  field supporting the prominence give rise to the complex dynamics of
  the plasma. The plasma with the high velocities (70 km s<SUP>-1</SUP> to
  100 km s<SUP>-1</SUP> if we take into account the transverse velocities)
  may correspond to condensation of plasma along more or less horizontal
  threads of the arch-shape structure visible in 304 Å. The steady
  flows (5 km s<SUP>-1</SUP>) would correspond to a more quiescent plasma
  (cool and prominence-corona transition region) of the prominence packed
  into dips in horizontal magnetic field lines. The very weak secondary
  peaks in the Mg II profiles may reflect the turbulent nature of parts
  of the prominence. <P />Movies are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201423922/olm">http://www.aanda.org</A>

Title: Erratum: "Propagating Waves Transverse to the Magnetic Field
    in a Solar Prominence" <A href="/abs/2013ApJ...777..108S">(2013,
    ApJ, 777, 108)</A>
Authors: Schmieder, B.; Kucera, T. A.; Knizhnik, K.; Luna, M.;
   Lopez-Ariste, A.; Toot, D.
Bibcode: 2014ApJ...781..129S
Altcode:
  No abstract at ADS

Title: Propagating waves transverse to the magnetic field in a
    solar prominence
Authors: Kucera, Therese; Schmieder, Brigitte; Knizhnik, Kalman;
   Lopez-Ariste, Arturo; Luna, Manuel; Toot, David
Bibcode: 2014IAUS..300..435K
Altcode:
  We have observed a quiescent prominence with the Hinode Solar Optical
  Telescope (SOT) (Ca II and Hα lines), Sacramento Peak Dunn Solar
  Telescope using the Universal Birefringent Filter (DST/UBF, in Hα,
  Hβ and Sodium-D lines), THEMIS (Télescope Héliographique pour l
  Etude du Magnétisme et des Instabilités Solaires/MTR (Multi Raies)
  spectromagnetograph (He D<SUB>3</SUB>), and the Solar Dynamics
  Observatory Atmospheric Imaging Assembly (SDO/AIA) in EUV over a 4
  hour period on 2012 October 10. The small fields of view of the SOT,
  DST, and MTR are centered on a large prominence footpoint extending
  towards the surface. This feature appears in the larger field of view
  of the AIA/304 Å filtergram as a large, quasi-vertical pillar with
  loops on each side. The THEMIS/MTR data indicate that the magnetic
  field in the pillar is essentially horizontal and the observations in
  the optical domain show a large number of horizontally aligned features
  in the pillar. The data are consistent with a model of cool prominence
  plasma trapped in the dips of horizontal field lines. The SOT and DST
  data show what appear to be moving wave pulses. These pulses, which
  include a Doppler signature, move vertically, perpendicular to the
  field direction, along quasi-vertical columns of horizontal threads in
  the pillar. The pulses have a velocity of propagation of about 10 km/s,
  a wavelength about 2000 km in the plane of the sky, and a period about
  280 sec. We interpret these waves in terms of fast magnetosonic waves.

Title: Dynamics of a prominence observed in Mg II lines by IRIS
Authors: Schmieder, Brigitte; Mein, Pierre; Dalmasse, Kévin; Tian,
   Hui; Kucera, Therese; Lopez-Ariste, Arturo
Bibcode: 2014cosp...40E2927S
Altcode:
  In September 2013 several prominences were observed with the IRIS
  spectrograph during a 60 day-long international program. We will present
  one set of observations obtained using multiple instruments on September
  24. SDO/AIA and IRIS slit jaws provided images of the prominence
  corresponding to different physical conditions of the transition
  region between the cool plasma and the corona. The vector magnetic
  field was derived from THEMIS (Tenerife) observations using the He D3
  depolarisation due to the magnetic field. The inversion code (CPA) takes
  into account the Hanle and the Zeeman effects. Movies from SDO/AIA in
  304 A and Hinode/SOT in Ca II show the dynamics of the fine structures
  in the plane of the sky. From Mg II and Si IV line spectra observed by
  IRIS and H-alpha observed by the Multi-channel subtractive spectrograph
  (MSDP) in the Meudon solar tower we derived the Dopplershifts of the
  fine structures. The profiles of the Mg II lines are narrow (FHWM =0.15
  A) and not reversed, contrary to the predictions of the theoretical
  models (Paletou et al 1993). We could resolve the velocity of several
  structures along the LOS with Dopplershifts as high as 60 km/s.

Title: Propagating Waves Transverse to the Magnetic Field in a
    Solar Prominence
Authors: Schmieder, B.; Kucera, T. A.; Knizhnik, K.; Luna, M.;
   Lopez-Ariste, A.; Toot, D.
Bibcode: 2013ApJ...777..108S
Altcode: 2013arXiv1309.1568S
  We report an unusual set of observations of waves in a large prominence
  pillar that consist of pulses propagating perpendicular to the
  prominence magnetic field. We observe a huge quiescent prominence with
  the Solar Dynamics Observatory Atmospheric Imaging Assembly in EUV on
  2012 October 10 and only a part of it, the pillar, which is a foot or
  barb of the prominence, with the Hinode Solar Optical Telescope (SOT;
  in Ca II and Hα lines), Sac Peak (in Hα, Hβ, and Na-D lines), and
  THEMIS ("Télescope Héliographique pour l' Etude du Magnétisme et des
  Instabilités Solaires") with the MTR (MulTi-Raies) spectropolarimeter
  (in He D<SUB>3</SUB> line). The THEMIS/MTR data indicates that
  the magnetic field in the pillar is essentially horizontal and the
  observations in the optical domain show a large number of horizontally
  aligned features on a much smaller scale than the pillar as a whole. The
  data are consistent with a model of cool prominence plasma trapped in
  the dips of horizontal field lines. The SOT and Sac Peak data over
  the four hour observing period show vertical oscillations appearing
  as wave pulses. These pulses, which include a Doppler signature,
  move vertically, perpendicular to the field direction, along thin
  quasi-vertical columns in the much broader pillar. The pulses have
  a velocity of propagation of about 10 km s<SUP>-1</SUP>, a period of
  about 300 s, and a wavelength around 2000 km. We interpret these waves
  in terms of fast magnetosonic waves and discuss possible wave drivers.

Title: Dislocations in Magnetohydrodynamic Waves in a Stellar
    Atmosphere
Authors: López Ariste, A.; Collados, M.; Khomenko, E.
Bibcode: 2013PhRvL.111h1103L
Altcode: 2013arXiv1308.0145L
  We describe the presence of wave front dislocations in
  magnetohydrodynamic waves in stratified stellar atmospheres. Scalar
  dislocations such as edges and vortices can appear in Alfvén waves, as
  well as in general magnetoacoustic waves. We detect those dislocations
  in observations of magnetohydrodynamic waves in sunspots in the solar
  chromosphere. Through the measured charge of all the dislocations
  observed, we can give for the first time estimates of the modal
  contribution in the waves propagating along magnetic fields in solar
  sunspots.

Title: Improved Search of Principal Component Analysis Databases
    for Spectro-polarimetric Inversion
Authors: Casini, R.; Asensio Ramos, A.; Lites, B. W.; López Ariste, A.
Bibcode: 2013ApJ...773..180C
Altcode: 2013arXiv1307.0061C
  We describe a simple technique for the acceleration of
  spectro-polarimetric inversions based on principal component analysis
  (PCA) of Stokes profiles. This technique involves the indexing of the
  database models based on the sign of the projections (PCA coefficients)
  of the first few relevant orders of principal components of the
  four Stokes parameters. In this way, each model in the database can
  be attributed a distinctive binary number of 2<SUP>4n </SUP> bits,
  where n is the number of PCA orders used for the indexing. Each of
  these binary numbers (indices) identifies a group of "compatible"
  models for the inversion of a given set of observed Stokes profiles
  sharing the same index. The complete set of the binary numbers so
  constructed evidently determines a partition of the database. The
  search of the database for the PCA inversion of spectro-polarimetric
  data can profit greatly from this indexing. In practical cases it
  becomes possible to approach the ideal acceleration factor of 2<SUP>4n
  </SUP> as compared to the systematic search of a non-indexed database
  for a traditional PCA inversion. This indexing method relies on the
  existence of a physical meaning in the sign of the PCA coefficients
  of a model. For this reason, the presence of model ambiguities and of
  spectro-polarimetric noise in the observations limits in practice the
  number n of relevant PCA orders that can be used for the indexing.

Title: Propagating Waves Transverse to the Magnetic Field in a
    Solar Prominence
Authors: Kucera, Therese A.; Knizhnik, K.; Lopez Ariste, A.; Luna
   Bennasar, M.; Schmieder, B.; Toot, D.
Bibcode: 2013SPD....4410403K
Altcode:
  We have observed a quiescent prominence with the Hinode Solar Optical
  Telescope (SOT, in Ca II and H-alpha lines), Sacramento Peak Observatory
  (in H-alpha, H-beta and Sodium-D lines), and THEMIS/MTR (Télescope
  Héliographique pour l'Étude du Magnétisme et des Instabilités
  Solaires/MulTi Raies, providing vector magnetograms), and SDO/AIA (Solar
  Dynamics Observatory Atmospheric Imaging Assembly, in EUV) over a 4 hour
  period on 2012 October 10. The small fields of view of SOT, Sac Peak
  and THEMIS are centered on a large pillar-like prominence footpoint
  extending towards the surface. This feature appears in the larger
  field of view of the 304 Å band, as a large, quasi-vertical column
  with material flowing horizontally on each side. The THEMIS/MTR data
  indicate that the magnetic field in the pillar is essentially horizontal
  and the observations in the optical wavelengths show a large number of
  horizontally aligned features on a much smaller scale than the pillar
  as a whole. The data are consistent with a model of cool prominence
  plasma trapped in the dips of horizontal field lines. The SOT and Sac
  Peak data show what appear to be moving wave pulses. These pulses,
  which include a Doppler signature, move vertically, perpendicular to
  the field direction, along quasi-vertical columns. The pulses have
  a velocity of propagation of about 10 km/s, a period about 260 sec,
  and a wavelength around 2000 km. We interpret these waves in terms of
  fast magneto-sonic waves and discuss possible wave drivers.

Title: Mercury exosphere. III: Energetic characterization of its
    sodium component
Authors: Leblanc, Francois; Chaufray, Jean-Yves; Doressoundiram,
   Alain; Berthelier, Jean-Jacques; Mangano, Valeria; López-Ariste,
   Arturo; Borin, Patrizia
Bibcode: 2013Icar..223..963L
Altcode:
  Mercury's sodium exosphere has been observed only few times with high
  spectral resolution from ground based observatories enabling the
  analysis of the emission spectra. These observations highlighted
  the energetic state of the sodium exospheric atoms relative to
  the surface temperature. More recently, the Doppler shift of the
  exospheric Na atoms was measured and interpreted as consistent with
  an exosphere moving outwards from the subsolar point (Potter, A.E.,
  Morgan, T.H., Killen, R.E. [2009]. Icarus 204, 355-367). Using THEMIS
  solar telescope, we observed Mercury's sodium exosphere with very high
  spectral resolution at two opposite positions of its orbit. Using
  this very high spectral resolution and the scanning capabilities of
  THEMIS, we were able to reconstruct the 2D spatial distributions of
  the Doppler shifts and widths of the sodium atomic Na D<SUB>2</SUB>
  and D<SUB>1</SUB> lines. These observations revealed surprisingly
  large Doppler shift as well as spectral width consistent with previous
  observations. Starting from our 3D model of Mercury Na exosphere
  (Mercury Exosphere Global Circulation Model, Leblanc, F., Johnson,
  R.E. [2010]. Icarus 209, 280-300), we coupled this model with a 3D
  radiative transfer model described in a companion paper (Chaufray,
  J.Y., Leblanc, F. [2013]. Icarus, submitted for publication) which
  allows us to properly treat the non-maxwellian state of the simulated
  sodium exospheric population. Comparisons between THEMIS observations
  and simulations suggest that the previously observed energetic state
  of the Na exosphere might be essentially explained by a state of
  the Na exospheric atoms far from thermal equilibrium along with the
  Doppler shift dispersion of the Na atoms induced by the solar radiation
  pressure. However, the Doppler shift of the spectral lines cannot be
  explained by our modelling, suggesting either an exosphere spatially
  structured very differently than in our model or the inaccuracy of
  the spectral calibration when deriving the Doppler shift.

Title: Towards Measuring the Magnetic Energy Spectrum at
    Sub-Resolution Scales
Authors: López Ariste, A.; Sainz Dalda, A.
Bibcode: 2012ASPC..463..243L
Altcode:
  Area asymmetries in the Stokes V profile of the Zeeman-sensitive Fe
  I line at 630.25 nm shed light on gradients of velocity and magnetic
  field along the photon path. We use that information on observations
  of Hinode-SOT/SP of the quiet sun at different heliocentric angles
  to further investigate turbulent field models in those regions. The
  relationship between the asymmetry value and a correlation length
  scale for the field strength in the framework of stochastic radiative
  transfer for polarized light allows us to identify in the data
  subresolution scales of change of the magnetic field. From these we
  make a crude first attempt to determine the energy spectrum of the
  turbulent magnetic field down to the km scale as a proof-of-concept
  of the potential of this technique.

Title: Beyond the Solar Corona: Mercury's Magnetic Fields seen with
    Large Solar Telescopes
Authors: López Ariste, A.; Leblanc, F.
Bibcode: 2012ASPC..463..275L
Altcode:
  Solar telescopes have a potential in non-solar observations thanks to
  their instrumentation and their ability to observe in daylight, and
  despite their smaller mirror size compared to night-time telescopes. We
  review, as an illustration, THEMIS observations of Mercury's exosphere
  including the polarization of exospheric emission lines, and speculate
  that through analysis of this polarization made possible by the large
  photon-collecting capabilities of the upcoming large solar telescope
  we may expect to map Mercury's magnetosphere.

Title: Multi-purpose grating spectrograph for the 4-meter European
    Solar Telescope
Authors: Calcines, A.; Collados, M.; Feller, A.; Gelly, B.; Grauf, B.;
   Hirzberger, J.; López Ariste, A.; Lopez, R. L.; Mein, P.; Sayéde, F.
Bibcode: 2012SPIE.8446E..6TC
Altcode:
  This communication presents a family of spectrographs designed for
  the European Solar Telescope. They can operate in four different
  configurations: a long slit standard spectrograph (LsSS), two devices
  based on subtractive double pass (TUNIS and MSDP) and one based on
  an integral field, multi-slit, multi-wavelength configuration. The
  combination of them composes the multi-purpose grating spectrograph of
  EST, focused on supporting the different science cases of the solar
  photosphere and chromosphere in the spectral range from 3900 Å to
  23000 Å. The different alternatives are made compatible by using
  the same base spectrographs and different selectable optical elements
  corresponding to specific subsystems of each configuration.

Title: Resonance scattering polarization in the magnetosphere
    of Mercury
Authors: López Ariste, A.; Leblanc, F.; Casini, R.; Manso Sainz,
   R.; Gelly, B.; Le Men, C.
Bibcode: 2012Icar..220.1104L
Altcode:
  The conditions of the exosphere of Mercury constitute a prime example
  of a magnetosphere subject to space weather. We aim at improving the
  diagnostic of the physical conditions of Na atoms in the exosphere of
  Mercury, with particular emphasis in the possibility of inferring the
  magnetic field through polarimetry of the Na D emission lines. We
  performed spectropolarimetry of the Na D emission lines in the
  exosphere of Mercury at two different ranges of phases of the planet
  and interpreted them under present models of resonance scattering
  polarization applied to the Na atom. We measured successfully the
  polarization of the Na D<SUB>2</SUB> line while no polarization
  was seen in D<SUB>1</SUB>, as expected. The measured polarization
  varies with the phase angle but it is roughly a factor two smaller
  than expected from an isolated emitting atom. Depolarization due to
  the presence of optical depth explains this factor two and is the
  most probable explanation for this discrepancy. A framework for the
  observation of polarization in the emission lines of the exosphere and
  its interpretation is laid. This opens the possibility to use these
  measurements for diagnostic of the physical conditions in the exosphere
  of Mercury, and eventually to infer the magnetic field of Mercury and
  its variability from observations made with ground telescopes.

Title: Scales of the magnetic fields in the quiet Sun
Authors: López Ariste, A.; Sainz Dalda, A.
Bibcode: 2012A&A...540A..66L
Altcode: 2012arXiv1202.5436L
  Context. The presence of a turbulent magnetic field in the quiet Sun has
  been unveiled observationally using different techniques. The magnetic
  field is quasi-isotropic and has field strengths weaker than 100
  G. It is pervasive and may host a local dynamo. <BR /> Aims: We aim to
  determine the length scale of the turbulent magnetic field in the quiet
  Sun. <BR /> Methods: The Stokes V area asymmetry is sensitive to minute
  variations in the magnetic topology along the line of sight. Using
  data provided by Hinode-SOT/SP instrument, we performed a statistical
  study of this quantity. We classified the different magnetic regimes
  and infer properties of the turbulent magnetic regime. In particular
  we measured the correlation length associated to these fields for the
  first time. <BR /> Results: The histograms of Stokes V area asymmetries
  reveal three different regimes: one organized, quasi-vertical and
  strong field (flux tubes or other structures of the like); a strongly
  asymmetric group of profiles found around field concentrations; and a
  turbulent isotropic field. For the last, we confirm its isotropy and
  measure correlation lengths from hundreds of kilometers down to 10 km,
  at which point we lost sensitivity. A crude attempt to measure the
  power spectra of these turbulent fields is made. <BR /> Conclusions: In
  addition to confirming the existence of a turbulent field in the quiet
  Sun, we give further prove of its isotropy. We also measure correlation
  lengths down to 10 km. The combined results show magnetic fields with
  a large span of length scales, as expected from a turbulent cascade.

Title: Laboratory determination of Landé factors for the molecular
    radical FeH
Authors: Crozet, P.; Tourasse, G.; Ross, A.; Paletou, F.; López
   Ariste, A.
Bibcode: 2012EAS....58...63C
Altcode:
  We report laboratory measurements of the Zeeman response of lines in the
  0-0 Wing-Ford band of the F-X system (λ ~ 1 μm) of FeH, measured in
  magnetic fields 0.3 - 0.5 Tesla. New Landé factors are used to deduce
  the magnetic field in sunspots from Stokes V profiles recorded at the
  solar telescope THEMIS. The magnetic field deduced from atomic lines
  (Ti, Fe) is slightly higher than that found from FeH.

Title: Spectropolarimetric Comparison Between SDO/HMI and
    Hinode-SOT/SP Through THEMIS/MTR
Authors: Sainz Dalda, A.; Lopez Ariste, A.; Gelly, B.; Tarbell, T. D.;
   Centeno, R.; DeRosa, M. L.; Hoeksema, J. T.
Bibcode: 2011AGUFMSH31A1986S
Altcode:
  In the golden age of solar spacecraft observatories, the use of similar
  instruments observing same targets offers us the possibility to get
  more accurate information of the physical processes taking place on
  them. We present a comparison between the vector magnetic field and
  thermodynamic quantities obtained by three different spectropolarimetric
  instruments. We have used the simultaneous multi-wavelength capabilities
  of THEMIS/MTR as bridge between the observations at Fe I 6173 Å
  provided by SDO/HMI and at Fe I 6301 &amp; 6302 Å by Hinode-SOT/SP
  observations. The official inversion codes for these instruments (PCA
  based-on, VFISV and MERLIN respectively) have been used with the data
  properly arranged for them. Therefore, we compare the final products
  usually offered to the community, i.e. after the inversion, using
  different codes and these different wavelengths. The cross-calibration
  of these products shall allow us to go forward from one instrument
  result to other one in an easy, convenient way.

Title: Double-pass spectroimaging with spectral multiplexing: TUNIS
Authors: López Ariste, A.; Le Men, C.; Gelly, B.
Bibcode: 2011CoSka..41...99L
Altcode:
  Solar observations would benefit from simultaneous imaging and
  spectroscopy. To approach in an optimal manner such an ideal goal
  we have designed and built in THEMIS a spectro-imager based upon
  the concept of subtractive double pass through a diffraction grating
  spectrograph called TUNIS. In its basic design it produces an image
  at a wavelength which changes linearly in one of the directions of
  the image. To improve the simultaneity of the spectral coverage,
  we have implemented a spectral multiplexing, based upon Hadamard
  spectroscopy. We present the first observations of TUNIS and give the
  main specifications. TUNIS has been proposed for the European Solar
  Telescope (EST) project.

Title: The Polarization Optics for the European Solar Telescope
Authors: Bettonvil, F. C. M.; Collados, M.; Feller, A.; Gelly, B. F.;
   Keller, C. U.; Kentischer, T. J.; López Ariste, A.; Pleier, O.;
   Snik, F.; Socas-Navarro, H.
Bibcode: 2011ASPC..437..329B
Altcode:
  EST, the European Solar Telescope, is a 4-m class solar telescope,
  which will be located at the Canary Islands. It is currently in the
  conceptual design phase as a European funded project. In order to
  fulfill the stringent requirements for polarimetric sensitivity
  and accuracy, the polarimetry has been included in the design
  work from the very beginning. The overall philosophy has been to
  use a combination of techniques, which includes a telescope with
  low (and stable) instrumental polarization, optimal full Stokes
  polarimeters, differential measurement schemes, fast modulation
  and demodulation, and accurate calibration, and at the same time not
  giving up flexibility. The current baseline optical layout consists of a
  14-mirror layout, which is polarimetrically compensated and non-varying
  in time. In the polarization free F2 focus ample space is reserved for
  calibration and modulators and a polarimetric switch. At instrument
  level the s-, and p-planes of individual components are aligned,
  resulting in a system in which eigenvectors can travel undisturbed
  through the system.

Title: Unnoticed Magnetic Field Oscillations in the Very Quiet Sun
    Revealed by SUNRISE/IMaX
Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz,
   R.; Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; López Ariste,
   A.; Schmidt, W.; Barthol, P.; Gandorfer, A.
Bibcode: 2011ApJ...730L..37M
Altcode: 2011arXiv1103.0145M
  We present observational evidence for oscillations of magnetic flux
  density in the quiet areas of the Sun. The majority of magnetic
  fields on the solar surface have strengths of the order of or lower
  than the equipartition field (300-500 G). This results in a myriad of
  magnetic fields whose evolution is largely determined by the turbulent
  plasma motions. When granules evolve they squash the magnetic field
  lines together or pull them apart. Here, we report on the periodic
  deformation of the shapes of features in circular polarization observed
  at high resolution with SUNRISE. In particular, we note that the
  area of patches with a constant magnetic flux oscillates with time,
  which implies that the apparent magnetic field intensity oscillates
  in antiphase. The periods associated with this oscillatory pattern
  are compatible with the granular lifetime and change abruptly, which
  suggests that these oscillations might not correspond to characteristic
  oscillatory modes of magnetic structures, but to the forcing by granular
  motions. In one particular case, we find three patches around the same
  granule oscillating in phase, which means that the spatial coherence
  of these oscillations can reach 1600 km. Interestingly, the same kind
  of oscillatory phenomenon is also found in the upper photosphere.

Title: Eigenpolarimetry: An Algebraic Approach to Polarization-free
    Telescopes and Instruments
Authors: López Ariste, A.; Semel, M.
Bibcode: 2011ASPC..437..403L
Altcode:
  We have developed new algebraic tools based upon the symmetries of
  Mueller matrices that allow us to circumvent those problems while
  keeping the adjective polarization-free for our telescopes and not
  requiring expensive and delicate calibrations. The basic concept is
  to modulate entrance polarization into the eigenvectors of the system
  so that polarization information arrives unscathed at the analyzing
  beamsplitters, placed at the instrument or detector level. I will
  describe the basic concept, its application to THEMIS and the
  propositions for EST.

Title: Photospheric Hanle diagnostic of weak magnetic dipoles in stars
Authors: López Ariste, A.; Asensio Ramos, A.; González Fernández, C.
Bibcode: 2011A&A...527A.120L
Altcode: 2010arXiv1011.6288L
  <BR /> Aims: We propose and explore a new diagnostic technique
  based upon the linear polarization emitted in Hanle-sensitive lines
  in disk-integrated stars where a dipolar magnetic field breaks the
  rotational symmetry of the resonance scattering polarization <BR />
  Methods: A star with a simple dipolar field and a 1-0 spectral line were
  used to compute polarization amplitudes and angles. <BR /> Results:
  Predicted amplitudes are low but within reach of present instruments
  <BR /> Conclusions: A new application of the Hanle effect is proposed
  and analyzed as a tool that allows measuring of some of the weakest
  stellar magnetic fields.

Title: Detecting photons with orbital angular momentum in extended
astronomical objects: application to solar observations
Authors: Uribe-Patarroyo, N.; Alvarez-Herrero, A.; López Ariste, A.;
   Asensio Ramos, A.; Belenguer, T.; Manso Sainz, R.; Lemen, C.; Gelly, B.
Bibcode: 2011A&A...526A..56U
Altcode:
  Context. The orbital angular momentum (OAM) of the photon is a
  property of light from astronomical objects that has not yet been
  measured. We propose a method of measuring the OAM spectrum of light
  from an extended natural source, the Sun. Relating the OAM spectrum of
  different solar areas to its structures could lead to a novel remote
  sensing technique. <BR /> Aims: We present a method for measuring the
  OAM spectrum of solar photons. <BR /> Methods: The THEMIS (Télescope
  Héliographique pour l'Étude du Magnetisme et les Instabilités
  Solaires is a 0.9 m solar telescope property of the French CNRS-INSU
  at the Spanish Observatorio del Teide.) telescope is being used with
  a novel phase-diversity technique. A spatial light modulator is placed
  on one pupil image, and an ad-hoc optical setup allows the measurement
  of two simultaneous phase-diverse images in the same CCD, with equal
  optical paths. <BR /> Results: Preliminary results show that very good
  seeing is mandatory for this kind of observation. The method works in
  the laboratory, and good seeing conditions in the 2010 campaign are
  being awaited.

Title: The polarization optics for the European Solar Telescope (EST)
Authors: Bettonvil, F. C. M.; Collados, M.; Feller, A.; Gelly, B. F.;
   Keller, C. U.; Kentischer, T. J.; López Ariste, A.; Pleier, O.;
   Snik, F.; Socas-Navarro, H.
Bibcode: 2010SPIE.7735E..6IB
Altcode: 2010SPIE.7735E.214B
  EST (European Solar Telescope) is a 4-m class solar telescope, which
  is currently in the conceptual design phase. EST will be located at
  the Canary Islands and aims at observations with the best possible
  spectral, spatial and temporal resolution and best polarimetric
  performance, of the solar photosphere and chromosphere, using a
  suite of instruments that can efficiently produce two-dimensional
  spectropolarimetric information of the thermal, dynamic and magnetic
  properties of the plasma over many scale heights, and ranging from
  λ=350 until 2300 nm. In order to be able to fulfill the stringent
  requirements for polarimetric sensitivity and accuracy, from the very
  beginning the polarimetry has been included in the design work. The
  overall philosophy has been to use a combination of techniques, which
  includes a telescope with low (and stable) instrumental polarization,
  optimal full Stokes polarimeters, differential measurement schemes,
  fast modulation and demodulation, and accurate calibration. The
  current baseline optical layout consists of a 14-mirror layout,
  which is polarimetrically compensated and nonvarying in time. In the
  polarization free F2 focus ample space is reserved for calibration and
  modulators and a polarimetric switch. At instrument level the s-, and
  p-planes of individual components are aligned, resulting in a system
  in which eigenvectors can travel undisturbed through the system.

Title: Image reconstruction with analytical point spread functions
Authors: Asensio Ramos, A.; López Ariste, A.
Bibcode: 2010A&A...518A...6A
Altcode: 2010arXiv1004.3278A
  Context. The image degradation produced by atmospheric turbulence
  and optical aberrations is usually alleviated using post-facto image
  reconstruction techniques, even when observing with adaptive optics
  systems. <BR /> Aims: These techniques rely on the development of the
  wavefront using Zernike functions and the non-linear optimization of a
  certain metric. The resulting optimization procedure is computationally
  heavy. Our aim is to alleviate this computational burden. <BR />
  Methods: We generalize the extended Zernike-Nijboer theory to carry
  out the analytical integration of the Fresnel integral and present a
  natural basis set for the development of the point spread function when
  the wavefront is described using Zernike functions. <BR /> Results:
  We present a linear expansion of the point spread function in terms
  of analytic functions, which, in addition, takes defocusing into
  account in a natural way. This expansion is used to develop a very
  fast phase-diversity reconstruction technique, which is demonstrated
  in terms of some applications. <BR /> Conclusions: We propose that
  the linear expansion of the point spread function can be applied to
  accelerate other reconstruction techniques in use that are based on
  blind deconvolution.

Title: Spectrograph capabilities of the European Solar Telescope
Authors: Calcines, A.; Collados, M.; Feller, A.; Grauf, B.;
   Grivel-Gelly, C.; Hirzberger, J.; López Ariste, A.; López López,
   R.; Mein, P.; Sayède, F.
Bibcode: 2010SPIE.7735E..20C
Altcode: 2010SPIE.7735E..68C
  EST is a project for a 4-meter class telescope to be located in the
  Canary Islands. EST will be optimized for studies of the magnetic
  coupling between the photosphere and the chromosphere. This requires
  high spatial and temporal resolution diagnostics tools of properties of
  the plasma, by using multiple wavelength spectropolarimetry. To achieve
  these goals, visible and near-IR multi-purpose spectrographs are being
  designed to be compatible with different modes of use: LsSS (Long-slit
  Standard Spectrograph), multi-slit multi-wavelength spectrograph with
  an integral field unit, TUNIS (Tunable Universal Narrow-band Imaging
  Spectrograph), and new generation MSDP (Multi-channel Subtractive
  Double-pass Spectrograph). In this contribution, these different
  instrumental configurations are described.

Title: Double-pass spectro-imaging: TUNIS
Authors: López Ariste, A.; Le Men, C.; Gelly, B.; Asensio Ramos, A.
Bibcode: 2010AN....331..658L
Altcode:
  We present TUNIS, a double-pass spectro-imager built in THEMIS as a
  proof-of-concept for EST. Basic concepts and selected first results
  are shown. We introduce the concept of a Hadamard spectral mask as
  a proposition to move forward from the present implementation of a
  single-wavelength per image pixel to a more general one of multiplexed
  spectral information that improves the temporal coherence of the
  spectral measurement.

Title: The stochastic, intermittent nature of quiet Sun magnetism
Authors: Martinez González, M. J.; Manso Sainz, R.; López Ariste,
   A.; Bellot Rubio, L. R.; Bianda, M.
Bibcode: 2010iac..talk....1M
Altcode: 2010iac..talk..153M
  No abstract at ADS

Title: Statistical Analysis of the very Quiet Sun Magnetism
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
   A.; López Ariste, A.; Bianda, M.
Bibcode: 2010ApJ...711L..57M
Altcode: 2010arXiv1001.4551M
  The behavior of the observed polarization amplitudes with spatial
  resolution is a strong constraint on the nature and organization
  of solar magnetic fields below the resolution limit. We study the
  polarization of the very quiet Sun at different spatial resolutions
  using ground- and space-based observations. It is shown that 80% of the
  observed polarization signals do not change with spatial resolution,
  suggesting that, observationally, the very quiet Sun magnetism
  remains the same despite the high spatial resolution of space-based
  observations. Our analysis also reveals a cascade of spatial scales
  for the magnetic field within the resolution element. It is manifest
  that the Zeeman effect is sensitive to the microturbulent field usually
  associated with Hanle diagnostics. This demonstrates that Zeeman and
  Hanle studies show complementary perspectives of the same magnetism.

Title: Compressive sensing for spectroscopy and polarimetry
Authors: Asensio Ramos, A.; López Ariste, A.
Bibcode: 2010A&A...509A..49A
Altcode: 2009arXiv0909.4439A
  We demonstrate, through numerical simulations with real data, the
  feasibility of using compressive sensing techniques for the acquisition
  of spectro-polarimetric data. This allows us to combine the measurement
  and the compression process into one consistent framework. Signals are
  recovered using a sparse reconstruction scheme from projections of
  the signal of interest onto appropriately chosen vectors, typically
  noise-like vectors. The compressibility properties of spectral lines
  are analyzed in detail. The results shown in this paper demonstrate
  that, thanks to the compressibility properties of spectral lines, it
  is feasible to reconstruct the signals using only a small fraction of
  the information that is measured nowadays. We investigate in depth
  the quality of the reconstruction as a function of the amount of
  data measured and the influence of noise. This change of paradigm
  also allows us to define new instrumental strategies and to propose
  modifications to existing instruments in order to take advantage of
  compressive sensing techniques.

Title: Multiline Zeeman signatures through line addition
Authors: Semel, M.; Ramírez Vélez, J. C.; Martínez González,
   M. J.; Asensio Ramos, A.; Stift, M. J.; López Ariste, A.; Leone, F.
Bibcode: 2009A&A...504.1003S
Altcode: 2009arXiv0906.4226S
  Context: To obtain a significant Zeeman signature in the polarised
  spectra of a magnetic star, we usually “add” the contributions
  of numerous spectral lines; the ultimate goal is to recover the
  spectropolarimetric prints of the magnetic field in these line
  additions. <BR />Aims: Here we want to clarify the meaning of these
  techniques of line addition; in particular, we try to interpret the
  meaning of the “pseudo-line” formed during this process and to
  find out why and how its Zeeman signature is still meaningful. <BR
  />Methods: We create a synthetic case of line addition and apply well
  tested standard solar methods routinely used in research on magnetism
  in the Sun. <BR />Results: The results are convincing and the Zeeman
  signatures well detected; Solar methods are found to be quite efficient
  for stellar observations. We statistically compare line addition
  with least-squares deconvolution and demonstrate that they both give
  very similar results, as a consequence of the special statistical
  properties of the weights. <BR />Conclusions: The Zeeman signatures
  are unequivocally detected in this multiline approach. We suggest
  that magnetic field detection is reliable well beyond the weak-field
  approximation. Linear polarisation in the spectra of solar type stars
  can be detected when the spectral resolution is sufficiently high.

Title: Multi-Line Stokes Inversion for Prominence Magnetic-Field
    Diagnostics
Authors: Casini, R.; López Ariste, A.; Paletou, F.; Léger, L.
Bibcode: 2009ApJ...703..114C
Altcode: 2009arXiv0906.2144C
  We present test results on the simultaneous inversion of the Stokes
  profiles of the He I lines at 587.6 nm (D<SUB>3</SUB>) and 1083.0 nm
  in prominences (90° scattering). We created data sets of synthetic
  Stokes profiles for the case of quiescent prominences (B &lt; 200 G),
  assuming a conservative value of 10<SUP>-3</SUP> of the peak intensity
  for the polarimetric sensitivity of the simulated observations. In
  this work, we focus on the error analysis for the inference of the
  magnetic field vector, under the usual assumption that the prominence
  can be assimilated to a slab of finite optical thickness with uniform
  magnetic and thermodynamic properties. We find that the simultaneous
  inversion of the two lines significantly reduces the errors on the
  inference of the magnetic field vector, with respect to the case of
  single-line inversion. These results provide a solid justification for
  current and future instrumental efforts with multi-line capabilities
  for the observations of solar prominences and filaments.

Title: Variability of the polarization profiles of the Ba II D{2}
    line in the solar atmosphere
Authors: López Ariste, A.; Asensio Ramos, A.; Manso Sainz, R.;
   Derouich, M.; Gelly, B.
Bibcode: 2009A&A...501..729L
Altcode:
  Resonance-scattering polarization and the Hanle effect are powerful
  but seldom exploited probes into the magnetism of the quiet solar
  atmosphere. They are also very interesting checks of the quantum theory
  of atomic polarization. The Ba ii D2 line has been known for more than
  20 years as presenting a conspicuous signal of resonance scattering
  polarization thanks to its atomic configuration and the presence
  of five different isotopes of Ba, two of which present a hyperfine
  structure.A model that considers most of the known ingredients of the
  atomic polarization of Ba ii related to the formation of the D2 line
  was presented in 2007. We intend to observe all the variability of the
  Stokes profiles of this line in conditions of resonance scattering
  to verify the general validity of the model and to ascertain the
  use of the model for magnetic field diagnostics in the quiet solar
  chromosphere and in spicules. The new CCD cameras at THEMIS and the
  recently commissioned tip-tilt tracking system gave us the opportunity
  to perform the required observations with unprecedented precision
  and reliability, resulting in data ready to confront the present
  theory.The Stokes Q profiles, both off-limb and on disk, appear to
  verify this theory in qualitative terms. The observed departures in
  terms of overall signal amplitude and relative ratios among the three
  spectral features point to a refinement of the theory for quantitative
  purposes, perhaps including radiative-transfer effects. We observed,
  on the other hand, anomalous Stokes V profiles in the absence of Zeeman
  effect that remain unexplained.The qualitative agreement between the
  theory and the observations encourages an increased effort to also
  match the observations from a quantitative point of view, including
  the observed anomalous Stokes V profiles.

Title: Centre to Limb Variation of the Internetwork Magnetism
Authors: Martínez González, M. J.; Asensio Ramos, A.; López
   Ariste, A.
Bibcode: 2009ASPC..405..219M
Altcode:
  We analyse 1.5 μm spectropolarimetric observations of quiet
  regions at different positions on the solar surface. Neither the
  circular and linear polarization signals nor the area and amplitude
  asymmetries present a clear trend with the position on the Sun. This
  fact discards a network-like scenario for the internetwork magnetic
  fields. An isotropically distributed magnetic field would reproduce the
  observations, whatever its organisation (myriads of low-lying loops,
  turbulent field, etc.).

Title: Flux Cancellation in the Solar Photosphere: a near-IR Line
    of Mn I as a Diagnostic Tool
Authors: Asensio Ramos, A.; Martínez González, M. J.; López Ariste,
   A.; Trujillo Bueno, J.; Collados, M.
Bibcode: 2009ASPC..405..215A
Altcode:
  Recently, \cite{asensio_2_asensio_mn07} pointed out that the near-IR
  line of Mn I at 15262.702 Å provides a new diagnostic window for
  exploring the magnetism of the quiet Sun. In contrast with previously
  considered Mn I lines located at visible wavelengths this near-IR
  line has the advantage that the shape of its intensity profile is very
  sensitive to the presence of magnetic fields. This enhanced magnetic
  sensitivity is produced by the coincidence of two favorable facts:
  the enhanced Zeeman sensitivity of near-IR lines and because this line
  is subjected to particularly strong Paschen-Back perturbations due to
  the hyperfine structure of manganese. Of great diagnostic interest is
  that the intensity profile itself give us information on the unsigned
  magnetic flux, while the polarization profiles are sensitive to the
  net flux. An application to spectropolarimetric observations with the
  Tenerife Infrared Polarimeter has allowed us to obtain the first flux
  cancellation map in an enhanced network region.

Title: Short-term variations of Mercury's Na exosphere observed with
    very high spectral resolution
Authors: Leblanc, F.; Doressoundiram, A.; Schneider, N.; Massetti,
   S.; Wedlund, M.; López Ariste, A.; Barbieri, C.; Mangano, V.;
   Cremonese, G.
Bibcode: 2009GeoRL..36.7201L
Altcode: 2009GeoRL..3607201L
  Short time variations of Mercury's exosphere cannot be tracked
  easily from ground based observatories because of the difficulty
  of distinguishing them from Earth atmospheric effects. On July 13th
  2008, using THEMIS solar telescope, we were able to simultaneously
  measure brightness, Doppler shift and width of the exospheric sodium
  D<SUB>2</SUB> emission line during half a day with a resolving power
  of ∼370,000. Mercury's exosphere displayed an emission brightness
  peak in the Northern hemisphere which vanished in few hours and a
  more persistent Southern Hemispheric peak. The bulk Doppler shift of
  the exosphere suggests a period of strong escape from Mercury. The
  global changes of the Doppler shift and of the Doppler width suggest
  that a cloud of sodium atoms ejected before or at the beginning of our
  sequence of observations passed through THEMIS field of view moving
  anti-sunward. A preferentially southern ejection of sodium atoms leading
  to the observed persistent southern emission peak is consistent with
  the orientation of the Interplanetary Magnetic Field during that period.

Title: Multiline Zeeman signatures as demonstrated through the
    Pseudo-line
Authors: Semel, M.; Ramirez Velez, J. C.; Stift, M. J.; Martinez
   Gonzalez, M. J.; Lopez Ariste, A.; Leone, F.
Bibcode: 2008arXiv0810.3543S
Altcode:
  In order to get a significant Zeeman signature in the polarised spectra
  of a magnetic star, we usually 'add' the contributions of numerous
  spectral lines; the ultimate goal is to recover the spectropolarimetric
  prints of the magnetic field in these line additions. Here we want to
  clarify the meaning of these techniques of line addition; in particular,
  we try to interpret the meaning of the 'pseudo-line' formed during
  this process and to find out why and how its Zeeman signature is still
  meaningful. We create a synthetic case of lines addition and apply
  well tested standard solar methods routinely used in the research
  on magnetism in our nearest star. The results are convincing and the
  Zeeman signatures well detected; Solar methods are found to be quite
  efficient also for stellar observations. The Zeeman signatures are
  unequivocally detected in this multiline approach. We may anticipate
  the outcome magnetic fields to be reliable well beyond the weak-field
  approximation. Linear polarisation in the spectra of solar type stars
  can be detected when the spectral resolution is sufficiently high.

Title: Design and implementation of an image stabilization device
    at the THEMIS solar telescope
Authors: Gelly, Bernard; Le Men, Claude; López Ariste, Arturo;
   Delaigue, Cyril; Gónzalez Perez, Ruyman
Bibcode: 2008ExA....22...67G
Altcode: 2008ExA...tmp....1G
  An image stabilizer has been inserted into the optical path of the
  THEMIS solar telescope. THEMIS is a Ritchey Chretien reflector telescope
  using an altazimuthal mount and closely tied to its spectrograph. The
  optical and mechanical design, implementation and system tests are
  described, and emphasis is put on the complexity of situations that this
  stabilizer must accept, including the scanning of the solar surface
  while stabilizing. The current closed-loop crossover frequency of the
  device is 65 Hz at - 3 dB on all typical solar scenes.

Title: High latitude peaks in Mercury's sodium exosphere: Spectral
    signature using THEMIS solar telescope
Authors: Leblanc, F.; Doressoundiram, A.; Schneider, N.; Mangano, V.;
   López Ariste, A.; Lemen, C.; Gelly, B.; Barbieri, C.; Cremonese, G.
Bibcode: 2008GeoRL..3518204L
Altcode:
  We present the first combined intensity and temperature maps of sodium
  in Mercury's exosphere, made possible by the use of the THEMIS solar
  telescope on Tenerife in the Canary Islands. The intensity maps clearly
  show high-latitude peaks, and temperatures inferred from spectral line
  widths suggest that these regions are either slightly hotter than the
  rest of the exosphere or much smaller than observed. These brighter,
  warmer regions are also observed, for the first time, to appear within
  few Earth hours which strongly suggest that they are produced by
  solar wind sputtering. This highly capable instrument obtained these
  data during daylight, highlighting the unique potential for THEMIS to
  undertake continuous multi-hour and multi-day datasets in conjunction
  with the MESSENGER mission to Mercury.

Title: Venus wind map at cloud top level with the MTR/THEMIS visible
spectrometer, I: Instrumental performance and first results
Authors: Gaulme, Patrick; Schmider, François-Xavier; Grec, Catherine;
   López Ariste, Arturo; Widemann, Thomas; Gelly, Bernard
Bibcode: 2008P&SS...56.1335G
Altcode: 2010arXiv1011.2680G
  Solar light gets scattered at cloud top level in Venus' atmosphere,
  in the visible range, which corresponds to the altitude of 67 km. We
  present Doppler velocity measurements performed with the high resolution
  spectrometer MTR of the Solar telescope THEMIS (Teide Observatory,
  Canary Island) on the sodium D2 solar line (5890A˚). Observations
  lasted only 49 min because of cloudy weather. However, we could assess
  the instrumental velocity sensitivity, 31ms<SUP>-1</SUP> per pixel of
  1 arcsec, and give a value of the amplitude of zonal wind at equator
  at 151±16ms<SUP>-1</SUP>.

Title: Strength distribution of solar magnetic fields in photospheric
    quiet Sun regions
Authors: Ramírez Vélez, J. C.; López Ariste, A.; Semel, M.
Bibcode: 2008A&A...487..731R
Altcode: 2008arXiv0806.1326R
  Context: The magnetic topology of the solar photosphere in its quietest
  regions is hidden by the difficulties to disentangle magnetic flux
  through the resolution element from the field strength of unresolved
  structures. The observation of spectral lines with strong coupling with
  hyperfine structure, like the observed Mn i line at 553.7 nm, allows
  such differentiation. <BR />Aims: To analyse the distribution of field
  strengths in the network and intranetwork of the solar photosphere
  through inversion of the Mn i line at 553.7 nm. <BR />Methods: An
  inversion code for the magnetic field using the principal component
  analysis (PCA) has been developed. Statistical tests are run on
  the code to validate it. The code has to draw information from the
  small-amplitude spectral feature appearing in the core of the Stokes
  V profile of the observed line for field strengths below a certain
  threshold, coinciding with lower limit of the Paschen-Back effect in
  the fine structure of the involved atomic levels. <BR />Results: The
  inversion of the observed profiles, using the circular polarisation (V)
  and the intensity (I), shows the presence of magnetic fields strengths
  in a range from 0 to 2 kG, with predominant weak strength values. Mixed
  regions with mean strength field values of 1130 and 435 Gauss are
  found associated with the network and intranetwork, respectively. <BR
  />Conclusions: The Mn i line at 553 nm probes the field strength
  distribution in the quiet sun and shows the predominance of weak,
  hectoGauss fields in the intranetwork, and strong, kiloGauss fields in
  the network. It also shows that both network and intranetwork are to be
  understood at our present spatial resolutions as field distributions,
  of which we hint at the mean properties.

Title: Comet McNaught C/2006 P1: observation of the sodium emission
    by the solar telescope THEMIS
Authors: Leblanc, F.; Fulle, M.; López Ariste, A.; Cremonese, G.;
   Doressoundiram, A.; Sainz Dalda, A.; Gelly, B.
Bibcode: 2008A&A...482..293L
Altcode:
  Comet McNaught C/2006 P1 was the brightest comet of the last forty
  years when reaching its perihelion at an heliocentric distance of
  0.17Â AU. Two days before this perihelion, at an heliocentric distance
  of 0.2Â AU, Themis, a French-Italian solar telescope in the Canary
  Islands, Spain, observed the Comet sodium emission of McNaught. The
  measured maximum sodium brightness of the D2Â emission line peaked
  at 900Â Mega-Rayleigh. The spatial distribution of the sodium
  emission with respect to the nucleus of the comet is in agreement
  with previous observations. It displays a clear sunward-tailward
  asymmetry that suggests a dichotomy of the sodium sources between
  a source close to the nucleus and an extended source most probably
  corresponding to the dust tail. The spatial distribution along the
  slit of the width and speed of the Doppler NaÂ distribution also
  suggests such a dichotomy. The sodium ejection rate inferred from this
  observation agrees with the value of the ejection rate extrapolated
  from comet Hale-Bopp, taking into account the heliocentric distance
  of comet McNaught and its significantly larger dust release. If
  we suppose a similar concentration of sodium atoms in both comets,
  this observation suggests that the sodium ejection rate from comets
  McNaught and Hale-Bopp is proportional to the solar flux. Therefore
  the most probable ejection mechanisms are photo-sputtering, solar wind
  sputtering, or cometary ion sputtering, and not thermal desorption.

Title: Near-IR internetwork spectro-polarimetry at different
    heliocentric angles
Authors: Martínez González, M. J.; Asensio Ramos, A.; López Ariste,
   A.; Manso Sainz, R.
Bibcode: 2008A&A...479..229M
Altcode: 2007arXiv0710.5219M
  Aims:The analysis of near infrared spectropolarimetric data at the
  internetwork in different regions on the solar surface could offer
  constraints that reject current modeling of these quiet areas. <BR
  />Methods: We present spectro-polarimetric observations of very
  quiet regions for different values of the heliocentric angle for the
  Fe i lines at 1.56 μm, from disc centre to positions close to the
  limb. The spatial resolution of the data is 0.7{-}1''. We analyse
  direct observable properties of the Stokes profiles as the amplitude
  of circular and linear polarisation, as well as the total degree of
  polarisation. The area and amplitude asymmetries are also studied. <BR
  />Results: We do not find any significant variation in the properties of
  the polarimetric signals with the heliocentric angle. This means that
  the magnetism of the solar internetwork remains the same regardless
  of the position on the solar disc. This observational fact discards
  the possibility of modeling the internetwork as a network-like
  scenario. The magnetic elements of internetwork areas seem to be
  isotropically distributed when observed at our spatial resolution.

Title: Spectrophotometric analysis of Ellerman bombs in the Ca II,
    Hα, and UV range
Authors: Pariat, E.; Schmieder, B.; Berlicki, A.; Deng, Y.; Mein,
   N.; López Ariste, A.; Wang, S.
Bibcode: 2007A&A...473..279P
Altcode:
  Context: Even if Ellerman bombs have been observed in the Hα line
  within emerging magnetic flux regions since the early 20th century,
  their origin and the mechanisms that lead to their formation have been
  strongly debated. Recently, new arguments in favor of chromospheric
  magnetic reconnection have been advanced. Ellerman bombs seem to be
  the signature of reconnections that take place during the emergence
  of the magnetic field. <BR />Aims: We have observed an active region
  presenting emergence of magnetic flux. We detected and studied
  Ellerman bombs in two chromospheric lines: Ca ii 8542 Å and Hα. We
  investigated the link between Ellerman bombs and other structures and
  phenomena appearing in an emerging active region: UV bright points,
  arch filament systems, and magnetic topology. <BR />Methods: On August
  3, 2004, we performed multi-wavelength observations of the active
  region NOAA 10655. This active region was the target of SoHO Joint
  Observation Program 157. Both SoHO/MDI and TRACE (195 Å and 1600 Å)
  were used. Simultaneously, we observed in the Ca ii and Na D1 lines
  with the spectro-imager MSDP mode of THEMIS. Alternately to the MSDP,
  we used the MTR spectropolarimeter on THEMIS to observe in Hα and
  in the Fe i doublet at 6302 Å. We derived the magnetic field vectors
  around some Ellerman bombs. <BR />Results: We present the first images
  of EBs in the Ca ii line and confirm that Ellerman bombs can indeed
  be observed in the Ca ii line, presenting the same “moustache”
  geometry profiles as in the Hα line, but with a narrower central
  absorption in the Ca ii line, in which the peaks of emission are
  around ±0.35 Å. We noticed that the Ellerman bombs observed in the
  wings of Ca ii line have an elongated shape - the length about 50%
  greater than the width. We derived mean semi-axis lengths of 1.4''
  × 2.0''. In the UV time profiles of the Ellerman bombs, we noticed
  successive enhanced emissions. The distribution of lifetimes of these
  individual impulses presents a strong mode around 210 s. Study of the
  magnetic topology shows that 9 out of the 13 EBs are located on the
  inversion line of the longitudinal field and that some typical examples
  might be associated with a bald patch topology. <BR />Conclusions: We
  provide new arguments in favor of the reconnection origin of Ellerman
  bombs. The different individual impulses observed in UV may be related
  to a bursty mode of reconnection. We also show that this Ca ii 8542
  Å chromospheric line is a good indicator of Ellerman bombs and can
  bring new information about these phenomena.

Title: What can We learn about Filaments from Vector Magnetograms?
Authors: Schmieder, B.; Aulanier, G.; Lopez Ariste, A.
Bibcode: 2007ASPC..369..137S
Altcode:
  Theoretical MHD models of filaments have predicted the existence
  of dips in magnetic field lines supporting the cool filament
  plasma. Condensation would be one of the possible mechanism of filament
  formation. The controversy is still alive. Filament fine structures are
  of dynamical nature and injection of cold material (like surges) from
  the photosphere would be the proposed mechanism. With the presence of
  dips, the magnetic lines are predicted to be tangent to the photosphere,
  near the barbs, footpoints of the prominences. The injection model
  predicted nearly vertical structures of the barbs rooted in the
  minority polarities. <P />Measurements of vector magnetic field in
  filament channels can answer to these questions and will be possible
  with Solar-B. <P />Recent results have been obtained with THEMIS vector
  magnetograph in Tenerife, suggesting that dips are really present in
  filament channel leading to the conclusion that the observed magnetic
  topology in the photosphere fully supports models of prominences based
  on dips.

Title: Turbulent fields in the quiet sun from Hanle and Zeeman
    effects with THEMIS
Authors: López Ariste, A.; Malherbe, J. M.; Manso Sainz, R.; Asensio
   Ramos, A.; Ramírez Vélez, J. C.; Martínez González, M.
Bibcode: 2007sf2a.conf..596L
Altcode:
  The measurement of the Sr I line with full polarimetry and spatial
  resolution with THEMIS has provided the unexpected result of an apparent
  correlation between the Hanle effect signals and the Zeeman effect
  signals on this line. Traditionnally, Hanle effect signals in linear
  polarisation have been interpreted as the signature of unorganized,
  turbulent fields, while Zeeman effect signals -mostly in circular
  polarisation- were interpreted as structured fields. The correlation
  between both observed by THEMIS requires a change of mind respect to the
  picture of the quiet sun, with fewer structured fields and a turbulent
  field visible also in deep magnetograms. This picture is supported also
  by recent results with Mn lines with strong coupling with its hyperfine
  structure and of center-to-limb histograms of Zeeman amplitudes.

Title: Chromospheric reversals in the emergence of an ephemeral region
Authors: Sainz Dalda, A.; López Ariste, A.
Bibcode: 2007A&A...469..721S
Altcode:
  Context: The behaviour of both ephemeral regions and moving magnetic
  features has been often described for the photospheric layer, but
  not for the chromosphere. Both magnetic structures are related to the
  decay of active regions, but their actual role is not yet clear. <BR
  />Aims: Our aim is to observe and understand the behaviour of these
  structures in the chromosphere. <BR />Methods: We performed simultaneous
  photospheric and chromospheric spectropolarimetric observations of an
  ephemeral region and a moving magnetic feature. A new code developed
  for the reduction of spectropolarimetric data of several wavelengths
  observed simultaneously was used here for the first time and will be
  described here for future reference. The resulting Stokes profiles are
  analysed in detail. <BR />Results: The Stokes V profiles of Fe i 6301
  and 6302 Å are reversed in polarity with respect to the chromospheric
  Ca ii 8498 and 8542 Å in the cases shown. They reveal a reversed
  magnetic field topology between these layers for both structures. The
  time evolution of the ephemeral region results in a cancellation of
  the chromospheric signal, while the ratio of Stokes V amplitudes of
  Fe i 6301 and 6302 Å simultaneously informs us of a strengthening of
  the photospheric field. A scenario of the evolution of the ephemeral
  region is consequently suggested. <BR />Conclusions: The moving magnetic
  feature reveals itself as an exclusively photospheric feature, similar
  to the ephemeral region in the initial stages of its emergence. As
  the loop emerges into the chromosphere it is slowed down and does not
  reach the corona. Most probably, the opposite polarity background in
  the chromosphere prevents ascent into corona.

Title: Unveiling the Magnetic Field Topology of Prominences
Authors: López Ariste, A.; Aulanier, G.
Bibcode: 2007ASPC..368..291L
Altcode:
  A renewed effort is being dedicated to infer magnetic fields in
  prominences through spectropolarimetry of emission lines like the He
  D<SUB>3</SUB> and 10830 Å lines. We will briefly review those present
  observational efforts pointing at how do they improve or disregard past
  attempts to measure magnetic fields in prominences and at how they try
  to answer some of the standing questions from theoretical models. A
  brief and gentle introduction to the Hanle effect and other subtle
  quantum phenomena is given, as they are key to present diagnostic
  techniques and, finally, an overview of future observational goals
  and wishes is presented.

Title: Spectrophotometry of Ellerman Bombs with THEMIS
Authors: Pariat, E.; Schmieder, B.; Berlicki, A.; López Ariste, A.
Bibcode: 2007ASPC..368..253P
Altcode:
  During coordinated campaigns with THEMIS and space missions (TRACE,
  SOHO) emerging flux was observed in multi-wavelengths. Ellerman bombs
  (EBs) have been identified in TRACE 1600 Å and in chromospheric
  lines. The Hα and Ca II 8542 Å lines present two enhanced emission
  peaks respectively at 1 Å and at 0.35 Å which are signatures of
  EBs. Vector magnetic field measured in the photosphere are consistent
  with previous results indicating the presence of bald-patches under
  the EBs and consequently the emergence of horizontal flux tubes from
  below the photosphere.

Title: The Intrinsic Dimensionality of Spectropolarimetric Data
Authors: Asensio Ramos, A.; Socas-Navarro, H.; López Ariste, A.;
   Martínez González, M. J.
Bibcode: 2007ApJ...660.1690A
Altcode: 2007astro.ph..1604A
  The amount of information available in spectropolarimetric data is
  estimated. To this end, the intrinsic dimensionality of the data is
  inferred with the aid of a recently derived estimator based on nearest
  neighbor considerations and obtained applying the principle of maximum
  likelihood. We show in detail that the estimator correctly captures the
  intrinsic dimension of artificial data sets with known dimension. The
  effect of noise in the estimated dimension is analyzed thoroughly,
  and we conclude that it introduces a positive bias that needs to be
  accounted for. Real simultaneous spectropolarimetric observations
  in the visible 630 nm and the near-infrared 1.5 μm spectral regions
  are also investigated in detail, showing that the near-infrared data
  set provides more information of the physical conditions in the solar
  atmosphere than the visible data set. Finally, we demonstrate that the
  amount of information present in an observed data set is a monotonically
  increasing function of the number of available spectral lines.

Title: A Near-Infrared Line of Mn I as a Diagnostic Tool of the
    Average Magnetic Energy in the Solar Photosphere
Authors: Asensio Ramos, A.; Martínez González, M. J.; López Ariste,
   A.; Trujillo Bueno, J.; Collados, M.
Bibcode: 2007ApJ...659..829A
Altcode: 2006astro.ph.12389A
  We report on spectropolarimetric observations of a near-IR line of Mn I
  located at 15262.702 Å whose intensity and polarization profiles are
  very sensitive to the presence of hyperfine structure. A theoretical
  investigation of the magnetic sensitivity of this line uncovers several
  interesting properties. The most important one is that the presence
  of strong Paschen-Back perturbations due to the hyperfine structure
  produces an intensity line profile whose shape changes according to the
  absolute value of the magnetic field strength. A line ratio technique is
  developed from the intrinsic variations of the line profile. This line
  ratio technique is applied to spectropolarimetric observations of the
  quiet solar photosphere in order to explore the probability distribution
  function of the magnetic field strength. Particular attention is given
  to the quietest area of the observed field of view, which was encircled
  by an enhanced network region. A detailed theoretical investigation
  shows that the inferred distribution yields information on the average
  magnetic field strength and on the spatial scale at which the magnetic
  field is organized. A first estimation gives ~250 G for the mean field
  strength and a tentative value of ~0.4" for the spatial scale at which
  the observed magnetic field is horizontally organized.

Title: Determination of field strengths in the quiet Sun
Authors: López Ariste, A.; Martínez González, M. J.; Ramírez
   Vélez, J. C.
Bibcode: 2007A&A...464..351L
Altcode:
  The magnetism in the quiet regions of the solar photosphere carries
  information on the dynamo processes and its interaction with the
  convection of the outer layers of the sun. Unfortunately, the scales
  of the magnetic structures on these regions are mostly unresolved. It
  is therefore instrumental to tell apart the intrinsic field strengths
  in those regions from the flux through the resolution element. This
  disentanglement has been far from obvious, leading to opposing views
  of the magnetic topology in the unresolved structures of the quiet
  Sun. Our study contributes to the disentanglement of field strength from
  flux in the quiet Sun, at least, through the use of new observational
  constrains in the form of spectropolarimetry of Mn i lines observed
  in the solar spectrum.The chosen Mn lines present a strong coupling
  with hyperfine structure resulting in spectral features, present or
  absent as a function of field strength alone. We observe one of those
  lines simultaneously and co-spatially with the Fe i lines at 630 nm,
  at the core of the previous measurements.The inversion of the observed
  Fe lines results in either strong or weak fields depending on the
  initializations of the inversion algorithm. All the solutions show
  nevertheless equally good values for the σ parameter and are therefore
  equally valuable as solutions. The Mn however selects unambiguously
  strong or weak fields, sometimes agreeing with the inversions of the Fe
  lines, but half the time disagreeing with them.The Fe i lines at 630 nm,
  in the conditions found in the quiet Sun, carry no binding information
  on field strength. A proper analysis of quiet Sun magnetism should
  necessarily pass through its simultaneous and co-spatial observation
  with other lines imposing constraints on field strength, as the Mn i
  lines here analyzed. Ultimately, the magnetic topology of the quiet Sun
  shall arise from the coherent analysis of all these lines, sensitive
  to the Zeeman effect.

Title: Magnetic strength analysis in the quiet Sun regions using
    the Mn I line at 553 nm.
Authors: Ramírez Vélez, J. C.; López Ariste, A.
Bibcode: 2007MmSAI..78...54R
Altcode:
  In this work we present a quantitative analysis of the magnetic field
  stength in quiet Sun regions using Mn I lines. The hyperfine structure
  (HFS) of this atom results for its 553 nm line in a change of profile if
  the magnetic strength is inferior to one kiloGauss. Spectropolarimetric
  data obtained at the THEMIS telescope is presented and analysed. With
  this purpose, we have developped and tested an inversion code based on
  a Milne-Eddington atmospheric model. Close to half the total amount of
  data shows enough signal in circular polarization (V) to be inverted. We
  have found that quiet sun regions are dominated by weak magnetic
  strengths (hG) but that strong intensities (kG) are not absent, being
  these last ones close to one quarter of the total of inverted profiles.

Title: Using reduction and inversion tools for THEMIS-MTR data:
    chromospheric reversals of a moving magnetic feature and an ephemeral
    region .
Authors: Sainz Dalda, A.; López Ariste, A.
Bibcode: 2007MmSAI..78..154S
Altcode:
  New tools have been developed for THEMIS spectropolarimetric data. In
  this paper we present how these tools work and can be used in order
  to understand two interesting observed phenomena: a moving magnetic
  feature and an ephemeral region.

Title: THEMIS observational hits in 2006.
Authors: López Ariste, A.
Bibcode: 2007MmSAI..78...27L
Altcode:
  We describe some of the most remarkable observational results reached
  by THEMIS in 2006, stressing the instrumental improvements that
  have allowed them, in particular the arrival of the new IXON CCD
  detectors, and the T3 tip-tilt system. The selected observations are
  the measurement of Na emission in Mercury's exosphere during daylight,
  the polarimetry at 10<SUP>-5</SUP> on the 2nd solar spectrum, the
  observation of the spatial variation over the photosphere in the
  resonance scattering polarization of the SrI line and the simultaneous
  observation of the two He lines D_3 and 10830 over prominences.

Title: Spectral Line Selection for HMI
Authors: Norton, A. A.; Pietarila Graham, J. D.; Ulrich, R. K.;
   Schou, J.; Tomczyk, S.; Liu, Y.; Lites, B. W.; López Ariste, A.;
   Bush, R. I.; Socas-Navarro, H.; Scherrer, P. H.
Bibcode: 2006ASPC..358..193N
Altcode:
  We present information on two spectral lines, Fe I 6173 Å and Ni I 6768
  Å, that were candidates for use in the Helioseismic and Magnetic Imager
  (HMI) instrument. Both Fe I and Ni I profiles have clean continuum and
  no blends that threaten performance. The higher Landé factor of Fe
  I means its operational velocity range in regions of strong magnetic
  field is smaller than for Ne I. Fe I performs better than Ni I for
  vector magnetic field retrieval. Inversion results show that Fe I
  consistently determines field strength and flux more accurately than
  the Ni I line. Inversions show inclination and azimuthal errors are
  recovered to ≈2° above 600 Mx/cm<SUP>2</SUP> for Fe I and above
  1000 Mx/cm<SUP>2</SUP> for Ni I. The Fe I line was recommended, and
  ultimately chosen, for use in HMI.

Title: Quiet-Sun Magnetism Seen with a Mn Line: Km-Sized Magnetic
    Structures
Authors: López Ariste, A.; Ramírez Vélez, J. C.; Tomczyk, S.;
   Casini, R.; Semel, M.
Bibcode: 2006ASPC..358...54L
Altcode:
  We observed Manganese lines with large hyperfine structure and used them
  to disentangle strength from flux in the measurement of photospheric
  magnetic fields. In observations of the quiet sun with both ASP and
  THEMIS, we measure flux from the amplitude of Stokes V in Fe lines, and
  the Mn line, crudely analyzed, places the field strength either above
  or below a threshold of 600 G, which is set by the atomic structure. In
  the case of THEMIS observations, having determined magnetic flux and
  field strength for every pixel, one can estimate filling factors of
  the magnetic field and determine characteristic scales. Structures at
  scales smaller than 50 km are revealed.

Title: Inference of the Magnetic Field in Spicules from
    Spectro-Polarimetry of He I D<SUB>3</SUB>
Authors: López Ariste, A.; Casini, R.
Bibcode: 2006ASPC..358..443L
Altcode:
  Spectro-polarimetric observations of the He I D<SUB>3</SUB> line in
  spicules over the solar limb have been analyzed, and the magnetic field
  direction determined. Up to a 90° ambiguity the field appears to be
  aligned with the visible spicular structures. The anomalous broadening
  of the observed line prevents the retrieval of other information at
  this stage. The only further constraint we were able to place was on
  the field strength: fields stronger than 40 G must be statistically
  present in our observations, but not much stronger than that.

Title: First observation of bald patches in a filament channel and
    at a barb endpoint
Authors: López Ariste, A.; Aulanier, G.; Schmieder, B.; Sainz
   Dalda, A.
Bibcode: 2006A&A...456..725L
Altcode:
  The 3D magnetic field topology of solar filaments/prominences is
  strongly debated, because it is not directly measureable in the
  corona. Among various prominence models, several are consistent
  with many observations, but their related topologies are very
  different. We conduct observations to address this paradigm. We
  measure the photospheric vector magnetic field in several small
  flux concentrations surrounding a filament observed far from disc
  center. Our objective is to test for the presence/absence of magnetic
  dips around/below the filament body/barb, which is a strong constraint
  on prominence models, and that is still untested by observations. Our
  observations are performed with the THEMIS/MTR instrument. The four
  Stokes parameters are extracted, from which the vector magnetic fields
  are calculated using a PCA inversion. The resulting vector fields
  are then deprojected onto the photospheric plane. The 180° ambiguity
  is then solved by selecting the only solution that matches filament
  chirality rules. Considering the weakness of the resulting magnetic
  fields, a careful analysis of the inversion procedure and its error
  bars was performed, to avoid over-interpretation of noisy or ambiguous
  Stokes profiles. Thanks to the simultaneous multi-wavelength THEMIS
  observations, the vector field maps are coaligned with the Hα image of
  the filament. By definition, photospheric dips are identifiable where
  the horizontal component of the magnetic field points from a negative
  toward a positive polarity. Among six bipolar regions analyzed in the
  filament channel, four at least display photospheric magnetic dips,
  i.e. bald patches. For barbs, the topology of the endpoint is that of
  a bald patch located next to a parasitic polarity, not of an arcade
  pointing within the polarity. The observed magnetic field topology in
  the photosphere tends to support models of prominence based on magnetic
  dips located within weakly twisted flux tubes. Their underlying and
  lateral extensions form photospheric dips both within the channel and
  below barbs.

Title: Quiet sun magnetic field diagnostics with a Mn line
Authors: López Ariste, A.; Tomczyk, S.; Casini, R.
Bibcode: 2006A&A...454..663L
Altcode:
  Context: .The Mn line at 553 nm shows strong spectral features in both
  intensity and polarization profiles due to the hyperfine structure of
  the atom. These features, their presence or absence, are known to be
  dependent on the magnetic regime to which the Mn atom is subject.<BR
  /> Aims: .Our objective is to disentangle strong kilo-Gauss (kG)
  fields from relatively weak hecto-Gauss (hG) fields in the quiet sun,
  and compute relative filling factors on the resolution element.<BR />
  Methods: .We observed the 553 nm Mn line in a quiet sun area with the
  Advanced Stokes Polarimeter, and we introduce an in-line ratio between
  different spectral features. Filling factors can be retrieved from
  the measurement of this ratio and the total longitudinal flux.<BR />
  Results: .In the photospheric network the kG dominate the magnetic flux,
  although out of the higher concentration areas the hG fields dominate in
  surface coverage. For the internetwork (granules and lanes confounded)
  the hG are dominant both in surface and total flux. <BR />

Title: Inversion of solar magnetic fields in photospheric quiet
    regions.
Authors: Ramírez Vélez, J. C.; López Ariste, A.; Semel, M.
Bibcode: 2006IAUJD...3E..13R
Altcode:
  Spectropolarymetric observational data and the correspondent
  magnetic field inversion of the solar photosphere in quiet regions are
  presented. The presence of atomic coupling between the nuclei momentum
  (I) and total electron momentum ( J[LS]), known as hyperfine structure
  atomic regime where the total momentum F = I + J, was founded in the
  observed data for the atom MnI at 553.7 nm. The signature of these
  hyperfine structure is clearly detected in circular polarisation in form
  of protuberance in the central part of the profile. We have developed
  an inversion code of the magnetic field using the Principal Component
  Analysis (PCA). An statistical test is applied to the code obtaining
  satisfactory inversion results. The inversion of the observed profiles,
  using the circular polarisation (V) and the intensity (I), shows mixed
  regions with values of |B| higher than 800 Gauss and lower than 600
  Gauss associated with the network and internetwork respectively.

Title: The THEMIS solar telescope in 2006: situation and perspectives
Authors: Gelly, B.; López Ariste, A.
Bibcode: 2006sf2a.conf..539G
Altcode:
  Recent technical improvements like a new image stabilizer, coupled to a
  set of comprehensive progress in data analysis and inversions software
  and have quite significantly raised the level of performances achieved
  at THEMIS. Spectropolarimetry at a relative precision of 10<SUP>-5</SUP>
  is now accessible, opening whole new fields of discovery, and exceeding
  by far the original specs of 10<SUP>-3</SUP>. In addition, and despite
  a lack of background, speckle imaging has proven to be possible and
  of obvious interest to couple with the current spectropolarimetry
  capabilities.

Title: Magnetic Fields Measured in Spicules.
Authors: López Ariste, A.; Casini, R.
Bibcode: 2005ESASP.596E..21L
Altcode: 2005ccmf.confE..21L
  No abstract at ADS

Title: Observational Evidence of Photoshperic Magnetic Dips in
    Filament Channels
Authors: Schmieder, B.; López Ariste, A.; Aulanier, G.
Bibcode: 2005ESASP.596E..19S
Altcode: 2005ccmf.confE..19S
  No abstract at ADS

Title: Inference of the magnetic field in spicules from
    spectropolarimetry of He I D3
Authors: López Ariste, A.; Casini, R.
Bibcode: 2005A&A...436..325L
Altcode:
  We present observations of spicules in the He I D3 line with full-Stokes
  spectropolarimetry, which were done with the Advanced Stokes Polarimeter
  at the Dunn Solar Telescope of the Sacramento Peak Observatory. The line
  profiles appear to be significantly broadened by non-thermal processes,
  which we interpret using the hypothesis of a distribution of velocities
  inside the spicule. The possibility of inferring the magnetic field
  in those conditions is tested on synthetic data, and the results are
  generalized to the interpretation of the observed data. We conclude
  that the magnetic field is aligned with the visible structure of the
  spicule, with strengths above 30 G in some cases (for heights between
  3000 and 5000 km above the photosphere).

Title: Principal Component Analysis of the He I D<SUB>3</SUB>
    Polarization Profiles from Solar Prominences
Authors: Casini, R.; Bevilacqua, R.; López Ariste, A.
Bibcode: 2005ApJ...622.1265C
Altcode:
  In this paper we discuss some general aspects of Stokes profile
  inversion in prominences using principal component analysis
  (PCA). First, we address the problem of departure of the formation of
  He I D<SUB>3</SUB> from the optically thin approximation, dealing in
  a heuristic way with the inversion of the two visible components of
  the line, which are differently affected by radiative transfer. This
  modified inversion does not alter recently published results on
  the magnetic structure of quiescent prominences, in particular
  confirming the presence of magnetic fields significantly larger than
  average in limited regions. Second, we investigate the effect of
  line-of-sight integration of the polarization signals from different
  magnetic configurations, to determine whether PCA inversion could
  interpret an observed signal in terms of a single magnetic model
  with unrealistically large field strength. Our results show that this
  possibility is not statistically significant, and in addition the few
  models that are wrongly inverted show very peculiar geometries that
  are not found in our inversion of real data. This result also supports
  the reality of stronger than average fields in prominences. Finally,
  we further investigated the importance of full Stokes inversion for
  a correct diagnostic of magnetic fields in prominences. We show how
  the exclusion of Stokes V induces the appearance of a 90° ambiguity
  in the position angle of the magnetic field in the plane of the sky
  (in addition to the well-known 180° ambiguity). Inclusion of Stokes V
  proves to be important at all field strengths, even though Stokes Q and
  U should in theory be sufficient to retrieve the complete vector field
  information for field strengths in the Hanle depolarization regime. We
  also show how the inversion noise determined by the discreteness of
  the PCA database may be an additional source of the 90° ambiguity,
  even in the presence of Stokes V, therefore imposing a lower bound on
  the model density of the database.

Title: Full Stokes Spectropolarimetry of Hα in Prominences
Authors: López Ariste, A.; Casini, R.; Paletou, F.; Tomczyk, S.;
   Lites, B. W.; Semel, M.; Landi Degl'Innocenti, E.; Trujillo Bueno,
   J.; Balasubramaniam, K. S.
Bibcode: 2005ApJ...621L.145L
Altcode:
  We report on spectropolarimetric observations of Hα in prominences
  made with the Télescope Héliographique pour l'Etude du Magnétisme et
  des Instabilités Solaires and the High Altitude Observatory/Advanced
  Stokes Polarimeter. Stokes Q and U show the expected profile shape from
  resonance scattering polarization and the Hanle effect. In contrast,
  most of the time, Stokes V does not show the antisymmetric profile shape
  typical of the Zeeman effect but a profile that indicates the presence
  of strong atomic orientation in the hydrogen levels, to an extent that
  cannot be explained by invoking the alignment-to-orientation transfer
  mechanism induced by the prominence magnetic field. We found that the
  largest signal amplitudes of Stokes V (comparable to that of Stokes
  Q and U) could be produced by a process of selective absorption of
  circularly polarized radiation from the photosphere, which requires
  that the prominence be in the vicinity of an active region. Although
  recent observations of active region filaments indicate such a
  selective absorption mechanism as a plausible explanation of the
  anomalous signals observed, the particular set of conditions that
  must be met suggest that a different explanation may be required
  to explain the almost ubiquitous symmetric V signal observed in Hα
  prominences. Therefore, we speculate that an alternative mechanism
  inducing strong atomic orientation at the observed level could be due
  to the presence of electric fields inducing an electric Hanle effect
  on Hα. Although we are still working toward a careful modeling of
  this effect, including both electric and magnetic fields, we present
  some preliminary considerations that seem to support this possibility.

Title: PCA-Interpolation Methods for Inversion of Solar Stokes
    Profiles. I. Inversion of Photospheric Profiles
Authors: Eydenberg, M. S.; Balasubramaniam, K. S.; López Ariste, A.
Bibcode: 2005ApJ...619.1167E
Altcode:
  We present a method for the analysis of solar Stokes profiles based on
  the technique of principal component analysis (PCA). We consider the
  development of a PCA/interpolation technique for obtaining estimates of
  the solar magnetic field vector when the underlying physical parameter
  distribution is not well determined. Unlike previous applications
  of PCA, we consider separate distributions of the PCA components as
  applied to the source of the underlying plasma, namely, the umbra,
  penumbra, and quiet photosphere. We also isolate the PCA for the Fe I
  λ6301.5 line. In some previous work, PCA was applied to the entire
  6300-6305 Å spectral region that includes the telluric lines. The
  method is tested on photospheric (Fe I λ6301.5) observations of NOAA
  Active Region 9240 and compared with the results of the traditional
  High Altitude Observatory Milne-Eddington inversion code. We also
  consider a PCA-perturbation analysis of the data and how it can assist
  the interpretation of the results, discuss current challenges faced by
  the inversion codes, and suggest important areas for future development.

Title: An orthonormal set of Stokes profiles
Authors: del Toro Iniesta, J. C.; López Ariste, A.
Bibcode: 2003A&A...412..875D
Altcode:
  A family of well-known orthonormal functions, the set of Hermite
  functions, is proposed as a suitable basis for expanding the
  Stokes profiles of any spectral line. An expansion series thus
  provides different degrees of approximation to the Stokes spectrum,
  depending on the number of basis elements used (or on the number
  of coefficients). Hence, an usually large number of wavelength
  samples, may be substituted by a few such coefficients, thus reducing
  considerably the size of data files and the analysis of observable
  information. Moreover, since the set of Hermite functions is an
  universal basis, it promises to help in modern inversion techniques
  of the radiative transfer equation that infer the solar physical
  quantities from previously compiled look-up tables or artificial neural
  networks. These features appear to be particularly important in modern
  solar applications producing huge amounts of spectropolarimetric data
  and on near-future, on-line applications aboard spacecrafts.

Title: Magnetic maps of prominences
Authors: Casini, R.; Lopez Ariste, A.; Tomczyk, S.; Lites, B.
Bibcode: 2003AGUFMSH42D..05C
Altcode:
  We present the first magnetic maps of a prominence obtained by applying
  our PCA inversion approach to prominence spectropolarimetric data
  in the He I D3 line. Our results indicate the presence of organized
  structures in the prominence plasma embedded in magnetic field that are
  significantly larger than average (50 G and higher). We reaffirm the
  need for a Hanle-based diagnostics of prominence magnetism using full
  Stokes spectropolarimetry, and the importance of improved, multi-line
  observations, ideally involving both He I D3 and 10830.

Title: Dynamo processes and activity cycles of the active stars AB
    Doradus, LQ Hydrae and HR 1099
Authors: Donati, J. -F.; Collier Cameron, A.; Semel, M.; Hussain,
   G. A. J.; Petit, P.; Carter, B. D.; Marsden, S. C.; Mengel, M.;
   López Ariste, A.; Jeffers, S. V.; Rees, D. E.
Bibcode: 2003MNRAS.345.1145D
Altcode:
  In this paper, we present new brightness and magnetic surface images of
  the young K0 dwarfs AB Doradus and LQ Hydrae, and of the K1 subgiant
  of the RS CVn system HR 1099 (=V711 Tauri), reconstructed from
  Zeeman-Doppler imaging spectropolarimetric observations collected
  at the Anglo-Australian Telescope during five observing campaigns
  (totalling 50 nights), from 1998 January to 2002 January. Along with
  the older images of the same stars (published in previous papers),
  our complete data set represents the first long-term series on temporal
  fluctuations of magnetic topologies of very active stars. <P />All of
  the magnetic images presented here indicate that large regions with
  predominantly azimuthal magnetic fields are continuously present at
  the surfaces of these stars. We take this as further evidence that the
  underlying dynamo processes that produce them are probably distributed
  throughout the entire convective zone (and not confined at its base,
  as in the Sun). We speculate that the radial and azimuthal field
  maps that we recover correspond, respectively, to the poloidal and
  toroidal components of the large-scale dynamo field. <P />We find, in
  particular, that some signatures, for instance the relative fraction
  of magnetic energy stored in the large-scale poloidal and toroidal
  field components, and the polarity of the axisymmetric component of
  the field, are variable with time, and provide potentially fruitful
  diagnostics for investigating magnetic cycles in active stars other
  than the Sun. We report here the detection of partial polarity switches
  in some of the axisymmetric field components of two of our programme
  stars (AB Dor and LQ Hya), suggesting that the dynamo operating in
  these stars may be cyclic.

Title: Magnetic Maps of Prominences from Full Stokes Analysis of
    the He I D3 Line
Authors: Casini, R.; López Ariste, A.; Tomczyk, S.; Lites, B. W.
Bibcode: 2003ApJ...598L..67C
Altcode:
  We present the first magnetic maps of a prominence, derived from
  inversion of spectropolarimetric data in He I D3 using the principal
  component analysis of all four Stokes profiles. This prominence,
  along with several others, was observed in 2002 May using the Dunn
  Solar Telescope of the National Solar Observatory/Sacramento Peak
  Observatory, equipped with the High Altitude Observatory Advanced
  Stokes Polarimeter. The use of an unocculted instrument allowed us to
  map the prominence magnetic fields down to the chromospheric limb. Our
  analysis indicates that the average magnetic field in prominences
  is mostly horizontal and varies between 10 and 20 G, thus confirming
  previous findings. However, our maps show that fields significantly
  stronger than average, even as large as 60 or 70 G, can often be found
  in clearly organized plasma structures of the prominence.

Title: The effect of atomic polarization on early estimates of
    prominence magnetic field strengths
Authors: Brown, A.; López Ariste, A.; Casini, R.
Bibcode: 2003SoPh..215..295B
Altcode:
  In this paper we present our results of the application of the
  magnetograph formula to synthetic Stokes V profiles in prominences. We
  investigate both the Zeeman and atomic-polarization signatures within
  the V profile and question why previous attempts to determine magnetic
  field strength, on average, were correct, even if the magnetograph
  formula does not apply in general.

Title: Polarimeter for the study of magnetic fields in prominences
Authors: Lopez Ariste, Arturo; Tomczyk, Steven; Semel, Meir;
   Darnell, A.
Bibcode: 2003SPIE.4853..235L
Altcode:
  We report on a new spectropolarimeter for the measurement of vector
  magnetic fields on prominences using the HeD<SUB>3</SUB> line in the
  Evans Solar Facility at the Sacramento Peak Observatory.

Title: Quick-Look Polarization Inversion Using PCA Decomposition. The
    Effect of Telluric Lines
Authors: Skumanich, A.; Toledano, O.; López Ariste, A.
Bibcode: 2003ASPC..286...53S
Altcode: 2003ctmf.conf...53S
  No abstract at ADS

Title: On the Invariance of PCA Quick-Look Polarization Analysis to
    Different Magnetic Activity Samples
Authors: Skumanich, A.; López Ariste, A.
Bibcode: 2003ASPC..307..109S
Altcode:
  No abstract at ADS

Title: Hyperfine Structure as a Diagnostic Tool of Solar Magnetic
    Fields
Authors: López Ariste, A.; Tomczyk, S.; Casini, R.
Bibcode: 2003ASPC..307..115L
Altcode:
  No abstract at ADS

Title: The Helioseismic and Magnetic Imager (HMI) on SDO: Full Vector
    Magnetography with a Filtergraph Polarimeter
Authors: Graham, J. D.; Norton, A.; López Ariste, A.; Lites, B.;
   Socas-Navarro, H.; Tomczyk, S.
Bibcode: 2003ASPC..307..131G
Altcode:
  No abstract at ADS

Title: PCA Inversion of Stokes Profiles in Solar Prominences
Authors: Casini, R.; López Ariste, A.
Bibcode: 2003ASPC..307..109C
Altcode:
  No abstract at ADS

Title: The Advanced Technology Solar Telescope
Authors: Paletou, F.; López Ariste, A.; Meunier, N.; Molodij, G.
Bibcode: 2003sf2a.conf...97P
Altcode: 2003sf2a.confE..30L
  The Advanced Technology Solar Telescope (ATST) is a 4-m aperture solar
  telescope project lead by the NSO and supported by all major solar US
  research institutes. Also a European initiative gathering 17 countries,
  so far, is currently being prepared in support of the project. ATST,
  intended for first light in 2010, will provide the high-angular
  resolution required for nowadays photospheric studies; but its large
  aperture and controlled scattered-light design makes it also a very
  promising tool for high sensitivity spectropolarimetric observations
  needed throughout the whole solar atmosphere up to the corona, and in
  spectral windows ranging from the visible to the thermal infrared.

Title: Improved Estimate of the Magnetic Field in a Prominence
Authors: López Ariste, A.; Casini, R.
Bibcode: 2003ApJ...582L..51L
Altcode:
  We present new results on the inversion of spectropolarimetric data
  of the He I D3 line in a prominence. These data were taken with the
  High Altitude Observatory Stokes II polarimeter on 1980 August 5. Full
  Stokes profiles in I, Q, U, and V were recorded at 13 locations of a
  prominence at the northeast limb. This prominence erupted 3 hr later,
  giving rise to the day of year 218 Solar Maximum Mission coronal
  transient. The data were originally analyzed in a paper by Querfeld
  et al. Here we analyze the data using the new inversion algorithm
  proposed by López Ariste &amp; Casini based on pattern recognition
  techniques. This method employs full Stokes profiles, and in tests
  on synthetic data, it proved to be much more precise than previous
  diagnostic procedures. When applied to the Stokes data of the prominence
  under study, it gives magnetic field strengths at least 2 times larger
  than previous estimates.

Title: Estimating the Effects of JPEG Compression and Radiation on
    the Accuracy of Vector Magnetic Fields Measurements for Solar-B
Authors: Lites, B.; Shine, R. A.; López Ariste, A.; Tarbell, T. D.
Bibcode: 2002AGUFMSH52A0471L
Altcode:
  The Japanese Solar-B satellite, currently scheduled for launch in
  September 2005, includes a spectro-polarimeter (SP) to precisely measure
  the full Stokes polarization vector (I,Q,U,V) in the Fe I lines at
  6302Å. These will be processed to produce vector magnetograms of the
  solar surface using algorithms based on those for the Advanced Stokes
  Polarimeter (ASP) as described in Skumanich, et al, 1997, ApJ Suppl
  110. Accumulations of the raw images into time averaged I,Q,U,V images
  will be done on board and the results will be 12 bit JPEG compressed to
  make the best use of the available telemetry. Hence a single radiation
  hit in a raw image affects the entire time average at that point. Also,
  radiation spikes affect JPEG compression performance. Because of
  concerns about these effects, we simulated them separately and in
  combination using ASP data and radiation level measurements from the
  TRACE satellite. Like TRACE, Solar-B will fly in a high inclination,
  sun synchronous orbit and be exposed to radiation from the polar
  radiation belts as well as the SAA. Since the SP detector will be
  better shielded than that on TRACE, we hope that these will be an over
  estimate of the effects. The results from the simulations are very
  encouraging. We find that for active region magnetic fields we can use
  JPEG to compress the data volume by more than a factor of 10 without
  compromising the accuracy of the inferred magnetic field vector. The
  radiation in the polar regions has little effect and even the much
  stronger SAA radiation causes average perturbations that are less than
  the formal errors for sunspot fields and about twice the formal errors
  for plage fields. However, very weak field measurements will benefit
  from less lossy compression and periods of low radiation. Of course,
  the very strong radiation hits always produce artifacts. Compression
  performance is affected only slightly so it will not be necessary to
  avoid observations in the SAA because of excessive telemetry usage. This
  work was supported by NASA contract NAS8-01002.

Title: Hyperfine Structure as a Diagnostic of Solar Magnetic Fields
Authors: López Ariste, A.; Tomczyk, S.; Casini, R.
Bibcode: 2002ApJ...580..519L
Altcode:
  We propose the use of the Zeeman effect of the hyperfine structure
  (HFS) as a diagnostic of solar photospheric magnetic fields. The HFS
  induces unique signatures in the polarized spectra of particular atoms,
  which can be used to directly get information on the magnetic field
  strength. To explore the effects of HFS, we adopt a numerical model of
  line formation and radiative transfer of polarized light in a magnetized
  Milne-Eddington atmosphere. We concentrate our studies on several lines
  of Mn I, which show particularly striking HFS signatures in the solar
  atlas. For these lines, anomalous Stokes profiles are produced for
  fields in the range 0-1000 G. The presence of these unusual profiles
  for weak magnetic fields constitutes a novel and potentially powerful
  diagnostic of the strength of solar magnetic fields in the quiet
  photosphere and plages.

Title: Magnetic Fields in Prominences: Inversion Techniques for
    Spectropolarimetric Data of the He I D<SUB>3</SUB> Line
Authors: López Ariste, A.; Casini, R.
Bibcode: 2002ApJ...575..529L
Altcode:
  We propose the use of principal component analysis (PCA) to invert
  spectropolarimetric data from prominences. Observation of the
  full Stokes profiles in prominences is very important for a deeper
  understanding of magnetic-field topology in these solar structures,
  and for the testing of theoretical models. The line formation problem,
  however, is complicated by the special conditions of prominences:
  anisotropy of light, low magnetic intensities, temperature and density
  ranges, etc. We created a code to solve this problem in the limit of
  optically thin plasma and of a collisionless regime, and use it in
  combination with PCA techniques to invert synthetic data. The results
  show that inversion is feasible.

Title: Inference of Solar Magnetic Field Parameters from Data with
    Limited Wavelength Sampling
Authors: Graham, Jonathan D.; López Ariste, Arturo; Socas-Navarro,
   Hector; Tomczyk, Steven
Bibcode: 2002SoPh..208..211G
Altcode:
  We investigate the diagnostic potential of polarimetric measurements
  with filtergraph instruments. Numerical simulations are used to explore
  the possibility of inferring the magnetic field vector, its filling
  factor, and the thermodynamics of model atmospheres when only a few
  wavelength measurements are available. These simulations assume the
  magnetic Sun to be represented by Milne-Eddington atmospheres. The
  results indicate that two wavelength measurements are insufficient
  to reliably determine the magnetic parameters, regardless of whether
  magnetograph techniques or least-squares fitting inversions are
  used. However, as few as four measurements analyzed with the inversion
  technique provide enough information to retrieve the intrinsic magnetic
  field with an accuracy better than 10% in most cases.

Title: Inference of Solar Vector Magnetic Fields with Filtergraph
    Instruments
Authors: Graham, J. D.; Lites, B. W.; López Ariste, A.; Norton, A.;
   Socas-Navarro, H.; Tomczyk, S.
Bibcode: 2002AAS...200.5611G
Altcode: 2002BAAS...34..736G
  We investigate the diagnostic potential of polarimetric measurements
  with filtergraph instruments. Numerical simulations are used to
  explore the possibility of inferring the magnetic field vector, its
  filling factor, and the thermodynamics of model atmospheres when
  only a few wavelength measurements are available. The simulations
  assume the magnetic sun to be represented by Milne-Eddington (ME)
  atmospheres. The results indicate that two wavelength measurements
  are insufficient to reliably determine the magnetic parameters,
  regardless of whether magnetograph techniques or least-squares fitting
  inversions are used. However, as few as four measurements analyzed
  with the inversion technique provide enough information to retrieve the
  intrinsic magnetic field with an accuracy better than 10 generated by
  more general models and ASP data passed through a simulated filtergraph.

Title: The Physical Content of the Leading Orders of Principal
    Component Analysis of Spectral Profiles
Authors: Skumanich, A.; López Ariste, A.
Bibcode: 2002ApJ...570..379S
Altcode:
  We consider the principal component analysis (PCA) method of
  expanding Stokes intensity and net polarization profiles in terms of
  eigenfunctions (or principal components) of the spectral covariance
  matrix. The expansion is ordered by the magnitude of the relevant
  eigenvalue from largest to smallest. We find that the ordering
  represents a perturbation expansion. This allows us to examine the
  physical content of the first few orders of the basis set for 40,000
  profiles for each Stokes parameter for a solar active region. For the
  intensity profile, we find that the expansion represents a Taylor
  series with the highest ranked, or first, eigenfunction being the
  zeroth order term, the second as the (scaled) first derivative of
  the zeroth term, and the third as the (scaled) second derivative
  term. Thus, we can derive a velocity from the coefficients of the
  first derivative term and a magnetic splitting parameter from those
  of the second using the standard velocity perturbation and weak-field
  expansion. For the net polarization profiles, we find that the zeroth
  order terms yield, using the weak-field expansion, the vector magnetic
  field. A comparison with a Stokes profile fitting inversion shows
  that the thus-estimated velocity and magnetic parameters are in good
  agreement with the more time-consuming profile fitting values, but
  do show a roll-off, or saturation, for sufficiently large values. We
  also find that the bright quiet-Sun points have an upflow signature,
  while the dark regions have a downflow-one in good agreement with that
  derived by traditional analysis.

Title: On the Universality of the Leading Orders of PCA. Spectral
    Eigenprofiles for the Sun.
Authors: Skumanich, A.; López Ariste, A.
Bibcode: 2002AAS...200.5511S
Altcode: 2002BAAS...34..733S
  In a previous Principal Component Analysis (PCA) of the spectral
  region containing the intensity profiles of the FeI λ630 .15 &amp;
  λ630 .25 nm lines for a solar active region, Ref(1), the issue of the
  universality or invariance of the eigenprofiles (principal components)
  to changes in the magnetic composition of the studied region was
  raised. To address this issue we have performed a series of PCA studies
  in which we incrementally increased the threshold Zeeman polarization
  for the included profiles for a given region. This reduces the relative
  roles of the various components such as the quiet sun and elements of
  the active sun, viz. network, pores, plage and spot. We find that the
  Taylor series representation of the highest ranked eigenprofiles in the
  expansion of the intensity, which yield an estimate of the velocity and
  magnetic splitting (first and second derivative terms respectively,
  Ref(1)), continues to hold. However increased eigenprofile 'mixing'
  occurs. Consequently to obtain eigenprofiles which represent the
  derivatives of the zeroth order eigenprofile one requires significant
  rotations of the basis in the PCA space. The implication for derived
  velocity and magnetic parameters will be discussed. - Skumanich,
  A. &amp; López Ariste, A. 2002, "The Physical Content of the Leading
  Orders of Principal Component Analysis of Spectral Profiles", ApJ
  570. In press

Title: New Polarization Diagnostics for the Solar Atmosphere
Authors: Casini, R.; López Ariste, A.; Tomczyk, S.; Lites, B.
Bibcode: 2002AAS...200.3403C
Altcode: 2002BAAS...34..690C
  We present relatively new diagnostics of ``weak" magnetic fields in the
  solar atmosphere. The first diagnostic is suggested by recent advances
  in the inversion of Stokes profiles of lines formed by resonance
  scattering in the weakly magnetized plasma of prominences (Hanle
  effect and level-crossing; 0 to 100 G). Use of pattern recognition
  techniques (PCA) in this field has marked a sensible progress with
  respect to previous diagnostic procedures. The second diagnostic is
  the modelling of hyperfine structured (HFS) lines that can be observed
  in the spectrum of the quiet photosphere. This allows to investigate
  relatively weak photospheric fields (200 G to 1000 G), in which regime
  the HFS induces peculiar signatures in the Stokes profiles, including
  the appearance of subcomponents and net circular polarization. The third
  diagnostic is suggested by interesting polarization properties of the
  Na I D1 line formed by resonance scattering: the atomic polarization
  in the upper level of D1, which is responsible of a characteristic
  antisymmetric (i.e., V-like) signature in the core of Stokes Q, is
  rapidly suppressed for B &gt; 10 G, irrespective of the magnetic field
  direction. A common denominator of these three diagnostics is their
  sensitivity to the actual strength of the magnetic field, instead
  of the magnetic flux within the resolution element. Another common
  aspect is that all require (or would profit from) high polarization
  sensitivity, which will be one of the strengths of ATST. For the
  diagnostics of prominence magnetic fields, the possibility of multiline
  spectropolarimetry could be decisive. Simultaneous observations of
  He I D3 (5876A) and 10830A, or of He I D3 and the Na I D lines (all
  within a 20A spectral range!), would increase the inversion accuracy
  of PCA. The high spatial resolution capabilities of ATST would be
  advantageous mostly to diagnose weak photospheric fields, already at
  the present time. Because of the complexity of radiative transfer in
  complicated structures like prominences, high spatial resolution in
  these structures is not the highest priority. However, we hope that
  when ATST will become operative, this complicated problem will have
  been attacked succesfully.

Title: On the Asymmetry of Stokes Profiles
Authors: López Ariste, A.
Bibcode: 2002ApJ...564..379L
Altcode:
  We find all the sources of asymmetries in Stokes profiles available for
  a unidimensional line of sight. The analytical solution for the transfer
  equation for polarized light obtained by López Ariste &amp; Semel
  is used for this purpose. Three sources of asymmetries are obtained:
  the first is the well-known case of depth-dependent velocity fields;
  the two others are due to wavelength asymmetries in the absorption
  matrix and the emission vector. These emission vector asymmetries
  can be produced by the presence of a polarized (but symmetric) source
  function. Some formulae are derived describing the asymmetries owing
  to depth-dependent velocity fields in Stokes V for the case of the
  Zeeman effect in a multiplet, as well as for the net area of the
  Stokes V profile, the so-called net circular polarization (NCP). An
  implication of this last formula is that Milne-Eddington atmospheric
  models, even when generalized to velocity and magnetic fields that
  vary linearly with optical depth, cannot account for the observed NCPs.

Title: Pattern recognition techniques and the measurement of solar
    magnetic fields
Authors: Lopez Ariste, Arturo; Rees, David E.; Socas-Navarro, Hector;
   Lites, Bruce W.
Bibcode: 2001SPIE.4477...96L
Altcode:
  Measuring vector magnetic fields in the solar atmosphere using the
  profiles of the Stokes parameters of polarized spectral lines split
  by the Zeeman effect is known as Stokes Inversion. This inverse
  problem is usually solved by least-squares fitting of the Stokes
  profiles. However least-squares inversion is too slow for the new
  generation of solar instruments (THEMIS, SOLIS, Solar-B, ...) which will
  produce an ever-growing flood of spectral data. The solar community
  urgently requires a new approach capable of handling this information
  explosion, preferably in real-time. We have successfully applied pattern
  recognition and machine learning techniques to tackle this problem. For
  example, we have developed PCA-inversion, a database search technique
  based on Principal Component Analysis of the Stokes profiles. Search is
  fast because it is carried out in low dimensional PCA feature space,
  rather than the high dimensional space of the spectral signals. Such
  a data compression approach has been widely used for search and
  retrieval in many areas of data mining. PCA-inversion is the basis of
  a new inversion code called FATIMA (Fast Analysis Technique for the
  Inversion of Magnetic Atmospheres). Tests on data from HAO's Advanced
  Stokes Polarimeter show that FATIMA isover two orders of magnitude
  faster than least squares inversion. Initial tests on an alternative
  code (DIANNE - Direct Inversion based on Artificial Neural NEtworks)
  show great promise of achieving real-time performance. In this paper
  we present the latest achievements of FATIMA and DIANNE, two powerful
  examples of how pattern recognition techniques can revolutionize data
  analysis in astronomy.

Title: Full-Stokes spectropolarimetry of solar prominences
Authors: Paletou, F.; López Ariste, A.; Bommier, V.; Semel, M.
Bibcode: 2001A&A...375L..39P
Altcode:
  We report on first spectropolarimetric observations of solar prominences
  made at THEMIS. Hereafter we shall emphasis on the preliminary analysis
  of He I D<SUB>3</SUB> full-Stokes observations. Our measurements
  clearly show manifestations of both the Hanle and Zeeman effects. More
  generally, we demonstrate the very favourable performances of the THEMIS
  multi-line spectropolarimetric mode for further new investigations of
  the magnetic field in solar prominences. Based on observations made at
  THEMIS, operated on the Island of Tenerife by CNRS-CNR in the Spanish
  Observatorio del Teide of the Instituto de Astrofísica de Canarias.

Title: Fast Inversion of Spectral Lines Using Principal Components
    Analysis. II. Inversion of Real Stokes Data
Authors: Socas-Navarro, H.; López Ariste, A.; Lites, B. W.
Bibcode: 2001ApJ...553..949S
Altcode:
  The principal components analysis (PCA) technique is used to develop
  an extremely fast and stable Stokes inversion code, suitable for
  application to large data sets from instruments that operate under
  standard conditions. This paper provides some tips on reducing the
  dimensionality of the problem, which are then used to develop a
  practical implementation of PCA for the automatic analysis of Stokes
  profiles. The resulting code is tested using real spectropolarimetric
  observations of the pair of Fe I lines at 6302 Å at a sunspot.

Title: The Physical Content of the Leading Orders of Principle
    Component Analysis of Spectral Profiles
Authors: Skumanich, A.; Lopez Ariste, A.
Bibcode: 2001AAS...198.0303S
Altcode: 2001BAAS...33..786S
  We consider the PCA (Principle Component Analysis) method of profile
  analysis introduced by Ref (1). In this method one constructs a
  covariance matrix C<SUB>ij</SUB> given by the product of the profile
  amplitudes at wavelengths λ <SUB>i</SUB> and λ <SUB>j</SUB> averaged
  over all profile samples (either over space or time). The matrix
  is 'diagonalized' and ordered by Singular Value Decomposition. The
  resulting orthonormal eigenfunctions over wavelength space are then
  used as a basis for the expansion of the observed profiles at each
  space(time) point. The eigenvalue ordering is by magnitude of the
  mean square over space(time) of the expansion coefficients and is
  given by the product of the frequency of occurrence of the particular
  eigenfunction and its intrinsic mean square amplitude. An error based
  truncation scheme yields a coefficient set which is a compression of
  the original data set. We have applied the PCA method to the 40,000
  profiles for each Stokes component for a Solar active region. In the
  case of the intensity profile we find that the expansion appears to
  be similar to a Taylor series with the 0th efunction being the zero
  order term, the 1st efunction as the first derivative of the zero term
  and the 2nd efunction as the second derivative term. Thus we derive a
  line-of-sight velocity from the coefficients of the first derivative
  term and a magnetic signature, using the weak field Milne-Eddington
  approximation, from the second. A comparison with a Stokes profile
  inversion shows that the thus estimated velocity and magnetic parameters
  are in good agreement with the more time consuming profile fitting
  values but do show a "roll-off" for sufficiently large values. One also
  finds that the bright 'quiet' Sun points have an upflow while the dark
  have a downflow similar to that derived by other analysis. 1) Rees,
  D., López Ariste, A., Thatcher, J. &amp; Semel, M. 2000, A &amp; A,
  355, 759 The National Center for Atmospheric Research is sponsored by
  the National Science Foundation.

Title: Observation of Linear Polarization in the Infrared Ca II
    Triplet Lines during Umbral Flashes
Authors: López Ariste, A.; Socas-Navarro, H.; Molodij, G.
Bibcode: 2001ApJ...552..871L
Altcode:
  We report on Télescope Héliographique pour l'Etude du Magnétisme et
  des Instabilités Solaires (THEMIS)2 observations of linear polarization
  events associated with umbral flashes observed in the Ca II infrared
  (IR) triplet lines. The observed signals are usually delayed in time
  and shifted in space when compared to the intensity and circular
  polarization signals from the flash. The observations are compatible
  with a scenario whereby flashes are produced by a perturbation
  propagating along the magnetic field lines as they bend out toward
  the penumbra. Only a fraction of the resolution element appears to
  be emitting flashlike profiles, as if the waves were propagating only
  within localized magnetic field lines. This localization, however, does
  not impede the apparent propagation of the perturbation horizontally
  within the umbra.

Title: Group Theory Approach to the Transfer of Polarized Light
Authors: López Ariste, A.
Bibcode: 2001ASPC..236..529L
Altcode: 2001aspt.conf..529L
  No abstract at ADS

Title: Real-Time Inversion of Spectropolarimetric Data
Authors: López Ariste, A.
Bibcode: 2001ASPC..248..621L
Altcode: 2001mfah.conf..621L
  No abstract at ADS

Title: Current Ground-Based Stellar Spectropolarimeters and their
    Use for Zeeman-Doppler Imaging
Authors: Semel, M.; López Ariste, A.
Bibcode: 2001ASPC..248..575S
Altcode: 2001mfah.conf..575S
  No abstract at ADS

Title: Inversion Codes based on Pattern Recognition
Authors: López Ariste, A.
Bibcode: 2001ASPC..236..521L
Altcode: 2001aspt.conf..521L
  No abstract at ADS

Title: Scattering Polarization Observations with the Tenerife Gregory
    Coudé Telescope
Authors: Dittmann, O.; Trujillo Bueno, J.; Semel, M.; López Ariste, A.
Bibcode: 2001ASPC..236..125D
Altcode: 2001aspt.conf..125D
  No abstract at ADS

Title: Fast inversion of spectral lines using principal component
    analysis. I. Fundamentals
Authors: Rees, D. E.; López Ariste, A.; Thatcher, J.; Semel, M.
Bibcode: 2000A&A...355..759R
Altcode:
  This paper presents PCA inversion, a novel application of Principal
  Component Analysis to the problem of spectral line inversion,
  ie. solar/stellar atmospheric model parameter estimation from spectral
  lines. For a given type of spectral line we compute a database of
  synthetic spectral profiles using a large number of models. Inversion
  of an observed profile to obtain an atmospheric model is equivalent
  to a problem in pattern recognition, finding the nearest profile in
  the synthetic profile database. To reduce dimensionality we use the
  synthetic data as a PCA training set to decompose each synthetic (and
  observed) profile into a sum of a small number of principal components,
  or eigenprofiles. The coefficients of this decomposition can be regarded
  as elements of a low-dimensional eigenfeature vector. The eigenfeatures
  are smooth functions of model parameters, indicating that eigenfeatures
  for parameters not in the training set could be easily estimated by
  interpolation. Search for the nearest profile is fast because it is
  done in the eigenfeature vector space. We illustrate the method using
  several types of synthetic spectra: unpolarised intensity profiles of
  a line formed in a Milne-Eddington model atmosphere; unpolarised Hα
  flux profiles of a line formed in non-Local Thermodynamic Equilibrium
  in the chromosphere of a cool star; and polarised Stokes parameter
  profiles of a line split by the Zeeman effect in the presence of a
  magnetic field. We also apply PCA to a set of Stokes data observed
  in a sunspot region by the High Altitude Observatory Advanced Stokes
  Polarimeter. PCA inversion is proposed as a fast alternative to
  non-linear least squares inversion commonly used for solar magnetic
  field measurements based on such Stokes data.

Title: First results from THEMIS spectropolarimetric mode
Authors: López Ariste, A.; Rayrole, J.; Semel, M.
Bibcode: 2000A&AS..142..137L
Altcode:
  We present here the very first spectropolarimetric results obtained with
  the multiline spectroscopy mode (MTR) of THEMIS. The principal problems
  found during the data analysis are exposed. A first characterisation
  of the quality of this observing mode is given. Based on observations
  made with THEMIS operated on the island of Tenerife by CNRS-CNR in
  the Spanish Observatorio del Teide of the Instituto de Astrofísica
  de Canarias.

Title: La spectropolarimétrie en astrophysique. Application au
    diagnostic des champs magnétiques solaires et stellaire.
Authors: Lopez Ariste, A.
Bibcode: 1999JAF....60Q..40L
Altcode:
  No abstract at ADS

Title: DIAGONAL: A numerical solution of the Stokes transfer equation
Authors: López Ariste, A.; Semel, M.
Bibcode: 1999A&AS..139..417L
Altcode:
  In a previous paper \cite[(Semel &amp; López Ariste 1999]{art1},
  hereafter referred to as Paper I) the authors found a new solution for
  the integration of the radiative transfer equation for polarized light
  (RTE). The present paper is a continuation of that work, and shows how
  this solution has been incorporated into a new numerical code for the
  integration of the RTE. This code, called DIAGONAL, is characterized
  by an analytical handling of most model atmospheres, differing in that
  from other codes. It gives exact solutions in new, non-trivial cases. In
  the general case when no complete analytical solution is available,
  the code resorts to a multi-layer technique, thus providing a solution
  that combines an approximated analytical model and a minimized residual
  treated linearly. The algorithm is shown both to be very stable and
  to reach a high precision even for a small number of integration
  layers. This results in a very fast code. A comparison with DELO has
  been carried out. DIAGONAL is available, upon request to the authors.

Title: Analytical solution of the radiative transfer equation for
    polarized light
Authors: López Ariste, A.; Semel, M.
Bibcode: 1999A&A...350.1089L
Altcode: 1999astro.ph..9232A; 1999astro.ph..9232L
  A new formalism is introduced for the transfer of polarized
  radiation. Stokes parameters are shown to be four-vectors in a
  Minkowski-like space and, most strikingly, the radiative transfer
  equation (RTE) turns out to be an infinitesimal transformation under
  the Poincaré (plus dilatations) group. A solution to the transfer
  equation as a finite element of this group is proposed.

Title: Integration of the radiative transfer equation for polarized
light: the exponential solution
Authors: Semel, M.; López Ariste, A.
Bibcode: 1999A&A...342..201S
Altcode: 1998astro.ph.11153S
  The radiative transfer equation (RTE) for polarized light accepts a
  convenient exponential solution when the absorption matrix commutes
  with its integral. We characterize some of the matrix depth variations
  which are compatible with the commutation condition. Eventually
  the vector solution may be diagonalized and one may obtain four
  independent scalar solutions with four optical depths, complex in
  general. When the commutation condition is not satisfied, one must
  resort to a determination of an appropriate evolution operator, which
  is shown to be well determined mathematically, but whose explicit
  form is, in general, not easy to apply in a numerical code. However,
  we propose here an approach to solve a general case not satisfying
  the commutation condition.

Title: Exponential solutions to the radiative transfer equation for
    polarized light
Authors: López Ariste, A.; Semel, M.
Bibcode: 1999ASSL..243..157L
Altcode: 1999sopo.conf..157L
  No abstract at ADS

