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
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.
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.
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−1 to heights where this velocity is comparable to the escape velocity.
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.

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 >= 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.
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.
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.
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.
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
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.
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.
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.
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.

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.
Aims: We seek to make the CLIMSO images easier to use and more profitable for the scientific community.
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.
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-2 sr-1 nm-1. 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-1 at sunrise and 70 m s-1 at noon in the equatorial region. Regarding meridional winds, we detect an equator-to-pole meridional flow peaking at 45 m s-1 at mid-latitudes, i.e., about twice as large as what has been reported so far.

Tables A.1-A.3 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A82 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.

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.

(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 >=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
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.
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.
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-1, 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.
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.

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-1 at sunrise and 70 m s-1 at noon in the equatorial region. Regarding meridional winds, we detect an equator-to-pole meridional flow peaking at 45 m s-1 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
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.
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.
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.
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.
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.
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.
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.
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.

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 cm2 . 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.
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.
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.
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 × 104 km). Knots run along more or less horizontal threads with velocities reaching 65 km s-1. The dominant driving force is the gas pressure.

Movies associated to Figs. 1, 9, 10, and 13 are available at http://www.aanda.org 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.
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.
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 D3 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.
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 k3 Doppler shift is found to be between ±10 km s-1 everywhere. Electron densities at a temperature of 1.5 × 106 K are found to be around 109 cm-3. 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.
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 D3 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 D3) multiplet aiming to measure the spectral characteristics of the He I D3 profiles and to find for them an adequate fitting model. The component characteristics of the He I D3 Stokes I profiles are measured by the fitting system by approximating them with a double Gaussian. This model yields an He I D3 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−1, with a standard deviation of ± 1.7 km s−1 around the peak value of 0.4 km s−1. 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 D3 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 D3 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 D3 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 D3 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 D3 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. Betelgeuse is an M supergiant that harbors spots and giant granules at its surface and presents linear polarization of its continuum.
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 Betelgeuse.
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.
Results: We have discovered the existence of a linearly polarized spectrum for Betelgeuse, detecting a rather strong signal (at a few times 10-4 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 Betelgeuse. 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 Betelgeuse obtained over a period of 1.4 yr to study the evolution of the spots and of the atmosphere.
Conclusions: Our study of the linearly polarized spectrum of Betelgeuse 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.

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.
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.
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.
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.
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 D3 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.
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.
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.
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.
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.

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.
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.
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.
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.
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-1 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 D3 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?
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.
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.
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.
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.

Appendix A is available in electronic form at http://www.aanda.org 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 D3 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.
Aims: The aim of this work is to study the dynamics of the prominence fine structures in multiple wavelengths to understand their formation.
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 D3 line depolarization (Hanle effect combined with the Zeeman effect).
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-1 (a quasi-steady component) to 60-80 km s-1. 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.
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-1 to 100 km s-1 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-1) 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.

Movies are available in electronic form at http://www.aanda.org Title: Erratum: "Propagating Waves Transverse to the Magnetic Field in a Solar Prominence" (2013, ApJ, 777, 108) 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 D3), 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 D3 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-1, 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 24n 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 24n 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 D2 and D1 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 D2 line while no polarization was seen in D1, 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.
Aims: We aim to determine the length scale of the turbulent magnetic field in the quiet Sun.
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.
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.
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 & 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
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
Methods: A star with a simple dipolar field and a 1-0 spectral line were used to compute polarization amplitudes and angles.
Results: Predicted amplitudes are low but within reach of present instruments
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.
Aims: We present a method for measuring the OAM spectrum of solar photons.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 (D3) and 1083.0 nm in prominences (90° scattering). We created data sets of synthetic Stokes profiles for the case of quiescent prominences (B < 200 G), assuming a conservative value of 10-3 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 D2 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-1 per pixel of 1 arcsec, and give a value of the amplitude of zonal wind at equator at 151±16ms-1. 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.
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.
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.
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.
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 0.17 AU. Two days before this perihelion, at an heliocentric distance of 0.2 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 D2 emission line peaked at 900 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 Na 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.
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.
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.
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.
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.
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.
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.

Measurements of vector magnetic field in filament channels can answer to these questions and will be possible with Solar-B.

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.
Aims: Our aim is to observe and understand the behaviour of these structures in the chromosphere.
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.
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.
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 D3 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 10-5 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/cm2 for Fe I and above 1000 Mx/cm2 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 D3 Authors: López Ariste, A.; Casini, R. Bibcode: 2006ASPC..358..443L Altcode: Spectro-polarimetric observations of the He I D3 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.
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.
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.
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.
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 10-5 is now accessible, opening whole new fields of discovery, and exceeding by far the original specs of 10-3. 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 D3 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 D3 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.

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.

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 HeD3 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 & 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 D3 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 & 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. & 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 > 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 & 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 D3 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 Cij given by the product of the profile amplitudes at wavelengths λ i and λ j 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. & Semel, M. 2000, A & 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 & 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