Magnetism, Polarization and Radiative Transfer in Astrophysics

    General
    Description

    Magnetic fields pervade all astrophysical plasmas and govern most of the variability in the Universe at intermediate time scales. They are present in stars across the whole Hertzsprung-Russell diagram, in galaxies, and even perhaps in the intergalactic medium. Polarized light provides the most reliable source of information at our disposal for the remote sensing of astrophysical magnetic fields, including those on the Sun. In particular, the diagnostics of solar and stellar magnetic fields requires the measurement and physical interpretation of polarization signatures in spectral lines, which are induced by various physical mechanisms taking place at the atomic level. In addition to the familiar Zeeman effect, polarization can also be generated by various other physical processes, such as atomic level polarization induced by anisotropic pumping mechanisms, quantum interference between fine-structured or hyperfine-structured energy levels, the Hanle effect, etc. Interestingly, the polarization produced by such mechanisms is sensitive to the physical conditions of the astrophysical plasma under consideration and, in particular, to the presence of magnetic fields in a parameter domain that goes from field intensities as small as 1 micro-G to many thousands of Gauss.

    The main objective of this project is to explore in depth the physics and origin of polarized radiation in astrophysical plasmas as well as its diagnostic use for understanding cosmical magnetic fields, with emphasis on the magnetism of the extended solar atmosphere. Our investigations deal with:

    -the theoretical understanding of relevant polarization physics, which requires new insights into the quantum theory of polarized light scattering in the presence of magnetic and electric fields.

    -the development of plasma diagnostic tools for the investigation of astrophysical magnetic fields, with emphasis on the magnetism of the extended solar atmosphere, circumstellar envelopes and planetary nebulae.

    -spectropolarimetric observations and their physical interpretation.

    -radiative transfer in three-dimensional models of stellar atmospheres, resulting from magneto-hydrodynamical simulations.

    -atomic and molecular spectroscopy and spectro-polarimetry, with applications in several fields of astrophysics.

    This research project is formed by a group of scientists convinced of the importance of complementing theoretical and observational investigations in order to face some of the present challenges of 21st century Astrophysics.

    Principal investigator
    1. We applied deep learning techniques to the analysis of observations. Using convolutional neural networks, we developed techniques for the deconvolution of observational data. These techniques were also used to accelerate the deconvolution process of ground-based observations, achieving a cadence of around a hundred images processed per second.
    2. We developed an inference technique based on bayesian statistics in order to interpret the observations provided by the CLASP international experiment. By parametrizing a state-of-the-art magneto-hydrodynamical model of the solar atmosphere, we found that the geometrical complexity of the transition region must be much more complex than the one provided by the model.
    3. We solved the problem of polarized radiation transfer in magneto-convection simulations that account for small-scale dynamo action for the Sr I line at 460.7 nm. We found that the model with most of the convection zone magnetized close to the equipartition and a surface mean field strength of 170G is compatible with the available observations.
    4. We studied the magnetic sensitivity of the Ca I line at 422.7nm. The linear polarization at the core is sensitive to the Hanle effect, while the linear polarization in the wings is sensitive to the magneto-optical effects, as a consequence of the newly found effect resulting from the joint action of partial redistribution and the Zeeman effects.
    5. We studied the formation of the H-alpha, Mg II h-k, and Ca II H-K and 845.2nm in a model atmosphere of a flaring bipolar active region, solving the radiation transfer problem taking into account partial redistribution in full 3D geometry and out of local thermodynamical equilibrium. We succeeded in reproducing common observational features of such flaring regions.

    Related publications

    • How to infer the Sun's global magnetic field using the Hanle effect

      We present a different approach to determine the characteristics of the global magnetic field of the Sun based on the study of the Hanle signals. The Hanle effect of a stellar dipole produces a surface asymmetric pattern of linear polarization that depends on the strength and geometry of this global field. Moreover, if the dipole is misaligned with

      Vieu, T. et al.

      Advertised on:

      3
      2017
      Citations
      1
    • A distinct magnetic property of the inner penumbral boundary. II. Formation of a penumbra at the expense of a pore

      Context. We recently presented evidence that stable umbra-penumbra boundaries are characterised by a distinct canonical value of the vertical component of the magnetic field, Bstablever. In order to trigger the formation of a penumbra, large inclinations in the magnetic field are necessary. In sunspots, the penumbra develops and establishes by

      Jurčák, J. et al.

      Advertised on:

      1
      2017
      Citations
      25
    • Small-scale magnetic flux emergence in a sunspot light bridge

      Context. Light bridges are convective intrusions in sunspots that often show enhanced chromospheric activity. Aims: We seek to determine the nature of flux emergence in a light bridge and the processes related to its evolution in the solar atmosphere. Methods: We analyse a sequence of high-resolution spectropolarimetric observations of a sunspot

      Louis, R. E. et al.

      Advertised on:

      12
      2015
      Citations
      25
    • Radiative Transfer Modeling of the Enigmatic Scattering Polarization in the Solar Na I D1 Line

      The modeling of the peculiar scattering polarization signals observed in some diagnostically important solar resonance lines requires the consideration of the detailed spectral structure of the incident radiation field as well as the possibility of ground level polarization, along with the atom's hyperfine structure and quantum interference between

      Belluzzi, L. et al.

      Advertised on:

      12
      2015
      Citations
      13
    • Multi-parametric Study of Rising 3D Buoyant Flux Tubes in an Adiabatic Stratification Using AMR

      We study the buoyant rise of magnetic flux tubes embedded in an adiabatic stratification using two-and three-dimensional, magnetohydrodynamic simulations. We analyze the dependence of the tube evolution on the field line twist and on the curvature of the tube axis in different diffusion regimes. To be able to achieve a comparatively high spatial

      Martínez-Sykora, J. et al.

      Advertised on:

      11
      2015
      Citations
      23
    • Bayesian least squares deconvolution

      Aims: We develop a fully Bayesian least squares deconvolution (LSD) that can be applied to the reliable detection of magnetic signals in noise-limited stellar spectropolarimetric observations using multiline techniques. Methods: We consider LSD under the Bayesian framework and we introduce a flexible Gaussian process (GP) prior for the LSD profile

      Asensio Ramos, A. et al.

      Advertised on:

      11
      2015
      Citations
      9
    • A deep look at the nuclear region of UGC 5101 through high angular resolution mid-IR data with GTC/CanariCam

      We present an analysis of the nuclear infrared (IR, 1.6-18 μm) emission of the ultraluminous IR galaxy UGC 5101 to derive the properties of its active galactic nucleus (AGN) and its obscuring material. We use new mid-IR high angular resolution (0.3-0.5 arcsec) imaging using the Si-2 filter (λC = 8.7 μm) and 7.5-13 μm spectroscopy taken with

      Martínez-Paredes, M. et al.

      Advertised on:

      12
      2015
      Citations
      12
    • Model Comparison for the Density Structure across Solar Coronal Waveguides

      The spatial variation of physical quantities, such as the mass density, across solar atmospheric waveguides governs the timescales and spatial scales for wave damping and energy dissipation. The direct measurement of the spatial distribution of density, however, is difficult, and indirect seismology inversion methods have been suggested as an

      Arregui, I. et al.

      Advertised on:

      10
      2015
      Citations
      22
    • Isotropic Inelastic Collisions in a Multiterm Atom with Hyperfine Structure

      A correct modeling of the scattering polarization profiles observed in some spectral lines of diagnostic interest, the sodium doublet being one of the most important examples, requires taking hyperfine structure (HFS) and quantum interference between different J-levels into account. An atomic model suitable for taking these physical ingredients

      Belluzzi, L. et al.

      Advertised on:

      10
      2015
      Citations
      4
    • CLASP: A UV Spectropolarimeter on a Sounding Rocket for Probing theChromosphere-Corona Transition Regio

      The wish to understand the energetic phenomena of the outer solar atmosphere makes it increasingly important to achieve quantitative information on the magnetic field in the chromosphere-corona transition region. To this end, we need to measure and model the linear polarization produced by scattering processes and the Hanle effect in strong UV

      Ishikawa, R. et al.

      Advertised on:

      8
      2015
      Citations
      1
    • Full Stokes observations in the He i 1083 nm spectral region covering an M3.2 flare

      We present an exceptional data set acquired with the Vacuum Tower Telescope (Tenerife, Spain) covering the pre-flare, flare, and post-flare stages of an M3.2 flare. The full Stokes spectropolarimetric observations were recorded with the Tenerife Infrared Polarimeter in the He i 1083.0 nm spectral region. The object under study was active region

      Kuckein, C. et al.

      Advertised on:

      10
      2015
      Citations
      6
    • Atomic Scattering Polarization. Observations, Modeling, Predictions

      This paper highlights very recent advances concerning the identification of new mechanisms that introduce polarization in spectral lines, which turn out to be key for understanding some of the most enigmatic scattering polarization signals of the solar visible spectrum. We also show a radiative transfer prediction on the scattering polarization

      Trujillo Bueno, J. et al.

      Advertised on:

      10
      2015
      Citations
      0
    • The impact of surface dynamo magnetic fields on the chemical abundance determination

      The solar abundances of Fe and of the CNO elements play an important role in addressing a number of important issues such as the formation, structure, and evolution of the Sun and the solar system, the origin of the chemical elements, and the evolution of stars and galaxies. Despite the large number of papers published on this issue, debates about

      Shchukina, N. G. et al.

      Advertised on:

      10
      2015
      Citations
      3
    • Formation of polarized spectral lines in atmospheres with horizontal inhomogeneities

      We study the problem of the generation and transfer of spectral line intensity and polarization in models of stellar atmospheres with horizontal plasma inhomogeneities. We solve the non-LTE radiative transfer problem in full 3D geometry taking into account resonant scattering polarization and its modification by magnetic fields via the Hanle effect

      Tichý, A. et al.

      Advertised on:

      10
      2015
      Citations
      2
    • Chromospheric Polarization in the Photospheric Solar Oxygen Infrared Triplet

      We present multilevel radiative transfer modeling of the scattering polarization observed in the solar O i infrared triplet around 777 nm. We demonstrate that the scattering polarization pattern observed on the solar disk forms in the chromosphere, far above the photospheric region where the bulk of the emergent intensity profiles originate. We

      Del Pino Alemán, T. et al.

      Advertised on:

      7
      2015
      Citations
      5
    • The impact of surface dynamo magnetic fields on the solar iron abundance

      Most chemical abundance determinations ignore that the solar photosphere is significantly magnetized by the ubiquitous presence of a small-scale magnetic field. A recent investigation has suggested that there should be a significant impact on the derived iron abundance, owing to the magnetically induced changes on the photospheric temperature and

      Trujillo Bueno, J. et al.

      Advertised on:

      7
      2015
      Citations
      13
    • Spatial deconvolution of spectropolarimetric data: an application to quiet Sun magnetic elements

      Context. One of the difficulties in extracting reliable information about the thermodynamical and magnetic properties of solar plasmas from spectropolarimetric observations is the presence of light dispersed inside the instruments, known as stray light. Aims: We aim to analyze quiet Sun observations after the spatial deconvolution of the data. We

      Quintero Noda, C. et al.

      Advertised on:

      7
      2015
      Citations
      29
    • Sparse inversion of Stokes profiles. I. Two-dimensional Milne-Eddington inversions

      Context. Inversion codes are numerical tools used to infer physical properties from observations. Despite their success, the quality of current spectropolarimetric observations and those expected in the near future presents a challenge to current inversion codes. Aims: The pixel-by-pixel strategy of inverting spectropolarimetric data that we

      Asensio Ramos, A. et al.

      Advertised on:

      5
      2015
      Citations
      32
    • Estimating the magnetic field strength from magnetograms

      A properly calibrated longitudinal magnetograph is an instrument that measures circular polarization and gives an estimation of the magnetic flux density in each observed resolution element. This usually constitutes a lower bound of the field strength in the resolution element, given that it can be made arbitrarily large as long as it occupies a

      Asensio Ramos, A. et al.

      Advertised on:

      5
      2015
      Citations
      2
    • The solar oxygen abundance from an empirical three-dimensional model

      The oxygen abundance in the solar photosphere, and consequently the solar metallicity itself, is still a controversial question with far-reaching implications in many areas of astrophysics. This paper presents a new determination obtained by fitting the forbidden O i line at 6300 Å with an observational 3D model. The approach presented here is

      Socas-Navarro, H.

      Advertised on:

      5
      2015
      Citations
      13

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