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

    • An open-source, massively parallel code for non-LTE synthesis and inversion of spectral lines and Zeeman-induced Stokes profiles

      With the advent of a new generation of solar telescopes and instrumentation, interpreting chromospheric observations (in particular, spectropolarimetry) requires new, suitable diagnostic tools. This paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, for synthesis and inversion of spectral lines and

      Socas-Navarro, H. et al.

      Advertised on:

      5
      2015
      Citations
      140
    • The Differences in the Torus Geometry between Hidden and Non-hidden Broad Line Active Galactic Nuclei

      We present results from the fitting of infrared (IR) spectral energy distributions of 21 active galactic nuclei (AGNs) with clumpy torus models. We compiled high spatial resolution (∼0.3–0.7 arcsec) mid-IR (MIR) N-band spectroscopy, Q-band imaging, and nuclear near- and MIR photometry from the literature. Combining these nuclear near- and MIR

      Ichikawa, K. et al.

      Advertised on:

      4
      2015
      Citations
      87
    • Three-dimensional Radiative Transfer Simulations of the Scattering Polarization of the Hydrogen Lyα Line in a Magnetohydrodynamic Model of the Chromosphere–Corona Transition Region

      Probing the magnetism of the upper solar chromosphere requires measuring and modeling the scattering polarization produced by anisotropic radiation pumping in UV spectral lines. Here we apply PORTA (a novel radiative transfer code) to investigate the hydrogen Lyα line in a three-dimensional model of the solar atmosphere resulting from a state of

      Štěpán, J. et al.

      Advertised on:

      4
      2015
      Citations
      45
    • Height Variation of the Vector Magnetic Field in Solar Spicules

      Proving the magnetic configuration of solar spicules has hitherto been difficult due to the lack of spatial resolution and image stability during off-limb ground-based observations. We report spectropolarimetric observations of spicules taken in the He i 1083 nm spectral region with the Tenerife Infrared Polarimeter II at the German Vacuum Tower

      Orozco Suárez, D. et al.

      Advertised on:

      4
      2015
      Citations
      28
    • Spectro-Polarimetric Imaging Reveals Helical Magnetic Fields in Solar Prominence Feet

      Solar prominences are clouds of cool plasma levitating above the solar surface and insulated from the million-degree corona by magnetic fields. They form in regions of complex magnetic topology, characterized by non-potential fields, which can evolve abruptly, disintegrating the prominence and ejecting magnetized material into the heliosphere

      Martínez González, M. J. et al.

      Advertised on:

      3
      2015
      Citations
      31
    • Chromospheric Diagnosis with Ca II Lines: Forward Modeling in Forward Scattering. I

      This paper presents a synthetic tomography of the quiet solar chromosphere formed by spatial maps of scattering polarization. It has been calculated for the Ca II 8498, 8542, and 3934 Å lines by solving the non-LTE radiative transfer problem of the second kind in a three-dimensional atmosphere model obtained from realistic magneto-hydrodynamical

      Carlin, E. S. et al.

      Advertised on:

      3
      2015
      Citations
      26
    • Magnetic and Dynamical Photospheric Disturbances Observed During an M3.2 Solar Flare

      This Letter reports on a set of full-Stokes spectropolarimetric observations in the near-infrared He i 10830 Å spectral region covering the pre-flare, flare, and post-flare phases of an M3.2 class solar flare. The flare originated on 2013 May 17 and belonged to active region NOAA 11748. We detected strong He i 10830 Å emission in the flare. The red

      Kuckein, C. et al.

      Advertised on:

      2
      2015
      Citations
      17
    • Constraining the shaping mechanism of the Red Rectangle through the spectro-polarimetry of its central star

      We carried out high-sensitivity spectro-polarimetric observations of the central star of the Red Rectangle protoplanetary nebula with the aim of constraining the mechanism that gives its biconical shape. The stellar light of the central binary system is linearly polarised since it is scattered on the dust particles of the nebula. Surprisingly, the

      Martínez González, M. J. et al.

      Advertised on:

      2
      2015
      Citations
      13
    • Hierarchical analysis of the quiet-Sun magnetism

      Standard statistical analysis of the magnetic properties of the quiet Sun rely on simple histograms of quantities inferred from maximum-likelihood estimations. Because of the inherent degeneracies, either intrinsic or induced by the noise, this approach is not optimal and can lead to highly biased results. We carried out a meta-analysis of the

      Asensio Ramos, A. et al.

      Advertised on:

      12
      2014
      Citations
      18
    • The Solar Internetwork. I. Contribution to the Network Magnetic Flux

      The magnetic network (NE) observed on the solar surface harbors a sizable fraction of the total quiet Sun flux. However, its origin and maintenance are not well known. Here we investigate the contribution of internetwork (IN) magnetic fields to the NE flux. IN fields permeate the interior of supergranular cells and show large emergence rates. We

      Gošić, M. et al.

      Advertised on:

      12
      2014
      Citations
      85
    • Synthetic Observations of Wave Propagation in a Sunspot Umbra

      Spectropolarimetric temporal series from Fe I λ6301.5 Å and Ca II infrared triplet lines are obtained by applying the Stokes synthesis code NICOLE to a numerical simulation of wave propagation in a sunspot umbra from MANCHA code. The analysis of the phase difference between Doppler velocity and intensity core oscillations of the Fe I λ6301.5 Å line

      Felipe, T. et al.

      Advertised on:

      11
      2014
      Citations
      26
    • Pair separation of magnetic elements in the quiet Sun

      The dynamic properties of the quiet Sun photosphere can be investigated by analyzing the pair dispersion of small-scale magnetic fields (i.e., magnetic elements). By using 25 h-long Hinode magnetograms at high spatial resolution (0.3 arcsec), we tracked 68 490 magnetic element pairs within a supergranular cell near the disk center. The computed

      Giannattasio, F. et al.

      Advertised on:

      9
      2014
      Citations
      24
    • High speed magnetized flows in the quiet Sun

      Context. We analyzed spectropolarimetric data recorded with Hinode/SP in quiet-Sun regions located at the disk center. We found single-lobed Stokes V profiles showing highly blue- and red-shifted signals. Oftentimes both types of events appear to be related to each other. Aims: We aim to set constraints on the nature and physical causes of these

      Quintero Noda, C. et al.

      Advertised on:

      9
      2014
      Citations
      12
    • Frequency Redistribution Function for the Polarized Two-term Atom

      We present a generalized frequency redistribution function for the polarized two-term atom in an arbitrary magnetic field. This result is derived within a new formulation of the quantum problem of coherent scattering of polarized radiation by atoms in the collisionless regime. The general theory, which is based on a diagrammatic treatment of the

      Casini, R. et al.

      Advertised on:

      8
      2014
      Citations
      28
    • Photospheric downward plasma motions in the quiet Sun

      Context. We analyze spectropolarimetric data taken with the Hinode spacecraft in quiet solar regions at the disk center. Distorted redshifted Stokes V profiles are found that show a characteristic evolution that always follows the same sequence of phases. Aims: We aim to characterize the statistical properties of these events and recover the

      Quintero Noda, C. et al.

      Advertised on:

      6
      2014
      Citations
      12
    • The magnetic field configuration of a solar prominence inferred from spectropolarimetric observations in the He i 10 830 Å triplet

      Context. Determining the magnetic field vector in quiescent solar prominences is possible by interpreting the Hanle and Zeeman effects in spectral lines. However, observational measurements are scarce and lack high spatial resolution. Aims: We determine the magnetic field vector configuration along a quiescent solar prominence by interpreting

      Orozco Suárez, D. et al.

      Advertised on:

      6
      2014
      Citations
      54
    • Depolarizing Collisions with Hydrogen: Neutral and Singly Ionized Alkaline Earths

      Depolarizing collisions are elastic or quasielastic collisions that equalize the populations and destroy the coherence between the magnetic sublevels of atomic levels. In astrophysical plasmas, the main depolarizing collider is neutral hydrogen. We consider depolarizing rates on the lowest levels of neutral and singly ionized alkali earths Mg I, Sr

      Manso Sainz, R. et al.

      Advertised on:

      6
      2014
      Citations
      15
    • On the Inversion of the Scattering Polarization and the Hanle Effect Signals in the Hydrogen Lyα Line

      Magnetic field measurements in the upper chromosphere and above, where the gas-to-magnetic pressure ratio β is lower than unity, are essential for understanding the thermal structure and dynamical activity of the solar atmosphere. Recent developments in the theory and numerical modeling of polarization in spectral lines have suggested that

      Ishikawa, R. et al.

      Advertised on:

      6
      2014
      Citations
      15
    • Determination of the cross-field density structuring in coronal waveguides using the damping of transverse waves

      Context. Time and spatial damping of transverse magnetohydrodynamic (MHD) kink oscillations is a source of information on the cross-field variation of the plasma density in coronal waveguides. Aims: We show that a probabilistic approach to the problem of determining the density structuring from the observed damping of transverse oscillations

      Arregui, I. et al.

      Advertised on:

      5
      2014
      Citations
      17
    • Upper Limits to the Magnetic Field in Central Stars of Planetary Nebulae

      More than about 20 central stars of planetary nebulae (CSPNs) have been observed spectropolarimetrically, yet no clear, unambiguous signal of the presence of a magnetic field in these objects has been found. We perform a statistical (Bayesian) analysis of all the available spectropolarimetric observations of CSPN to constrain the magnetic fields in

      Asensio Ramos, A. et al.

      Advertised on:

      6
      2014
      Citations
      15

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