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

    • Stokes inversion based on convolutional neural networks
      Context. Spectropolarimetric inversions are routinely used in the field of solar physics for the extraction of physical information from observations. The application to two-dimensional fields of view often requires the use of supercomputers with parallelized inversion codes. Even in this case, the computing time spent on the process is still very
      Asensio Ramos, A. et al.

      Advertised on:

      6
      2019
      Citations
      46
    • Spectropolarimetric analysis of an active region filament. I. Magnetic and dynamical properties from single component inversions
      Aims: The determination of the magnetic filed vector in solar filaments is made possible by interpreting the Hanle and Zeeman effects in suitable chromospheric spectral lines like those of the He I multiplet at 10 830 Å. We study the vector magnetic field of an active region filament (NOAA 12087). Methods: Spectropolarimetric data of this active
      Díaz Baso, C. J. et al.

      Advertised on:

      5
      2019
      Citations
      18
    • Spectropolarimetric analysis of an active region filament.. II. Evidence of the limitations of a single-component model
      Aims: Our aim is to demonstrate the limitations of using a single-component model to study the magnetic field of an active region filament. To do this, we analyzed the polarimetric signals of the He I 10830 Å multiplet, which were acquired with the infrared spectrograph GRIS of the GREGOR telescope (Tenerife, Spain). Methods: After a first analysis
      Díaz Baso, C. J. et al.

      Advertised on:

      5
      2019
      Citations
      10
    • Can a negative-mass cosmology explain dark matter and dark energy?
      A recent study by Farnes (2018, A&A, 620, A92) proposed an alternative cosmological model in which both dark matter and dark energy are replaced with a single fluid of negative mass. This paper presents a critical review of that model. A number of problems and discrepancies with observations are identified. For instance, the predicted shape and
      Socas-Navarro, H.

      Advertised on:

      6
      2019
      Citations
      16
    • Inference of magnetic field strength and density from damped transverse coronal waves
      A classic application of coronal seismology uses transverse oscillations of waveguides to obtain estimates of the magnetic field strength. The procedure requires information on the density of the structures. Often it ignores the damping of the oscillations. We computed marginal posteriors for parameters such as the waveguide density, the density
      Arregui, I. et al.

      Advertised on:

      5
      2019
      Citations
      10
    • Torus model properties of an ultra-hard X-ray selected sample of Seyfert galaxies
      We characterize for the first time the torus properties of an ultra-hard X-ray (14-195 keV) volume-limited (DL 40 Mpc) sample of 24 Seyfert (Sy) galaxies (BCS40 sample). The sample was selected from the Swift/BAT nine-month catalogue. We use high angular resolution nuclear infrared (IR) photometry and N-band spectroscopy, the CLUMPY torus models
      García-Bernete, I. et al.

      Advertised on:

      7
      2019
      Citations
      37
    • High-resolution spectroscopy of Boyajian's star during optical dimming events
      Boyajian's star is an apparently normal main-sequence F-type star with a very unusual light curve. The dipping activity of the star, discovered during the Kepler mission, presents deep, asymmetric, and aperiodic events. Here we present high-resolution spectroscopic follow-up during some dimming events recorded post-Kepler observations, from ground
      Martínez González, M. J. et al.

      Advertised on:

      6
      2019
      Citations
      3
    • Diagnostic potential of the Ca II 8542 Å line for solar filaments
      Aims: In this study we explore the diagnostic potential of the chromospheric Ca II line at 8542 Å for studying the magnetic and dynamic properties of solar filaments. We have acquired high spatial resolution spectropolarimetric observations in the Ca II 8542 Å line using the CRISP instrument at the Swedish 1 m Solar Telescope. Methods: We used the
      Díaz Baso, C. J. et al.

      Advertised on:

      3
      2019
      Citations
      16
    • Height Dependence of the Penumbral Fine-scale Structure in the Inner Solar Atmosphere
      We studied the physical parameters of the penumbra in a large and fully developed sunspot, one of the largest over the last two solar cycles, by using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional Spectrometer. Inverting measurements with the Non-LTE
      Murabito, M. et al.

      Advertised on:

      3
      2019
      Citations
      17
    • Signatures of Magnetic Reconnection at the Footpoints of Fan-shaped Jets on a Light Bridge Driven by Photospheric Convective Motions
      Dynamical jets are generally found on light bridges (LBs), which are key to studying sunspot decay. So far, their formation mechanism is not fully understood. In this paper, we used state-of-the-art observations from the Goode Solar Telescope, the Interface Region Imaging Spectrograph, the Spectro-polarimeter on board Hinode, and the Atmospheric
      Bai, X. et al.

      Advertised on:

      1
      2019
      Citations
      20
    • Real-time, multiframe, blind deconvolution of solar images
      The quality of images of the Sun obtained from the ground are severely limited by the perturbing effect of the Earth's turbulent atmosphere. The post-facto correction of the images to compensate for the presence of the atmosphere require the combination of high-order adaptive optics techniques, fast measurements to freeze the turbulent atmosphere
      Asensio Ramos, A. et al.

      Advertised on:

      12
      2018
      Citations
      26
    • Comparison of theoretical and observed Ca II 8542 Stokes profiles in quiet regions at the centre of the solar disc
      Context. Interpreting the Stokes profiles observed in quiet regions of the solar chromosphere is a challenging task. The Stokes Q and U profiles are dominated by the scattering polarisation and the Hanle effect, and these processes can only be correctly quantified if 3D radiative transfer effects are taken into account. Forward-modelling of the
      Jurčák, J. et al.

      Advertised on:

      11
      2018
      Citations
      7
    • The real time MCAO solar prototype for the EST
      The European Solar Telescope (EST) will be equipped with a MCAO system to provide an excellent correction within one arcminute field of view (FOV). A prototype of this system will be built with the same requirements except for the frame rate. This prototype will allow to test and verify new concepts for Solar MCAO, as, e.g., the integration of
      Montoya, Luzma M. et al.

      Advertised on:

      7
      2018
      Citations
      0
    • Three-dimensional simulations of solar magneto-convection including effects of partial ionization
      In recent decades, REALISTIC three-dimensional radiative-magnetohydrodynamic simulations have become the dominant theoretical tool for understanding the complex interactions between the plasma and magnetic field on the Sun. Most of such simulations are based on approximations of magnetohydrodynamics, without directly considering the consequences of
      Khomenko, E. et al.

      Advertised on:

      10
      2018
      Citations
      55
    • CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region
      The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 2015 September 3 measured the linear polarization produced by scattering processes in the hydrogen Lyα line of the solar disk radiation. The line-center photons of this spectral line radiation mostly stem from the chromosphere-corona transition
      Trujillo Bueno, J. et al.

      Advertised on:

      10
      2018
      Citations
      24
    • A Statistical Inference Method for Interpreting the CLASP Observations
      On 2015 September 3, the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) successfully measured the linear polarization produced by scattering processes in the hydrogen Lyα line of the solar disk radiation, revealing conspicuous spatial variations in the Q/I and U/I signals. Via the Hanle effect, the line-center Q/I and U/I amplitudes encode
      Štěpán, J. et al.

      Advertised on:

      9
      2018
      Citations
      3
    • Optical alignment of the high-precision UV spectro-polarimeter (CLASP2)
      Chromospheric LAyer Spectro-Polarimeter (CLASP2) is our next sounding rocket experiment after the success of Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP1). CLASP2 is scheduled to launch in 2019, and aims to achieve high precision measurements ( 0.1 %) of the linear and circular polarizations in the Mg ii h and k lines near the 280 nm
      Song, Donguk et al.

      Advertised on:

      7
      2018
      Citations
      5
    • Wavefront error measurements and alignment of CLASP2 telescope with a dual-band pass cold mirror coated primary mirror
      "Chromospheric LAyer Spectro-Polarimeter (CLASP2)" is the next sounding rocket experiment of the "Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP)" that succeeded in observing for the first time the linear polarization spectra in the hydrogen Lyman-α line (121.6 nm) and is scheduled to be launched in 2019. In CLASP2, we will carry out full
      Yoshida, Masaki et al.

      Advertised on:

      7
      2018
      Citations
      5
    • MOLPOP-CEP: an exact, fast code for multi-level systems
      We present MOLPOP-CEP, a universal line transfer code that allows the exact calculation of multi-level line emission from a slab with variable physical conditions for any arbitrary atom or molecule for which atomic data exist. The code includes error control to achieve any desired level of accuracy, providing full confidence in its results
      Asensio Ramos, A. et al.

      Advertised on:

      9
      2018
      Citations
      17
    • Self-similar Approach for Rotating Magnetohydrodynamic Solar and Astrophysical Structures
      Rotating magnetic structures are common in astrophysics, from vortex tubes and tornadoes in the Sun all the way to jets in different astrophysical systems. The physics of these objects often combine inertial, magnetic, gas pressure, and gravitational terms. Also, they often show approximate symmetries that help simplify the otherwise rather
      Luna, M. et al.

      Advertised on:

      8
      2018
      Citations
      2

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