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

    • CMAG: A Mission to Study and Monitor the Inner Corona Magnetic Field
      Measuring magnetic fields in the inner corona, the interface between the solar chromosphere and outer corona, is of paramount importance if we aim to understand the energetic transformations taking place there, and because it is at the origin of processes that lead to coronal heating, solar wind acceleration, and of most of the phenomena relevant
      Orozco Suárez, David et al.

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      11
      2023
      Citations
      0
    • Tomographic Reconstruction of the Solar K-Corona Using Neural Fields
      We explore the application of neural fields for tomographic reconstructions of the solar corona using data from the Large Angle and Spectrometric Coronagraph (LASCO)-C2 instrument. We first demonstrate their ability to recover the electron-density volume in a synthetic static case, utilizing a simulated 3D model of the corona. Our results show that
      Asensio Ramos, Andrés

      Advertised on:

      11
      2023
      Citations
      1
    • The Impact of Angle-dependent Partial Frequency Redistribution on the Scattering Polarization of the Solar Na I D Lines
      The long-standing paradox of the linear polarization signal of the Na I D 1 line was recently resolved by accounting for the atom's hyperfine structure and the detailed spectral structure of the incident radiation field. That modeling relied on the simplifying angle-averaged (AA) approximation for partial frequency redistribution (PRD) in
      Janett, Gioele et al.

      Advertised on:

      11
      2023
      Citations
      1
    • Polarized resonance line transfer in a spherically symmetric medium with angle-dependent partial frequency redistribution
      In a stellar atmosphere, the resonance line polarization arises from scattering of limb-darkened radiation field by atoms. This spectral line polarization gets affected particularly in the wings, when the line photons suffer scattering on electrons in thermal motion. Scattering of line photons by atoms and electrons are, respectively, described by
      Sampoorna, M. et al.

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      12
      2023
      Citations
      0
    • First Metis Detection of the Helium D<SUB>3</SUB> Line Polarization in a Large Eruptive Prominence
      Metis on board Solar Orbiter is the space coronagraph developed by an Italian-German-Czech consortium. It is capable of observing solar corona and various coronal structures in the visible-light (VL) and UV (hydrogen Lyα) channels simultaneously for the first time. Here we present observations of a large eruptive prominence on 2021 April 25-26, in
      Heinzel, Petr et al.

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      11
      2023
      Citations
      1
    • On the Magnetic Nature of Quiet-Sun Chromospheric Grains
      Ca II K grains, i.e., intermittent, short-lived (about 1 minute), periodic (2-4 minutes), pointlike chromospheric brightenings, are considered to be the manifestations of acoustic waves propagating upward from the solar surface and developing into shocks in the chromosphere. After the simulations of Carlsson and Stein, we know that hot shocked gas
      Martínez González, María Jesús et al.

      Advertised on:

      10
      2023
      Citations
      1
    • Magnetic Field Information in the Near-ultraviolet Fe II Lines of the CLASP2 Space Experiment
      We investigate theoretically the circular polarization signals induced by the Zeeman effect in the Fe II lines of the 279.3-280.7 nm spectral range of the CLASP2 space experiment and their suitability to infer solar magnetic fields. To this end, we use a comprehensive Fe II atomic model to solve the problem of the generation and transfer of
      Afonso Delgado, David et al.

      Advertised on:

      9
      2023
      Citations
      0
    • Magnetic field fluctuations in the shocked umbral chromosphere
      Context. Umbral chromospheric observations show the presence of magnetoacoustic shocks. Several recent studies have reported magnetic field fluctuations associated with these shock waves. The mechanism behind these periodic magnetic field changes is still an unresolved question. Aims: We aim to study the properties and origin of magnetic field
      Felipe, T. et al.

      Advertised on:

      8
      2023
      Citations
      2
    • Machine learning in solar physics
      The application of machine learning in solar physics has the potential to greatly enhance our understanding of the complex processes that take place in the atmosphere of the Sun. By using techniques such as deep learning, we are now in the position to analyze large amounts of data from solar observations and identify patterns and trends that may
      Asensio Ramos, Andrés et al.

      Advertised on:

      12
      2023
      Citations
      14
    • Accelerating Multiframe Blind Deconvolution via Deep Learning
      Ground-based solar-image restoration is a computationally expensive procedure that involves nonlinear optimization techniques. The presence of atmospheric turbulence produces perturbations in individual images that make it necessary to apply blind deconvolution techniques. These techniques rely on the observation of many short-exposure frames that
      Asensio Ramos, Andrés et al.

      Advertised on:

      7
      2023
      Citations
      1
    • iNNterpol: High-precision interpolation of stellar atmospheres with a deep neural network using a 1D convolutional auto encoder for feature extraction
      Context. Given the widespread availability of grids of models for stellar atmospheres, it is necessary to recover intermediate atmospheric models by means of accurate techniques that go beyond simple linear interpolation and capture the intricacies of the data. Aims: Our goal is to establish a reliable, precise, lightweight, and fast method for
      Westendorp Plaza, C. et al.

      Advertised on:

      7
      2023
      Citations
      2
    • Opacity for realistic 3D MHD simulations of cool stellar atmospheres
      Context. Realistic three-dimensional time-dependent simulations of stellar near-surface convection employ the opacity binning method for the efficient and accurate computation of the radiative energy exchange. The method provides several orders of magnitude of speedup, but its implementation includes a number of free parameters. Aims: Our aim is to
      Perdomo García, A. et al.

      Advertised on:

      7
      2023
      Citations
      4
    • The He I 10 830 Å line: Radiative transfer and differential illumination effects
      We study the formation of the Stokes profiles of the He I multiplet at 10 830 Å when relaxing two of the approximations that are typically considered in the modeling of this multiplet. Specifically, these are the lack of self-consistent radiation transfer and the assumption of equal illumination of the individual multiplet components. This He I
      Vicente Arévalo, Andrés et al.

      Advertised on:

      7
      2023
      Citations
      1
    • A study of the capabilities for inferring atmospheric information from high-spatial-resolution simulations
      In this work, we study the accuracy that can be achieved when inferring the atmospheric information from realistic numerical magneto-hydrodynamic simulations that reproduce the spatial resolution we will obtain with future observations made by the 4m class telescopes DKIST and EST. We first study multiple inversion configurations using the SIR code
      Quintero Noda, C. et al.

      Advertised on:

      7
      2023
      Citations
      5
    • The Magnetic Sensitivity of the (250-278 nm) Fe II Polarization Spectrum
      This paper presents a theoretical investigation of the polarization and magnetic sensitivity of the near-ultraviolet (near-UV) solar spectral lines of Fe II between 250 and 278 nm. In recent years, UV spectropolarimetry has become key to uncovering the magnetism of the upper layers of the solar chromosphere. The unprecedented data obtained by the
      Afonso Delgado, David et al.

      Advertised on:

      5
      2023
      Citations
      2
    • The Potential of the Wavelength-integrated Scattering Polarization of the Hydrogen Lyα Line for Probing the Solar Chromosphere
      The intensity and the linear scattering polarization profiles of the hydrogen Lyα line encode valuable information on the thermodynamic and magnetic structure of the upper layers of the solar chromosphere. The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) sounding rocket experiment provided unprecedented spectropolarimetric data of this
      Alsina Ballester, E. et al.

      Advertised on:

      4
      2023
      Citations
      1
    • Estimating the longitudinal magnetic field in the chromosphere of quiet-Sun magnetic concentrations
      Context. Details of the magnetic field in the quiet-Sun chromosphere are key to our understanding of essential aspects of the solar atmosphere. However, the strength and orientation of this magnetic field have not been thoroughly studied at high spatial resolution. Aims: We aim to determine the longitudinal magnetic field component (B ∥) of quiet
      Esteban Pozuelo, S. et al.

      Advertised on:

      4
      2023
      Citations
      3
    • Tomography of a Solar Plage with the Tenerife Inversion Code
      We apply the Tenerife Inversion Code (TIC) to the plage spectropolarimetric observations obtained by the Chromospheric LAyer SpectroPolarimeter (CLASP2). These unprecedented data consist of full Stokes profiles in the spectral region around the Mg II h and k lines for a single slit position, with around two thirds of the 196″ slit crossing a plage
      Trujillo Bueno, Javier et al.

      Advertised on:

      3
      2023
      Citations
      5
    • Evidence for the Operation of the Hanle and Magneto-optical Effects in the Scattering Polarization Signals Observed by CLASP2 across the Mg II h and k Lines
      Radiative transfer investigations of the solar Mg II h and k resonance lines around 280 nm have shown that, while their circular polarization (Stokes V) signals arise from the Zeeman effect, the linear polarization profiles (Stokes Q and U) are dominated by the scattering of anisotropic radiation and the Hanle and magneto-optical (MO) effects
      Ishikawa, Ryohko et al.

      Advertised on:

      3
      2023
      Citations
      2
    • Impact of opacity effects on chromospheric oscillations inferred from NLTE inversions
      Context. Spectropolarimetric inversions are a fundamental tool for diagnosing the solar atmosphere. Chromospheric inferences rely on the interpretation of spectral lines that are formed under nonlocal thermodynamic equilibrium (NLTE) conditions. In the presence of oscillations, changes in the opacity impact the response height of the spectral lines
      Felipe, T. et al.

      Advertised on:

      2
      2023
      Citations
      5

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