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

    Polarization Accuracy Verification of the Chromospheric LAyer SpectroPolarimeter 2022SoPh..297..135S
    Evidence of a flare ignited above a low-latitude spotted active region in the ultrafast rotator HK Aqr 2022MNRAS.517..744M
    Magnetic Field Diagnostics in the Solar Upper Atmosphere 2022ARA&A..60..415T
    Effects of Spectral Resolution on Simple Magnetic Field Diagnostics of the Mg II H and K Lines 2022ApJ...936..115C
    Influence of Thomson Electron Scattering Redistribution on Spectral Line Polarization Formed in Spherically Symmetric Extended and Expanding Atmospheres 2022ApJ...937...25S
    Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg II h and k Lines 2022ApJ...936...67R
    The transfer of polarized radiation in resonance lines with partial frequency redistribution, J-state interference, and arbitrary magnetic fields. A radiative transfer code and useful approximations 2022A&A...664A..76A
    TIC: A Stokes Inversion Code for Scattering Polarization with Partial Frequency Redistribution and Arbitrary Magnetic Fields 2022ApJ...933..145L
    Detection of Flare-induced Plasma Flows in the Corona of EV Lac with X-Ray Spectroscopy 2022ApJ...933...92C
    Searching for technosignatures in exoplanetary systems with current and future missions 2022AcAau.198..194H
    Accelerating Non-LTE Synthesis and Inversions with Graph Networks 2022ApJ...928..101V
    Spectropolarimetric observations of the solar atmosphere in the Hα 6563 Å line 2022A&A...659A.179J
    Bayesian Stokes inversion with normalizing flows 2022A&A...659A.165D
    Novel framework for the three-dimensional NLTE inverse problem 2022A&A...659A.137S
    Approximate Bayesian neural Doppler imaging 2022A&A...658A.162A
    Convolutional Neural Networks and Stokes Response Functions 2022ApJ...925..176C
    Empirical relations between the intensities of Lyman lines of H and He<SUP>+</SUP> 2022A&A...657A..86G
    The polarization angle in the wings of Ca I 4227: A new observable for diagnosing unresolved photospheric magnetic fields 2022A&A...657A..44C
    Magnetic imaging of the outer solar atmosphere (MImOSA) 2022ExA....54..185P
    Mapping solar magnetic fields from the photosphere to the base of the corona 2021SciA....7.8406I
    Modeling the scattering polarization of the solar Ca I 4227 Å line with angle-dependent partial frequency redistribution 2021A&A...655A..13J
    Polarization of the Lyα Lines of H I and He II as a Tool for Exploring the Solar Corona 2021ApJ...920..140H
    Newly formed downflow lanes in exploding granules in the solar photosphere 2021A&A...653A..96E
    Limitations of the Ca II 8542 Å Line for the Determination of Magnetic Field Oscillations 2021ApJ...918...47F
    On the (Mis)Interpretation of the Scattering Polarization Signatures in the Ca II 8542 Å Line through Spectral Line Inversions 2021ApJ...918...15C
    Solving the Paradox of the Solar Sodium D<SUB>1</SUB> Line Polarization 2021PhRvL.127h1101A
    Performance of solar far-side active region neural detection 2021A&A...652A.132B
    Naked emergence of an anti-Hale active region. I. Overall evolution and magnetic properties 2021A&A...652A..55W
    Exploring the Sun's upper atmosphere with neural networks: Reversed patterns and the hot wall effect 2021A&A...652A..78S
    Two-fluid simulations of Rayleigh-Taylor instability in a magnetized solar prominence thread. II. Effects of collisionality 2021A&A...650A.181P
    Probing Uncertainties in Diagnostics of a Synthetic Chromosphere 2021ApJ...913...71S
    Long Characteristics versus Short Characteristics in 3D Radiative Transfer Simulations of Polarized Radiation 2021ApJ...912...63D
    Rubidium abundances in solar metallicity stars 2021A&A...648A.107A
    Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST) 2021SoPh..296...70R
    Emergence of Internetwork Magnetic Fields through the Solar Atmosphere 2021ApJ...911...41G
    Evaluating the Reliability of a Simple Method to Map the Magnetic Field Azimuth in the Solar Chromosphere 2021ApJ...911...23J
    The Impact of Limited Time Resolution on the Forward-scattering Polarization in the Solar Sr I 4607 Å Line 2021ApJ...909..180D
    Temporal evolution of small-scale internetwork magnetic fields in the solar photosphere 2021A&A...647A.182C
    Long-term evolution of three light bridges developed on the same sunspot 2021A&A...647A.148G
    Concepts for future missions to search for technosignatures 2021AcAau.182..446S

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