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

    Height Dependence of the Penumbral Fine-scale Structure in the Inner Solar Atmosphere 2019ApJ...873..126M
    Signatures of Magnetic Reconnection at the Footpoints of Fan-shaped Jets on a Light Bridge Driven by Photospheric Convective Motions 2019ApJ...870...90B
    Real-time, multiframe, blind deconvolution of solar images 2018A&A...620A..73A
    Comparison of theoretical and observed Ca II 8542 Stokes profiles in quiet regions at the centre of the solar disc 2018A&A...619A..60J
    The real time MCAO solar prototype for the EST 2018SPIE10703E..48M
    Three-dimensional simulations of solar magneto-convection including effects of partial ionization 2018A&A...618A..87K
    CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region 2018ApJ...866L..15T
    A Statistical Inference Method for Interpreting the CLASP Observations 2018ApJ...865...48S
    Optical alignment of the high-precision UV spectro-polarimeter (CLASP2) 2018SPIE10699E..2WS
    Wavefront error measurements and alignment of CLASP2 telescope with a dual-band pass cold mirror coated primary mirror 2018SPIE10699E..30Y
    MOLPOP-CEP: an exact, fast code for multi-level systems 2018A&A...616A.131A
    Self-similar Approach for Rotating Magnetohydrodynamic Solar and Astrophysical Structures 2018ApJ...863..147L
    A Novel Investigation of the Small-scale Magnetic Activity of the Quiet Sun via the Hanle Effect in the Sr I 4607 Å Line 2018ApJ...863..164D
    Enhancing SDO/HMI images using deep learning 2018A&A...614A...5D
    Inversions of synthetic umbral flashes: Effects of scanning time on the inferred atmospheres 2018A&A...614A..73F
    High-resolution imaging and near-infrared spectroscopy of penumbral decay 2018A&A...614A...2V
    Small-scale Magnetic Flux Emergence in the Quiet Sun 2018ApJ...859L..26M
    The Magnetic Response of the Solar Atmosphere to Umbral Flashes 2018ApJ...860...28H
    Alfvén wave dissipation in the solar chromosphere 2018NatPh..14..480G
    On the Importance of the Nonequilibrium Ionization of Si IV and O IV and the Line of Sight in Solar Surges 2018ApJ...858....8N
    The magnetic nature of umbra-penumbra boundary in sunspots 2018A&A...611L...4J
    Possible Photometric Signatures of Moderately Advanced Civilizations: The Clarke Exobelt 2018ApJ...855..110S
    High-resolution Observations of Hα Spectra with a Subtractive Double Pass 2018SoPh..293...36B
    Magneto-optical Effects in the Scattering Polarization Wings of the Ca I 4227 Å Resonance Line 2018ApJ...854..150A
    Center-to-limb variation of the continuum intensity and linear polarization of stars with transiting exoplanets 2017KPCB...33..166S
    The First Post-Kepler Brightness Dips of KIC 8462852 2018ApJ...853L...8B
    Structure of sunspot light bridges in the chromosphere and transition region 2018A&A...609A..73R
    Ca II 8542 Å brightenings induced by a solar microflare 2017A&A...608A.117K
    Signatures of the impact of flare-ejected plasma on the photosphere of a sunspot light bridge 2017A&A...608A..97F
    The Temperature - Magnetic Field Relation in Observed and Simulated Sunspots 2017SoPh..292..188S
    Comparison of Solar Fine Structure Observed Simultaneously in Lyα and Mg ii h 2017ApJ...847..141S
    A Si I atomic model for NLTE spectropolarimetric diagnostics of the 10 827 Å line 2017A&A...603A..98S
    DeepVel: Deep learning for the estimation of horizontal velocities at the solar surface 2017A&A...604A..11A
    Search for torsional oscillations in isolated sunspots 2017A&A...604A..36G
    Numerical simulations of quiet Sun magnetic fields seeded by the Biermann battery 2017A&A...604A..66K
    A Hot Downflowing Model Atmosphere for Umbral Flashes and the Physical Properties of Their Dark Fibrils 2017ApJ...845..102H
    CLASP/SJ Observations of Rapid Time Variations in the Lyα Emission in a Solar Active Region 2017ApJ...846..127I
    The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry 2017SSRv..210..183T
    Design of an adaptable Stokes polarimeter for exploring chromospheric magnetism 2017AdSpR..60.1547L
    Chromospheric impact of an exploding solar granule 2017A&A...602L..12F

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