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

    Power Spectra of Velocities and Magnetic Fields on the Solar Surface and their Dependence on the Unsigned Magnetic Flux Density 2012ApJ...758..139K
    The Hanle Effect of Lyα in a Magnetohydrodynamic Model of the Solar Transition Region 2012ApJ...758L..43S
    LEMUR: Large European module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission 2012ExA....34..273T
    Unusual Stokes V profiles during flaring activity of a delta sunspot 2012A&A...547A..34F
    Signal detection for spectroscopy and polarimetry 2012A&A...547A.113A
    Anomalous Circular Polarization Profiles in the He I 1083.0 nm Multiplet from Solar Spicules 2012ApJ...759...16M
    Hanle Effect for Stellar Dipoles and Quadrupoles 2012ApJ...760....7M
    VFISV: Very Fast Inversion of the Stokes Vector for the Helioseismic and Magnetic Imager 2011SoPh..273..267B
    The Horizontal Magnetic Flux of the Quiet-Sun Internetwork as Observed with the Hinode Spectro-Polarimeter 2008ApJ...672.1237L
    Spectro-polarimetry in the era of large solar telescopes 2010AN....331..581S
    Non-local thermodynamic equilibrium inversions from a 3D magnetohydrodynamic chromospheric model 2012A&A...543A..34D
    Direct Imaging of Fine Structure in the Chromosphere of a Sunspot Umbra 2009ApJ...696.1683S
    Are solar chromospheric fibrils tracing the magnetic field? 2011A&A...527L...8D
    A New Approach to the Solar Oxygen Abundance Problem 2008ApJ...682L..61C
    A high-resolution three-dimensional model of the solar photosphere derived from Hinode observations 2011A&A...529A..37S
    Polar Field Reversal Observations with Hinode 2012ApJ...753..157S
    Pervasive Linear Polarization Signals in the Quiet Sun 2012ApJ...757...19B
    On the Distribution of Quiet-Sun Magnetic Fields at Different Heliocentric Angles 2012ApJ...746..182O
    Analysis of Quiet-Sun Internetwork Magnetic Fields Based on Linear Polarization Signals 2012ApJ...751....2O
    Resonance scattering polarization in the magnetosphere of Mercury 2012Icar..220.1104L
    Polarimetric Diagnostics of Unresolved Chromospheric Magnetic Fields 2009ApJ...701L..43C
    Magnetic field strength of active region filaments 2009A&A...501.1113K
    Wave Propagation and Shock Formation in Different Magnetic Structures 2009ApJ...692.1211C
    Three-dimensional Radiative Transfer Modeling of the Polarization of the Sun's Continuous Spectrum 2009ApJ...694.1364T
    The Influence of Coronal EUV Irradiance on the Emission in the He I 10830 Å and D3 Multiplets 2008ApJ...677..742C
    The Hanle Effect of the Hydrogen Lyα Line for Probing the Magnetism of the Solar Transition Region 2011ApJ...738L..11T
    Solar magnetism eXplorer (SolmeX). Exploring the magnetic field in the upper atmosphere of our closest star 2012ExA....33..271P
    Scattering Polarization of the Ca II IR Triplet for Probing the Quiet Solar Chromosphere 2010ApJ...722.1416M
    Scattering Polarization of Hydrogen Lines in Weakly Magnetized Stellar Atmospheres. I. Formulation and Application to Isothermal Models 2011ApJ...732...80S
    Scattering Polarization and Hanle Effect in Stellar Atmospheres with Horizontal Inhomogeneities 2011ApJ...743...12M
    On the Sensitivity of Partial Redistribution Scattering Polarization Profiles to Various Atmospheric Parameters 2010ApJ...722.1269S
    On the Probable Existence of an Abrupt Magnetization in the Upper Chromosphere of the Quiet Sun 2010ApJ...711L.133S
    Non-LTE Determination of the Silicon Abundance Using a Three-dimensional Hydrodynamical Model of the Solar Photosphere 2012ApJ...755..176S
    Determining the Magnetization of the Quiet Sun Photosphere from the Hanle Effect and Surface Dynamo Simulations 2011ApJ...731L..21S
    The Scattering Polarization of the Lyα Lines of H I and He II Taking into Account Partial Frequency Redistribution and J-state Interference Effects 2012ApJ...755L...2B
    The Polarization of the Solar Mg II h and k Lines 2012ApJ...750L..11B
    The Physical Origin and the Diagnostic Potential of the Scattering Polarization in the Li I Resonance Doublet at 6708 Å 2009ApJ...705..218B
    The Lyα Lines of H I and He II: A Differential Hanle Effect for Exploring the Magnetism of the Solar Transition Region 2012ApJ...746L...9T
    The Impact of Quantum Interference between Different J-levels on Scattering Polarization in Spectral Lines 2011ApJ...743....3B
    Spectropolarimetric forward modelling of the lines of the Lyman-series using a self-consistent, global, solar coronal model 2011A&A...529A..12K

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