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

    • Solar-cycle and Latitude Variations in the Internetwork Magnetism

      The importance of the quiet-Sun magnetism is that it is always there to a greater or lesser extent, being a constant provider of energy, independently of the solar cycle phase. The open questions about the quiet-Sun magnetism include those related to its origin. Most people claim that the local dynamo action is the mechanism that causes it. This

      Trelles Arjona, J. C. et al.

      Advertised on:

      2
      2023
      Citations
      0
    • Formation of the Mg II h and k Polarization Profiles in a Solar Plage Model and Their Suitability to Infer Magnetic Fields

      The Mg II h and k lines are among the strongest in the near-ultraviolet solar spectrum and their line core originates in the upper chromosphere, just below the transition region. Consequently, they have become one of the main targets for investigating the magnetism of the upper solar atmosphere. The recent Chromospheric Layer Spectropolarimeter

      Afonso Delgado, David et al.

      Advertised on:

      1
      2023
      Citations
      5
    • Towards the Identification and Classification of Solar Granulation Structures Using Semantic Segmentation

      Solar granulation is the visible signature of convective cells at the solar surface. The granulation cellular pattern observed in the continuum intensity images is characterised by diverse structures e.g., bright individual granules of hot rising gas or dark intergranular lanes. Recently, the access to new instrumentation capabilities has given us

      Díaz Castillo, S. M. et al.

      Advertised on:

      6
      2022
      Citations
      1
    • Polarimetric characterization of segmented mirrors

      We study the impact of the loss of axial symmetry around the optical axis on the polarimetric properties of a telescope with segmented primary mirror when each segment is present in a different aging stage. The different oxidation stage of each segment as they are substituted in time leads to non-negligible crosstalk terms. This effect is

      Collados, M. et al.

      Advertised on:

      6
      2022
      Citations
      1
    • Ultraviolet spectropolarimetry with Polstar: using Polstar to test magnetospheric mass-loss quenching

      Polstar is a proposed NASA MIDEX space telescope that will provide high-resolution, simultaneous full-Stokes spectropolarimetry in the far ultraviolet, together with low-resolution linear polarimetry in the near ultraviolet. This observatory offers unprecedented capabilities to obtain unique information on the magnetic and plasma properties of the

      Shultz, M. E. et al.

      Advertised on:

      12
      2022
      Citations
      5
    • Ultraviolet spectropolarimetry: investigating stellar magnetic field diagnostics

      Magnetic fields are important for stellar photospheres and magnetospheres, influencing photospheric physics and sculpting stellar winds. Observations of stellar magnetic fields are typically made in the visible, although infrared observations are becoming common. Here we consider the possibility of directly detecting magnetic fields at ultraviolet

      Folsom, C. P. et al.

      Advertised on:

      12
      2022
      Citations
      4
    • The Circular Polarization of the Mn 1 Resonance Lines around 280 nm for Exploring Chromospheric Magnetism

      We study the circular polarization of the Mn I resonance lines at 279.56, 279.91, and 280.19 nm (hereafter, UV multiplet) by means of radiative transfer modeling. In 2019, the CLASP2 mission obtained unprecedented spectropolarimetric data in a region of the solar ultraviolet including the Mg II h and k resonance lines and two lines of a subordinate

      del Pino Alemán, Tanausú et al.

      Advertised on:

      11
      2022
      Citations
      3
    • The polarization signals of the solar K I D lines and their magnetic sensitivity

      Aims: This work aims to identify the relevant physical processes in shaping the intensity and polarization patterns of the solar K I D lines through spectral syntheses, placing particular emphasis on the D2 line. Methods: The theoretical Stokes profiles were obtained by numerically solving the radiative transfer problem for polarized radiation

      Alsina Ballester, E.

      Advertised on:

      10
      2022
      Citations
      2
    • Polarization Accuracy Verification of the Chromospheric LAyer SpectroPolarimeter

      We have developed an advanced UV spectropolarimeter called Chromospheric LAyer SpectroPolarimeter (CLASP2), aimed at achieving very high accuracy measurements (<0.1% at 3 σ ) of the linear (Q /I and U /I ) and circular (V /I ) polarizations of the Mg II h and k lines (280 nm). CLASP2 was launched on board a NASA sounding rocket on April 11, 2019

      Song, Donguk et al.

      Advertised on:

      10
      2022
      Citations
      3
    • Evidence of a flare ignited above a low-latitude spotted active region in the ultrafast rotator HK Aqr

      We study the magnetic activity in the ultrafast rotator dMe HK Aqr using tomography techniques with high-resolution spectroscopy. We aim to characterize how this magnetic activity appears in a regime of very fast rotation without external forces, given that HK Aqr is, very likely, a single star. We find dark spots located at low latitudes. We also

      Martínez González, María Jesús et al.

      Advertised on:

      11
      2022
      Citations
      1
    • Magnetic Field Diagnostics in the Solar Upper Atmosphere

      The magnetic field is the main driver of the activity in the solar upper atmosphere, but its measurement is notoriously difficult. In order to determine the magnetic field in the chromosphere, transition region, and corona, we need to measure and interpret the polarization signals that the scattering of anisotropic radiation and the Hanle and

      Trujillo Bueno, J. et al.

      Advertised on:

      8
      2022
      Citations
      14
    • Effects of Spectral Resolution on Simple Magnetic Field Diagnostics of the Mg II H and K Lines

      We study the effects of finite spectral resolution on the magnetic field values retrieved through the weak-field approximation (WFA) from the cores of the Mg II h and k lines. The retrieval of the line-of-sight component of the magnetic field, B LOS, from synthetic spectra generated in a uniformly magnetized FAL-C atmosphere is accurate when

      Centeno, Rebecca et al.

      Advertised on:

      9
      2022
      Citations
      3
    • Influence of Thomson Electron Scattering Redistribution on Spectral Line Polarization Formed in Spherically Symmetric Extended and Expanding Atmospheres

      Scattering of line photons by ambient electrons in thermal motion in the stellar atmosphere modifies the wings of both intensity and linear polarization profiles of the spectral lines. The aim of the present paper is to investigate in detail the influence of Thomson electron scattering redistribution on resonance line polarization formed in

      Sampoorna, M. et al.

      Advertised on:

      9
      2022
      Citations
      1
    • Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg II h and k Lines

      The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg II h and k spectral region around 2800 Å along a 200″ slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar

      Rachmeler, L. A. et al.

      Advertised on:

      9
      2022
      Citations
      13
    • 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

      Aims: We present the theoretical framework and numerical methods we have implemented to solve the problem of the generation and transfer of polarized radiation in spectral lines without assuming local thermodynamical equilibrium, while accounting for scattering polarization, partial frequency redistribution (due to both the Doppler effect and

      Alsina Ballester, E. et al.

      Advertised on:

      8
      2022
      Citations
      7
    • TIC: A Stokes Inversion Code for Scattering Polarization with Partial Frequency Redistribution and Arbitrary Magnetic Fields

      We present the Tenerife Inversion Code (TIC), which has been developed to infer the magnetic and plasma properties of the solar chromosphere and transition region via full Stokes inversion of polarized spectral lines. The code is based on the HanleRT forward engine, which takes into account many of the physical mechanisms that are critical for a

      Li, H. et al.

      Advertised on:

      7
      2022
      Citations
      12
    • Detection of Flare-induced Plasma Flows in the Corona of EV Lac with X-Ray Spectroscopy

      Stellar flares are characterized by sudden enhancement of electromagnetic radiation from the atmospheres of stars. Compared to their solar counterparts, our knowledge on the coronal plasma dynamics of stellar flares and their connection to coronal mass ejections remains very limited. With time-resolved high-resolution spectroscopic observations

      Chen, Hechao et al.

      Advertised on:

      7
      2022
      Citations
      8
    • Searching for technosignatures in exoplanetary systems with current and future missions

      Technosignatures refer to observational manifestations of technology that could be detected through astronomical means. Most previous searches for technosignatures have focused on searches for radio signals, but many current and future observing facilities could also constrain the prevalence of some non-radio technosignatures. This search could

      Haqq-Misra, Jacob et al.

      Advertised on:

      9
      2022
      Citations
      7
    • Accelerating Non-LTE Synthesis and Inversions with Graph Networks

      The computational cost of fast non-LTE synthesis is one of the challenges that limits the development of 2D and 3D inversion codes. It also makes the interpretation of observations of lines formed in the chromosphere and transition region a slow and computationally costly process, which limits the inference of the physical properties on rather

      Vicente Arévalo, A. et al.

      Advertised on:

      4
      2022
      Citations
      3
    • Spectropolarimetric observations of the solar atmosphere in the Hα 6563 Å line

      We present novel spectropolarimetric observations of the hydrogen Hα line taken with the Zürich Imaging Polarimeter (ZIMPOL) at the Gregory Coudé Telescope of the Istituto Ricerche Solari Locarno (IRSOL). The linear polarization is clearly dominated by the scattering of anisotropic radiation and the Hanle effect, while the circular polarization is

      Jaume Bestard, J. et al.

      Advertised on:

      3
      2022
      Citations
      2

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