Ministerio de Ciencia, Innovación y Universidades Gobierno de Canarias Universidad de La Laguna CSIC Centro de Excelencia Severo Ochoa

Astrophysics Research Projects

The Sun and Solar System

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Magnetism, Polarization and Radiative Transfer in Astrophysics (P/300725)

TANAUSÚ DEL PINO ALEMAN

Javier Trujillo Bueno, Melania Cubas Armas, Nikolas Vitas, Supriya Hebbur Dayananda, Reza Rezaei, Andrii Sukhorukov, Jaume Bestard, Andrés Asensio Ramos, Fernando Moreno Insertis, Héctor Socas Navarro, Ernest Alsina Ballester, Ana Belén Griñón Marín 

IAC Collaborators: María Jesús Martínez González, Iñigo Arregui Uribe-Echevarria, Cristina Ramos Almeida, Carlos Allende Prieto, Manuel Collados Vera, Basilio Ruiz Cobo 

M. Bianda, L. Belluzzi, R. Ramelli (Istituto Solari Ricerche, Suiza); R. Casini, R. Centeno, J. de la Cruz Rodríguez (University of Stockholm, Suecia); N. Shchukina (Main Astronomical Observatory; Kiev; Ucrania); H. Uitenbroek (NSO, EEUU); D. Mckenzie (University of Alabama, Huntsville); R. Ishikawa (NAOJ); F. Auchére (IAS); J. Stepan (Astronomical Institute, Czech Republic)

Introduction 

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 interferences 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 aim 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.

Milestones

1) We have applied, for the first time in Solar Physics, deep learning techniques for the fast estimation of velocity fields parallel to the solar surface. For this, we developed a neural network that takes two consecutive images of solar granulation and returns the velocity vector. This method allows to measure velocity fields for each pixel in time intervals of 30 s, greatly improving previous methods (which need several minutes or hours and fields of view of several arcsec).

2) Spectro-polarimetric observations from the space telescope Hinode, processed with a novel analysis technique developed in this work, allowed us to confirm, for the first time, the theoretical prediction that linear polarization in forbidden lines has the opposite sign compared to normal permitted lines as a result of the line being dominated by a magnetic dipole transition. Our observations open a new window for solar oxygen abundance studies, offering an alternative method to disentangle the Ni I blend from the [O I] line at 630.03 nm that has the advantage of simple LTE formation physics.

3) Since the 1980s, solar astronomers have tried to detect the presence of sunspot torsional oscillations. According to theoretical models, if such oscillations exist they should have periods between several hours and days. Owing to the extremely long periods and the need to determine the magnetic field orientation, the problem is extremely challenging from the observational point of view. Several previous works claim to have found such oscillations with disparate characteristics, always very marginally, pushing the data to the limits. The HMI instrument onboard SDO allowed us, for the first time, to attack this problem with the right tools, delivering space-based observations with full time coverage. We conclude that these oscillations do not exist with amplitudes larger than a degree, refuting previous results as false detections.

4) The first results of the CLASP international experiment (Chromospheric Lyman-alpha Spectropolarimeter), which was motivated by theoretical research carried out by our group, have been published in The Astrophysical Journal Letters. This instrument, launched on September 3, 2015 by a NASA suborbital rocket, has allowed us to discover linear polarization signals in the radiation of the hydrogen Lyman-alpha line and thus confirm the theoretical predictions. These results open a new window for the exploration of the magnetization and the geometrical complexity of the enigmatic chromosphere-corona transition region.

5) Working within the framework of a rigorous quantum theory of spectral line polarization, we have developed a numerical code that solves the full non-LTE radiative transfer problem for polarized radiation, in one-dimensional models of the solar atmosphere, accounting for the combined action of the Hanle and Zeeman effects, as well as for partial frequency redistribution (PRD) phenomena. The application of this new diagnostic tool has allowed us to discover a new physical mechanism that introduces an interesting magnetic sensitivity in the wings of the linear polarization profiles of strong resonance lines, such as Ca I 422.7 nm and Mg II k.

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