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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - POLMAG (Polarized Radiation Diagnostics for Exploring the Magnetism of the Outer Solar Atmosphere )

Teaser

Summary

POLMAG aims at a true breakthrough in the development and application of polarized radiation diagnostic methods for exploring the magnetic fields of the chromosphere, transition region and corona of the Sun via the interpretation of the Stokes profiles produced by optically polarized atoms and the Hanle and Zeeman effects in ultraviolet (UV), visible and near-infrared spectral lines. To this end, POLMAG will combine and expand expertise on atomic physics, on the quantum theory of radiation, on high-precision spectropolarimetry, on advanced methods in numerical radiative transfer, and on the confrontation of spectropolarimetric observations with spectral synthesis in increasingly realistic three-dimensional (3D) numerical models of the solar atmosphere.

POLMAG targets the following very challenging issues:

- Which are the optimum spectral lines for probing the magnetism of the outer solar atmosphere ?
- How to compute efficiently the Stokes profiles taking into account partial frequency redistribution, J-state quantum
interference and the Hanle and Zeeman effects ?
- How to determine the magnetic, thermal and dynamic structure of the outer solar atmosphere through confrontations
with spectropolarimetric observations ?

POLMAG will go well beyond the current state of the art as follows:

- Applying and extending the quantum theory of light polarization
- Developing and applying efficient radiative transfer codes
- Modeling the Ly-alpha and Mg II h & k observations of our CLASP suborbital rocket experiments
- Developing novel coronal magnetometry methods by complementing for the first time the information provided by
forbidden and permitted lines
- Developing the plasma diagnostic techniques needed for the scientific exploitation of spectropolarimetric
observations with the new generation of solar telescopes and putting them at the disposal of the astrophysical
community

Work performed

During the first 18 months of the project we have created the group, which is now formed by one computer scientist, four post-doctoral researchers, two PhD students and the PI. In addition, there are a few external and IAC collaborators.

Main results achieved so far:

(1) We have carried out a novel investigation of the Hanle effect in the resonance line based on 3D radiative transfer calculations in a high-resolution magneto-convection model having most of the convection zone magnetized close to the equipartition and a surface mean field strength of 170 gauss. The Hanle effect produced by the modelâ€™s magnetic field depolarizes the zero-field scattering polarization signals significantly, to the extent that the center-to-limb variation of the calculated spatially averaged polarization amplitudes is compatible with the observations.

(2) We have shown that the magneto-optical (M-O) terms of the Stokes-vector transfer equation produce sizable signals in the wings of U/I and a clear sensitivity of the Q/I and U/I wings to the presence of photospheric magnetic fields with strengths similar to those that produce the Hanle effect in the line core.

(3) We find that the degraded theoretical profiles of the Ca II 854.2 nm line are qualitatively similar to the observed ones, but there is a fundamental difference in the width of all Stokes profiles: the observed lines are wider than the theoretical lines. We find that the amplitudes of the observed profiles are larger than those of the theoretical ones, which suggests that the symmetry breaking effects in the solar chromosphere are stronger than in the 3D model atmosphere. This means that the isosurfaces of temperature, velocity and magnetic field strength and orientation are more corrugated in the solar chromosphere than in the currently available 3D radiation-magneto-hydrodynamics simulations.

(4) In collaboration with USA and Japan, our POLMAG group participates at the PI level in a series of suborbital rocket experiments to investigate the enigmatic chromosphere-corona transition region of the Sun via ultraviolet spectropolarimetry.

(5) In spite of the fact that the Zeeman splitting induced by such weak fields is very small compared to the total width of the line, the magneto-optical terms that couple the transfer equations for Stokes Q and U are actually able to produce sizable changes in the Q/I and U/I wings. Our theoretical results further expand the diagnostic content of the unprecedented spectropolarimetric observations provided by the Chromospheric Lyman-Alpha Spectropolarimeter (CLASP).

Final results

(1) We have initiated two PhD theses. The topic of the first thesis is related with the WP-4 working package, specifically with the investigation of the polarization of chromospheric lines that can be observed with ground-based telescopes (e.g., Ca I 422.7 nm and the hydrogen H-alpha line). The topic of the second thesis, which we started a few months ago, is fully related with the POLMAG working packages WP-1 and WP-2, i.e., with the investigation of the diagnostic potential of the ultraviolet solar spectrum, whose observation requires instruments like CLASP.

(2) We have made a very significant progress with the development of a numerical code for calculating the intensity and polarization of the radiation scattered by the atoms and ions of the solar corona, both in permitted and forbidden spectral lines (see the POLMAG working package WP-3). The aim is to develop new diagnostic tools for probing the solar coronal plasma.

(3) We have carried out a complex radiative transfer investigation of the polarization produced by the joint action of scattering processes and the Hanle, Zeeman and magneto-optical effects in the resonance and subordinate lines of Mg II, which are located around 280 nm. Moreover, we have written a very detailed paper (del Pino AlemÃ¡n et al. 2019), which we are about to submit to The Astrophysical Journal.

(4) We have advanced with the development of an efficient multi-D, non-LTE inversion code that takes into account scattering polarization and the Hanle and Zeeman effects. Applying sparsity regularization, domain decomposition, and the massively parallel strategy of our PORTA radiative transfer code, we are able to efficiently infer the self-consistent thermal and magnetic properties of plasma structures embedded in the solar corona.

(5) Concerning WP-6, we are very close to complete the development of the public version of PORTA, our 3D radiative transfer code for modeling the intensity and polarization of spectral lines with massively parallel computers such as the MareNostrum. In addition, a new improved version of our HAZEL code has already been made available to the astrophysical community via the POLMAG website.

(6) A few months ago, on 11 April 2019, we launched CLASP-2 from the NASA center in the White Sands Missile Range, near Las Cruces (USA). This second suborbital rocket experiment has allowed us to measure for the first time the wavelength variation of the polarization produced by the joint action of scattering processes and the Hanle, Zeeman and magneto-optical effects across the Mg II h & k ultraviolet lines.

At the end of August 2019, we will present an advance of all such results at an international conference in GÃ¶ttingen (Germany). More recently, we have completed the writing of a 30 pages review paper on â€œOptically Polarized Atoms in the Solar Atmosphereâ€ and on 2019 July 1 we delivered an invited talk in a Symposium of the International Astronomical Union (IAU) in Chile.