Radiative transfer modeling of the spectral line polarization produced by optically pumped atoms in the solar atmosphere

Resumen

The main goal of this thesis is the study of the generation and transfer of polarized radiation in several spectral lines whose diagnostic potential has not been thoroughly investigated before. We aim at developing and applying reliable plasma diagnostic techniques, based on numerical radiative transfer calculations using realistic atomic and atmospheric models, suitable for modeling and interpreting the linear polarization due to scattering processes in weakly magnetized regions of the solar photosphere and chromosphere (with magnetic strengths B < 100 G). We also consider the circular polarization produced by the Zeeman effect. To this end, we have formulated the statistical equilibrium and radiative transfer equations within the framework of the quantum theory of spectral line polarization, and we have developed efficient numerical methods and radiative transfer codes without assuming Local Thermodynamic Equilibrium (LTE). The application of these radiative transfer techniques has allowed us to improve significantly our understanding of the true physical origin of the scattering polarization observed in several spectral lines, a crucial first step for inferring the physical properties and magnetic fields of the observed solar atmospheric region. The first research problem investigated in this thesis has been the influence of the non-coherence of the scattering with neutral hydrogen atoms and electrons in the scattering linear polarization of the spectral lines, a physical phenomenon that is usually approximated as coherent. To this end, we have developed an efficient numerical method to solve the problem of the generation and transfer of scattering polarization for a two-level atom taking into account the non-coherent interaction with neutral hydrogen atoms and electrons. We find that the non-coherent scattering is a mechanism that, under certain physical circumstances, can produce linear polarization signals in intrinsically unpolarizable spectral lines. Motivated by this interesting property, we have studied its potential to explain the "enigmatic" linear polarization signal that has been observed in the D1 line of ionized barium (Stenflo et al. 2000). Our second objective has been the understanding of the enigmatic linear polarization profiles observed in several spectral lines, via a detailed modeling of the generation and transfer of polarized radiation. To this aim, we have developed a radiative transfer code that can solve the problem of the generation and transfer of polarized radiation for any multilevel model atom in one-dimensional model atmospheres, taking into account the anisotropic radiation pumping, the Hanle effect and the Zeeman effect (for the circular polarization). Applying this radiative transfer tool, we have carried out several investigations with the aim of achieving a better understanding of the scattering polarization observed in the following spectral lines: the D1 and D2 resonance lines of ionized barium and the triplet whose lower levels are metastable, the infrared triplet of neutral oxygen and the multiplet 42 of neutral titanium. The emergent Stokes profiles of these spectral lines are sensitive to different regions of the solar atmosphere, going from the low photosphere to the chromosphere. Finally, we have studied the magnetic sensitivity of the k line of ionized magnesium. To this end, we have applied the code PORTA (see Stepan & Trujillo Bueno 2013) to solve the problem of the generation and transfer of polarized radiation in a three-dimensional solar model atmosphere that results from a magneto hydrodynamic simulation (Carlsson et al. 2015). Understanding the sensitivity of this very strong resonance line to the thermal, dynamic and magnetic structure of the upper solar chromosphere is of great scientific interest for planning future sub-orbital and orbital space missions (e.g., the second flight of the Chromospheric LAyer SPectropolarimeter, or CLASP-2) and for interpreting the novel spectropolarimetric observations that such space telescopes will make feasible.