Generation and transfer of polarized radiation in hydrodynamical models of the solar chromosphere

Resumen

The main goal of this thesis has been to investigate the effect that the macroscopic vertical velocity fields have on the scattering polarization signals formed in the solar chromosphere. We followed a theoretical approach based on the spectral synthesis of scattering polarization signals in dynamic models of the quiet Sun. Until now, the impact of macroscopic motions had never been considered in the treatment of the polarization signals produced by scattering processes and the Hanle effect. This is especially important in the solar chromosphere, given its strong dynamism and reduced magnetic field intensity. This investigation focuses in the analysis of the Ca ii IR triplet lines (at 8498, 8542 and 8662 A). The methodology of spectral synthesis allows to confront several chromospheric models with real observations. We solved the multilevel, non-LTE radiative problem of the generation and trans- fer of polarized radiation in increasingly realistic atmosphere models: Milne-Eddington atmospheres, atmospheres composed by two-level atoms, semiempirical models with ad-hoc velocities, hydrodynamical time-dependent models and a snapshot of a 3D MHD simulation. To such end, we included the action of the velocity fields on the atomic level polarization. Thus, we studied the impact of velocity gradients on the anisotropy of the radiation field, which controls the scattering polarization; and then, we showed the effects that the vertical velocity gradients have on the zero-field linear polarization profiles; finally, we synthetized the Stokes vectors in a forward scattering geometry including as well the Hanle effect produced by the magnetic field. Thus, we obtained the first tomographic view of a model quiet chromosphere that includes synthetic maps of linear polarization dominated by Hanle effect and of circular polarization dominated by Zeeman efffect. We focused on the use of such maps to diagnose the spatial topology of the magnetic field, the thermodynamical state of the atmosphere and the vertical stratification of velocity. We also studied the relevance of dynamic and thermodynamic in the calculation of the chromospheric magnetic field orientation in the presence of the 90º and 180º ambiguities. Furthermore, we simulated synthetic observations by degrading our polarization maps, as the space telescope Solar-C would do with real observations, and we reconstructed them by following several methods (e.g., Principal Component Analysis). We dedicated a chapter to the tools and technical procedures developed in this thesis: the RT code; an adaptative method for numerical grids that improves the convergence of the RT calculation; a PCA code; a program to calculate response functions for chromospheric lines; and finally, some advanced techniques for three- dimensional analysis and visualization.