OBSERVATOIRE DE PARIS ECOLE DOCTORALE ASTRONOMIE ET ASTROPHYSIQUE D’ILE-DE-FRANCE Doctorat ASTRONOMIE ET ASTROPHYSIQUE par Jǐŕı ŠTĚPÁN Etude du transfert du rayonnement polarisé dans l’atmosphère solaire

The adequate theoretical models of the NLTE radiative transfer are needed for interpretation of the complicated spectra emerging from the magnetized solar atmosphere. In the first part of the thesis, the quantum theory of polarization is reviewed along with the problem of NLTE radiative transfer of the 2nd kind. A general notation for a numerical treatment of the atomic density matrix is introduced. A lambda-operator splitting technique is presented and used for development of a novel preconditioning method analogous to the Jacobi iteration algorithm. Number of processes is taken into account: atomic polarization, level-crossings, Paschen-Back effect, Hanle and Zeeman effects, optical pumping, collisional (de)polarization. A nonlinear multigrid technique is developed in the framework of polarized radiative transfer theory and the efficiency of the method is discussed. The potential of this new solver is demonstrated on an example of transfer of the optically thick line of a multiterm atom in the Paschen-Back effect regime. In the second part of the thesis, the particular problem of impact polarization of the hydrogen H-alpha line in solar flares is considered. The semi-empirical models of the flaring chromosphere are used to find the differential effects of the proton beam on the line intensity and linear polarization profiles. Proton beams are shown to be an unlikely source of the observed linear polarization. In the last chapter, it is shown that the electric return currents are significant ingredient of the line formation and they are proposed as being a possible source of the observed linear polarization.