Scattering theory and radiative transfer in spherically symmetric moving atmospheres a thesis submitted for the award of degree of doctor of philosophy to the department of physics, Pondicherry University, Puducherry A. Megha [Thesis]

Ph.D. Thesis, Pondicherry University, Puducherry

Doctor of Philosophy Pondicherry University, Puducherry 2020

Magnetic fields are ubiquitous in astrophysical plasmas. They are responsible for most of the stellar activities and they manifest in numerous ways in the stellar atmospheres. The Sun being our nearest star is the natural laboratory to understand the causes and effects of the magnetic fields. The solar magnetic field couples the solar interior with its atmosphere. It drives the dynamic phenomena such as coronal mass ejections (CMEs) and solar flares. The magnetic field also plays a critical role in heating the solar upper chromosphere and corona as well as in accelerating the solar wind. A variety of techniques are used to infer these magnetic fields and subsequently map them into the layers of the solar atmosphere, from where the concerned observable originates. Studies of polarization properties of spectral lines formed in solar atmosphere serve as one of the best methods to determine the nature of solar magnetic fields (Stenflo 1994). The polarization is the result of breaking of symmetry in the source region. This symmetry breaking in the line forming regions can be attributed to the anisotropic illumination of the atoms and presence of magnetic fields. In the presence of magnetic fields the energy levels of the atoms split into different magnetic m sub-states. The transition from these split magnetic sub-states results in circularly or linearly or elliptically polarized light depending on the strength and orientation of the magnetic field with respect to the chosen line-of-sight (LOS). This effect discovered by Zeeman in the laboratory in 1896, is popularly known as Zeeman effect. It was first seen on Sun by Hale (1908) in sunspots. Using Zeeman effect in spectral lines it became possible to measure both the strength and orientation of the magnetic field vector (especially the strong fields of few kG). Zeeman effect cannot be used effectively in the presence of very weak fields (due to extremely small splitting) and also, it is not suitable to measure the turbulent fields (due to cancellation of the opposite polarities of the magnetic field within the finite spectral resolution). Under such conditions the magnetic field in the solar atmosphere can be determined by the linear polarization measurement of spectral lines with appropriate sensitivity to Hanle effect. Hanle effect is the result of quantum interferences between different magnetic m sub-states of a given atomic level involved in the transition. It is the modification: depolarization or repolarization and rotation of plane of linear polarization, of resonance scattering in the presence of weak magnetic fields. Therefore Zeeman and Hanle effects can be used to diagnose the magnetic field of the Sun in a very different and complementary parameter regimes. The aim of this thesis is to develop pure theoretical tools required for the determination of the solar magnetic fields using polarized spectral line formation theory.