Helioseismology and diagnostics of internal magnetic layers

Abstract

Solar magnetic fields, as well as temperature changes, introduce pressure deviations that play a significant role in modulating the resonant frequencies of p-mode oscillations. Those pressure deviations occurring in the atmosphere or sub-surface of the Sun can explain the frequency shifts observed on the timescale of the solar activity cycle. A separate study of the contribution of internal magnetic layers can clarify the relative importance of surface effects. Results from helioseismology provide realistic constraints for choosing parameters suitable to represent the magnetic layers buried in the solar interior and available for modelling, i.e. at the base of the convection zone and in the sunspots’ anchoring zone. Diagnostics of the internal magnetic layers are obtained through a schematic model in which the Sun is plane-stratified. The influence of a buried magnetic field on p-modes is explored, and the nature of various waves and instabilities that can arise on such a buried magnetic field is assessed. By treating the effects of internal magnetic layers, this thesis contributes to the building of a bridge between theories and observations. On the one hand, the theoretical analysis is explored carefully in the course of its formulation, which generates new hypotheses that were not obvious so far. On the other hand, observations help to understand which explanations of the solar cycle frequency shifts may apply.