Aspects of MHD wave propagation in solar atmospheric studies

Abstract

The theme of this thesis is ideal linear MHD wave propagation in structured media, using models relevant to structures in the solar atmosphere. We derive dispersion relations governing the ideal linear MHD modes for stationary states which are discretely structured in velocity and other plasma properties, in a direction transverse to the magnetic field, with field-aligned steady flow; the discrete structures considered are the single interface, uniform slab and uniform cylinder. This represents an extension of earlier models for the static case (Edwin 1984), by the inclusion of structured flows. The basic effects of flow are described, drawing on a discussion of the dispersion relations. The dispersion relations for the case of incompressible surface modes are examined in detail. We identify the qualitative effects of flow, including the onset of instability, by tracing the evolution of stable solutions and their propagation windows, as the relative flow is increased. Our analysis is presented in terms of a general formulation applicable to all three geometries (interface, slab and cylinder), revealing the combined role of dispersion and the ratio of densities in the two media. We go on to consider the relevance of the incompressible approximation to compressible surface modes, with particular reference to the static case of a single interface one side of which is field-free. We present and investigate analytical solutions for several special cases. The properties of the solutions obtained are compared with those for the equivalent incompressible case. Finally, we turn to the topic of global oscillations of quiescent prominences. A uniform slab model (Joarder 1993) yields, under conditions appropriate to the prominence-coronal inhomogeneity with the magnetic field threading the prominence being line-tied in the photosphere, modes which are analogous to the oscillations of a uniform string loaded with a point mass, and a formula approximating the period is given. We investigate the robustness of this formula for various plasma density profiles, assessing the applicability of the results from the uniform slab calculation to more realistic density profiles of the prominence-coronal inhomogeneity.