The role of mode conversion in solar active region oscillations

This thesis is concerned with conversion between the various magnetohydrodynamic wave modes occurring in magnetic structures that dapple the Sun's surface and fill its atmosphere. We pay particular attention to fast-to-slow conversion which takes place where the sound and Alfvén speeds coincide, and show that fast-to-Alfvén conversion can also transpire in three-dimensional structures near where the fast wave reflects. Implications are explored for various model atmospheres. Specifically, exact two-dimensional isothermal solutions are found to confirm previous ray-theoretic modelling, and provide useful insights on the ability of flare waves in the high atmosphere to penetrate the magnetic canopy and reach the photosphere, proffering a possible mechanism for exciting sunquakes. In the real Sun, and more realistic models, regions of strong inclined magnetic field act as `magnetic portals' allowing otherwise trapped acoustic waves to propagate into the solar atmosphere. At the point where these fast waves necessarily suffer reflection due to the exponentially increasing Alfvén speed, coupling with Alfvén waves can see them transmit through the transition region and into the solar corona. We explore the conditions that make this journey possible by following the fast wave as it impinges on the magnetoacoustic conversion layer, until incidence on the transition region. The fast-to-Alfvén mode conversion is quantified in a cold plasma and a more realistic warm plasma, both with an appended transition region. The phase of the partially reflected fast wave is altered by the mode conversion process, with profound implications for Time-Distance helioseismology. This is explored quantitatively with and without a transition region.