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Damping and stability of slow MHD waves in solar flux tubes

Boek - Dissertatie

The research presented in this thesis concerns the evolution of slow magnetohydrodynamic (MHD) waves in tubular atmospheric structures. The solar atmosphere is a highly dynamic medium in which MHD waves are an observable phenomenon. The outermost layer of the atmosphere, called the corona, has an unexpectedly high temperature on the order of 1M Kelvin. The solution of what is termed the coronal heating problem is not yet fully elucidated, but MHD waves are known to contribute to the heating by depositing some of their energy in the coronal plasma through various mechanisms. In addition, MHD waves can be used to probe the solar atmospheric plasma and derive some of its unobservable properties by matching observations with theory, in what is called atmospheric seismology. The study of slow MHD waves, present in every layer of the atmosphere, is a fundamental stepping stone towards the full understanding of the solar atmosphere. In particular, the assessment of the damping they undergo and the stability of the structure hosting them plays an important part in this process. This forms the subject of this thesis. We start by studying the resistive damping of slow sausage modes in a cylinder with a sharp boundary, and apply our analytical model to the case of a photospheric pore. Our results are found to match previous numerical investigations, in which it is claimed that electrical resistivity is more important than resonant absorption in the cusp continuum to damp slow sausage modes in these circumstances. We next study the local stability of a cylinder which undergoes a standing slow mode oscillation through a Cartesian model, where two plasmas with an oscillating bacground velocity are separated by a straight interface. It is found that, unlike in the case of a standing fast kink wave, the perturbations on the interface remain stable with respect to the Kelvin-Helmholtz instability for the slow mode. In another small chapter, we derive the formulas for the different energy and energy flux components of modes in a cylinder with a sharp boundary. Finally, we investigate slow modes which are resonantly absorbed in the cusp continuum (called quasimodes) through an analytical model where the solutions of the linear perturbations are represented by Frobenius and power series. Applying this to the case of a slow surface sausage mode in a photospheric pore, we find that the damping of this mode through resonant absorption in the cusp continuum is extremely weak. This is in line with the results of the first chapter, namely that electrical resistivity is more efficient than resonant absorption for damping slow sausage modes in pores.
Jaar van publicatie:2022
Toegankelijkheid:Open