TY - THES ID - 135113239 TI - Back-reaction of the plasma on coronal oscillations AU - Magyar, Norbert AU - Van Doorsselaere, Tom AU - KU Leuven. Faculty of science. Department of mathematics PY - 2018 PB - Leuven KU Leuven. Faculty of Science DB - UniCat UR - https://www.unicat.be/uniCat?func=search&query=sysid:135113239 AB - The Sun directly impacts many processes on our planet, from climate to thesustenance of life. While it needs no promoting, this makes it the most importantastrophysical object of study. Despite its obvious importance for us, it is safeto say that the Sun-Earth connection is far from being completely understood.The Sun exerts influence mostly by its luminosity, i.e. electromagnetic radiation,but also through its magnetic activity, leading to the solar wind and spaceweather, i.e. a variable but continuous flux of charged particles originating fromits dynamic atmosphere. Better understanding of space weather is essential aswe rely more and more on space-based technologies and as we fare further awayfrom Earth on space exploration missions. The driving force and origin of spaceweather is the magnetically dominated solar corona, which is still enigmatic froma physical point of view. Foremost, the almost 80-year-old mystery of how it isheated to multi-million degrees constitutes the famous coronal heating problem.The theories put forth to solve this conundrum can be put in two separateboxes (even if they might act together): the so-called direct-current models,in which the slow shear of the magnetic field lines leads to small reconnectionevents called nanoflares, and the alternative-current model, in which waves<br />generated by the turbulent convection at the Sun's surface get dissipated asthey propagate upwards.In this thesis, we focus on wave behaviour in the solar corona, withinthe framework of magnetohydrodynamics (MHD). The importance of a betterunderstanding of wave phenomena is twofold: on the one hand, as previouslymentioned, waves are a potential candidate for coronal heating; on the otherhand, as properties of waves hold clues about the medium they propagate in,they can be used as diagnostic tools for the elusive physical properties of the solarcorona, within the field of coronal seismology. The corona is not homogeneous,as the complex magnetic field configuration dictates its appearance. In thissense, we distinguish the open magnetic field corona, which are cooler regions,mostly situated at the Sun's poles, and the closed magnetic field corona, whichpresent the majestic coronal loops, arch-like plasma structures outlining themagnetic field. Coronal loops are central to coronal wave studies, as structuringintroduces many interesting phenomena, such as surface waves, wave damping,mode coupling, resonant absorption, phase mixing, and so on. The analyticalframework for these much-studied mechanisms is well developed, however,moving away from symmetric and linear problems to more realistic, nonlineardynamics is made possible with recent advances in numerical computing power.Much of the work carried out and presented in this thesis is thus concerning thenonlinear aspects of wave behaviour in the structured corona, using numericalsimulations, with implications for both the coronal heating problem and coronalseismology, the two prime outcomes of coronal wave studies.The first three studies presented in the results chapter focus on standingkink waves in coronal loops modeled as straight cylindrical flux tubes. Theeffects of radiative cooling, large amplitudes, and a twisted magneticfield on the oscillation properties are presented. In all cases, nonlinearities,among which the most prominent one is the development of the Kelvin-Helmholtzinstability around the loop, are shown to induce considerable deviations fromanalyitical results, e.g. in damping time and oscillation period. These have animpact on some seismological estimates that are based on these values, and onwave heating. Furthermore, it is hinting at the possibly complex and turbulentinternal structure of coronal loops, which is further explainedby studying the effect of propagating transverse waves in aninhomogeneous plasma. It is shown for the first time that turbulence can begenerated from unidirectionally propagating waves, constituting a paradigm shiftin MHD turbulence in general, not only for the coronal setting. Finally, thecapabilities of the promising and emerging field of dynamic coronal seismologyis evaluated. Based on the ubiquity of the transverse propagating Alfvénicwaves observed in the solar corona, the possibility of continuous diagnostics forphysical parameters such as magnetic field strengths would constitute advancesin our understanding of coronal evolution. It is shown that, despite boththeoretical and observational limitations, reliable magnetic field estimates can be achieved, robust to widely different simulated conditions, which are expectedto be present in the corona. ER -