Evolution and Non-Linear Consequences of Magnetic Twists in Flowing Plasmas

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Student thesis: Doctoral ThesisDoctor of Philosophy

Original languageEnglish
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Award date2017
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Abstract

In this thesis the overarching investigation is the interaction of Alfven waves with flowing plasmas. The main consequence of this interaction is that Alfven waves are amplified in proximity of negative flow gradients. This amplification is investigated using three numerical models.  The first and third studies implement fully non-linear Magnetohydrodynamics (MHD) systems, whereas the second study is a combination of a linear model and analytics. All studies adopt the thin flux tube approximation in 1.5-D MHD.
In the first study, the flow is supersonic yet sub-Alfvenic and undergoes a classical shock. The ow in this case is akin to the Evershed effect in sunspots. This is the first numerical confirmation that a shock may lead to Alfven wave amplification through over-reflection.  This over-reflection may only occur when an instability criterion is upheld. The amplification of the Alfvenic pulse leads to pressure perturbations which destabilise the shock. A global twist of the fluxtube is generated where B0~Bz.
The linear study investigates the wave-flow coupling for a deceleratingdownflow in an intergranular lane. This shows that a negative flow gradient leads to amplification of Alfven waves. The spatial and temporal evolution of the amplified Alfven waves in the presence of the smooth plasma flow are investigated.
The final study is an extension of the sunspot model and is applicableto prominence formation. Here, the wave-flow coupling is extended into the non-linear regime and the introduction of a second footpoint leads to ampfllication for any supersonic flow speed. The key results from this study is that the global twist generated by the mechanism now reaches B0~Bz. This causes a reversal of the flow in the descending leg of the structure due to the wave-flow coupling, leading to a density increase of about 30 times its original value