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Title: Alignment and structure in MHD dynamos
Author: Miller, D.
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2019
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Magnetic fields are ubiquitous within astrophysical settings. There is strong evidence to suggest that some of these magnetic fields, for example the Sun's, are maintained through a dynamo process whereby energy is exchanged between a flow and a magnetic field. Magnetohydrodynamics (MHD) is the branch of mathematics where this interaction is studied. The initial amplification of a weak seed field is often modelled using the kinematic dynamo approximation where the flow is not influenced by the magnetic field. This approximation to the early behaviour of a nonlinear dynamo problem, where the magnetic field grows exponentially during a kinematic phase and then saturates into a nonlinear regime, has the benefit of being far less computationally intensive. In this thesis, I examine three different topics within MHD dynamos. First, I examine how measuring alignment of the flow and magnetic field during a kinematic dynamo can reveal changes to the magnetic field structure. This I show to be useful both within individual simulations and when comparing magnetic fields within parameter studies. Secondly, I examine nonlinear dynamos where the flow and magnetic field are strongly aligned and have almost identical energies. I reproduce, and give an explanation for, a previously unexplained behaviour. Furthermore, I show that aligned flow and magnetic fields can exist for increasingly complex forcings and as such the aligned state is remark-ably robust. Finally, I consider a number of different nonlinear dynamos for a family of forcings with different magnetic field structures during their kinematic phase. Using Minkowski Functions to quantify the structures, I show that, where the magnetic field becomes sufficiently strong, the magnetic fields become (or remain) ribbon-like in the nonlinear regime. As such, the influence that stagnation points in the flow have on the magnetic field structure is less than in the kinematic dynamo equivalent.
Supervisor: Mason, J. ; Gilbert, A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: MHD ; Dynamo