MAGNETIC RECONNECTION: TRANSIENT DRIVING, PARTICLE ACCELERATION AND FILAMENT ERUPTION

Abstract

Magnetic reconnection, the restructuring of magnetic field topology accompanied by release of stored magnetic energy, is responsible for a wide range of astrophysical and laboratory plasma phenomena. This thesis is primarily concerned with extending the model of forced magnetic reconnection to more realistic driving disturbances, through MHD and test particle modelling, as well as the related problem of filament reconnection in tokamak plasmas. Forced magnetic reconnection, a reconnection event triggered by transient external perturbation, should be ubiquitous in the solar corona. Energy released during such cases can be much greater than that which was introduced by the perturbation. The exact dynamics of magnetic reconnection events are determined by the structure and complexity of the reconnection region: the thickness of reconnecting layers, the field curvature; the presence, shapes and sizes of magnetic islands. It is unclear how the properties of the external perturbation and the initial current sheet affect the reconnection region properties, and thereby the reconnection dynamics and energy release profile. The effect of the form of the external perturbation on the evolution of the reconnection region and the energy release process are investigated. A chief focus is the non-linear interactions between multiple, simultaneous perturbations, which represent more realistic scenarios. Fluid plasma simulations are performed using Lare2d, a 2.5D Lagrangian-remap solver for the visco-resistive MHD equations. The model of forced reconnection is extended to include superpositions of sinusoidal driving disturbances, including localised Gaussian perturbations. Island coalescence is shown to contribute significantly to energy release and involves rapid reconnection. Long wavelength modes in perturbations dominate the evolution, without the presence of which reconnection is either slow, as in the case of short wavelength modes, or the initial current sheet remains stable, as in the case of noise perturbations. Multiple perturbations combine in a highly non-linear manner: reconnection is typically faster than when either disturbance is applied individually, with multiple low-energy events contributing to the same total energy release. These MHD results are used as the basis for an investigation of how particles in fields undergoing forced reconnection are accelerated over the whole course of the reconnection event. Since different forms of perturbation result in radically different electric field distributions, this ought to have consequences for particle acceleration. Thermal test particles are injected into these fields and their trajectories are computed using the relativistic Guiding Centre Approximation. Unlike work using static fields, the evolution of the particle distribution is tracked from the equilibrium field through the full reconnection process to the final reconnected state. A small population of particles are accelerated to relativistic speeds, while most particles remain thermal. It is found that protons and electrons are asymmetrically ejected from reconnecting X-points, such that bands of high energy particles trace out opposite sides of the open field. Finally, in collaboration with CCFE, filament reconnection in tokamaks is considered. Edge Localised Modes (ELMs) have been noted to have many similarities with solar flares, from the production of hard X-rays to the plasma regime itself. A simplified model for an erupting filament is proposed and investigated using the same 2.5D MHD methods as in the forced reconnection study.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philippa Browning (Supervisor) & Mykola Gordovskyy (Supervisor)