Theoretical models of charged particle acceleration motivated by solar flares

Abstract

In this thesis we examine non-thermal particle behaviour in the presence of modelled electromagnetic fields motivated by various aspects of solar flares.

We first investigate particle dynamics in magnetic reconnection scenarios, in particular 2D reconnection in force-free current sheets and 3D separator reconnection. The electromagnetic fields are obtained by performing resistive magnetohydrodynamic (MHD) simulations with a non-zero anomalous resistivity specified in regions where the local current density exceeds a specified threshold. Test particle orbits and energy spectra are computed in the resulting electromagnetic fields using the relativistic guiding centre equations. Motivated by the enhanced anomalous resistivity, which is several orders of magnitude greater than the Spitzer resistivity, pitch angle scattering linked to the resistivity is introduced into guiding centre formalism when the test particle is located in regions of non-zero resistivity.

In 2D reconnection, pitch angle scattering modifies the particle trajectories, energy gain and orbit duration. In certain cases, pitch angle scattering allows test particles to gain more energy than would be possible in the absence of scattering due to particles traversing the reconnection region multiple times, hence experiencing a parallel electric field component along a greater portion of their orbits. We observe many of the same phenomena in 3D separator reconnection simulations, however changes in particle energy spectra are minimal in comparison to the 2D case.

We also investigate test particle behaviour in an analytical model of a collapsing magnetic trap with the inclusion of a jet braking region at the loop apex, which consists of an indentation in the loops caused by an interaction of a reconnection outflow with low lying magnetic field. New types of particle orbits that are not observed in the absence of the braking jet are characterised. The effects of different trap parameters on particle energisation and orbit behaviour are also examined.