Energy partition in magnetic reconnection and kinetic turbulence in weakly collisional plasmas

Abstract

This thesis presents the results of two studies detailing the dissipation of energy in magnetic reconnection and decaying turbulence in weakly collisional plasmas. One of the open questions regarding magnetic reconnection is how magnetic energy is dissipated into particle heating, kinetic energy, and supra-thermal particle acceleration; an understanding of energy partition in reconnection may be used to explain numerous physical phenomena such as the turbulent heating of the solar wind and the formation of high-energy astrophysical jets. We find the plasmoid dominated regime results in the formation of magnetised collisionless shocks which cause strong particle heating and the formation of spatially localised high temperature regions for electrons. The detection of these shocks, and the analysis of their role in thermalizing energy, is one of the most original contributions to this thesis; indeed, we are not familiar with any other study where this has previously been reported. We also find non-spatially localised heating for the ions with evidence to suggest that this heating occurs due to the onset of the ion-acoustic instability. The study finds that the particles which are accelerated to the highest energies are accelerated within plasmoids and form power law spectra with spectral slopes of E-1.6. This study proposes a novel mechanism to explain the particle acceleration: a magnetic mirror effect driven by the bulk velocity of the magnetic fields at the trailing edge of the plasmoid. This thesis also reports on the Kinetic Reduced Electron Heating Model (KREHM) using the Viriato code as part of a larger study comparing the limitations of validity of various reduced-kinetic models of plasmas against a fully-kinetic model. Two different ion betas are considered: 0.1 and 0.5. We find that there is excellent agreement between KREHM and the fully-kinetic model over a range far greater than the asymptotic [beta]i ~ me/mi limit would suggest. The KREHM model is also used to demonstrate the importance of electron Landau damping in kinetic turbulence, approaching the fully-kinetic spectral shape when it is included but deviating significantly and approaching the hybrid-kinetic spectra during isothermal runs when it is explicitly excluded. The limits and applicability of KREHM and the other reduced-kinetic approaches examined may provide insight into the pertinent kinetic effects in turbulent collisionless plasmas.