The Nature of Sub-Ion-Scale Collisionless Plasma Turbulence Energetics and Relative Ion-Electron Heating

Plasma systems are found ubiquitously throughout the universe, with many in a state of turbulence and weak collisionality. Indeed, our home planet, Earth, finds itself nested in one such system, where a stream of particles (mainly protons and electrons) are emanating from the Sun: The Solar Wind. Dissipation is believed to play a critical role in heating the solar wind, which is found to be hotter than predictions of adiabatic cooling. How energy is dissipated in turbulent plasma systems is not fully understood, with the nature of turbulence at very small scales an open question. Understanding turbulent plasma systems requires the coalescence of theory, experiments, observations, and simulations. Simulations require an appropriate choice of model. For weakly collisional plasmas, fluid closures such as viscosity are no longer appropriate, requiring the use of kinetic or higher-order fluid models. Higher-order fluid models suffer from a lack of closure, requiring approximations of higher-order quantities to close the systems. Closure approximations are typically validated against full kinetic models; however, the effect of these approximations on higher-order dynamics remains unclear. Moreover, to resolve electron-scale dynamics, within the limits of currently available computational resources, unrealistic values of parameters such as the ratio of the Alfv\'en speed to speed of light ($v_{A}/c$) or the ratio of ions to electron mass ($m_i/m_e$) are usually used in numerical simulations. However, these artificial values may introduce unphysical effects which could be significant at sub-ion scales, where kinetic effects are the most important. In this thesis, I use the 10-moment two-fluid model in the Gkeyll framework to perform controlled numerical experiments, investigating the effects of systematically varying $m_i/m_e$ on sub-ion dynamics, as well as the closure approximation used for the heat flux. In order to investigate dissipation and heating at these scales I use the pressure-strain interaction. I show that the sub-ion-scale dynamics and relative ion-electron heating are greatly affected by a departure from the realistic $m_i/m_e$ value. The dissipative process for both ions and electrons becomes less compressive in nature as $m_i/m_e$ is reduced, although ions do remain nearly incompressible at all $m_i/m_e$. Electron heating/dissipation is shown to be dominated by compressions/rarefactions for a realistic mass ratio, with smaller $m_i/m_e$ electron heating being dominated by incompressive viscous-like processes. I also show that as $m_i/m_e$ is increased towards its realistic value, the choice of closure parameter has less impact on lower-order dynamics, with almost no impact for the realistic mass ratio cases. That being said, the choice of closure parameter is shown to affect higher-order dynamics, even for realistic $m_i/m_e$. Finally, I present preliminary investigation into the scales at which electrons heat using a scale-filtering approach on the pressure-strain interaction.