Investigating electric fields and energy conversion in earth's turbulent magnetosheath

Turbulence plays a key role in plasma dynamics and heating throughout the Universe. Complex and unpredictable nonlinear motion leads to a transfer of energy from large to small scales, where it is dissipated into heat. In the absence of collisions, turbulence proceeds to microphysical scales where it is not known how reversible kinetic processes lead to net dissipation. Earth's magnetosheath provides an opportunity to directly probe this open question using in situ measurements. This thesis presents an investigation into the fundamental physics of turbulence using a database of magnetosheath observations by NASA's Magnetospheric Multiscale (MMS) mission. The comparative importance of different electric field contributions is studied for 60 magnetosheath intervals, revealing that how the nonlinear dynamics operate depends on both the ambient plasma conditions and the turbulence driver. The relationships uncovered provide observational constraints on turbulence theory, bounding the applicability of magnetohydrodynamic and kinetic Alfvén wave physics, with applications to other plasma regimes and simulation studies. The role of these electric field dynamics in facilitating turbulent dissipation is explored by quantifying the non-ideal energy conversion related to specific kinetic processes. It is found that energy conversion is enhanced in regions susceptible to anisotropy-driven kinetic microinstabilities, suggesting these mechanisms play a fundamental role in dissipating turbulent energy. The interpretation is that turbulence drives conditions to become locally unstable, triggering instabilities which redistribute the free energy in favour of growing wave modes. Despite these processes being reversible, the net transfer of energy is from the fields into the particles, consistent with net dissipation. Turbulence is also found to generate significantly non-Maxwellian velocity distributions, indicating that it provides additional sources of free energy whereby instabilities can thermalise the plasma.