Kinetic scale turbulence in collisionless magnetized plasmas: statistics, energy transfer and coherent structures

The term turbulence is used to describe a huge class of phenomena occurring in fluids, plasmas and other systems. Turbulence can be defined as a chaotic nonlinear process involving the transfer and reorganization of energy among different scales. Typically, turbulence is induced by a large scale driving force that provides energy to the system. Such energy is nonlinearly transferred to smaller and smaller scales, up to a point where it is finally dissipated, heating the system. This scale-to-scale transfer of energy is known as the turbulent cascade. The strong nonlinear coupling between different scales makes turbulence an extremely challenging problem to study. It is not an exaggeration to say that turbulence is one of the most important unsolved problems of modern physics.

The study of turbulence is an active research area in plasma physics since plasmas are ubiquitous in the universe and are often found to be in a turbulent state. Plasma turbulence is a multi-scale process, meaning that the turbulent cascade exhibits different properties in different ranges of scales. At very large scales, plasma turbulence has a fluid-like behavior but as the cascade of energy reaches ion and electron microscopic (kinetic) scales, the behavior of the system drastically changes. Understanding which mechanisms are responsible for the dynamical transitions in the turbulent cascade at kinetic scales is still an open problem and a frontier of modern research in plasma physics.

In this thesis, we study the properties of kinetic scale turbulence in collisionless magnetized plasmas by means of numerical simulations. In particular, we focus on investigating three main aspects of turbulence. The first aspect concerns the statistical properties of turbulence. Despite its chaotic nature, turbulence exhibits well defined and reproducible statistical features, such as energy spectra and other quantities describing various properties of turbulent fluctuations at different scales. The second aspect of plasma turbulence we investigate is the transfer of energy among scales as the turbulence develops. The idea is to track the path that the energy follows from large scales to kinetic scales, quantifying the different contributions of ions and electrons to the turbulent cascade and to dissipation at different scales. Finally, we investigate another fundamental property of plasma turbulence, which is its capability of generating kinetic scale coherent structures. The latter are typically associated with strong currents, intense heating and high energy particles, possibly playing a role in dissipation at kinetic scales.