Instabilities and Dissipation in Collisionless Magnetized Turbulence

Abstract

Many space and astrophysical plasmas, such as the solar wind, radiatively inefficient accretion flows onto black holes, and the intracluster medium of galaxy clusters (ICM), are hot and dilute, which makes them weakly collisional or even collisionless. We know from direct in-situ measurements (for the solar wind), radio and X-ray observations (for the ICM), and theoretical models alongside sub-mm observations (for the accretion flows) that these plasmas host a broadband spectrum of turbulent fluctuations.

Kinetic processes, occurring on scales much smaller than what could realistically be observed from Earth, can influence the observed emission from these systems in two main ways. First, the emission itself is radiated primarily by electrons, which are heated by the turbulent cascade. Therefore, understanding the energy dissipation mechanisms in turbulence is crucial for interpreting observations. Second, deviations from thermodynamic equilibrium caused by the turbulent motions could make the plasma unstable to a number of kinetic microinstabilities. These instabilities introduce an effective collisionality into otherwise collisionless plasmas and thereby impact the dynamics of turbulent fluctuations.

In the first part of this Thesis, we use a combination of theory and hybrid-kinetic particle-in-cell simulations to determine the mechanisms responsible for energy dissipation in collisionless turbulence across a range of plasma beta. If the magnetic energy is larger than the thermal energy, the ions are heated primarily by a combination of ion-cyclotron and stochastic heating. In the case of energetically sub-dominant magnetic fields, the main dissipation mechanism is anisotropic viscous heating. In each regime, we obtain the quasi-steady-state distribution functions and compare them with relevant solar-wind observations. We also motivate a model for energy partition amongst different particle species, which could be used for interpreting observations of black-hole accretion flows.

The second part is dedicated to the investigation of the interplay between large-scale turbulence and small-scale kinetics in the frontier regime of high plasma beta, in which ion-Larmor-scale instabilities play a role in regulating the MHD-scale viscous stress. We determine the effective collisionality in this regime, and compute the effective viscous scale of turbulence, at which the dynamics of turbulent fluctuations becomes different from that of collisional plasma.