Instabilities and Dissipation in Collisionless Magnetized Turbulence

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 emission from these systems in two main ways. First, the emission itself is radiated primarily by electrons, which are heated by the turbulent cascade. 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 this talk, I will highlight recent results of kinetic modeling of collisionless turbulence. For low and moderate values of plasma beta (the ratio of thermal and magnetic pressures), the dissipation of turbulent fluctuations occurs at sub-ion-Larmor scales primarily through a combination of stochastic and ion-cyclotron heating. Our simulations of this regime reproduce the observed preferential perpendicular ion heating and the development of non-thermal beams in the ion distribution function seen in the solar wind. With our high-beta simulations, we explore the interplay between kinetic micro-instabilities and the turbulent cascade. In particular, we obtain the effective collisionality and viscous scale in collisionless high-beta turbulence.