Wednesday, January 22, 2020
Many astrophysical plasmas, such as accretion flows around black holes, the intra-cluster medium, and the solar wind, are weakly collisional (or collisionless) and well magnetized. We know from X-ray observations (for intra-cluster medium), in-situ measurements (for solar wind), or from theoretical models (for accretion disks) that these plasmas host a spectrum of turbulent fluctuations. Although the large-scale processes that excite the turbulence are different for each system, the small-scale physics responsible for its dissipation is thought to be the same. This physics, however, is still poorly understood, despite decades of observations and theoretical models, due to the fundamentally non-linear nature of the problem, the wide scale separation, and a number of instabilities present in kinetic plasmas. In this talk, I will present the results from hybrid-kinetic particle-in-cell simulations of collisionless turbulence. Our low-beta simulations (where beta is the ratio of thermal and magnetic pressures) reproduce the observed preferential perpendicular ion heating and the development of non-thermal beams in the ion distribution function seen in the solar wind. Dissipation of turbulent fluctuations occurs at sub-ion-Larmor scales primarily through a combination of stochastic and ion-cyclotron heating. Our high-beta simulations focus on the effects of kinetic micro-instabilities on the turbulent cascade, in particular, on how they disrupt inertial-range Alfven waves and introduce an effective collisionality in an otherwise collisionless plasma.