Kinetic Turbulence in Collisionless High-Beta Plasmas

Abstract

We present results from three-dimensional hybrid-kinetic simulations of Alfvénic turbulence in a high-beta, collisionless plasma. The key feature of such turbulence is the interplay between local wave--wave interactions between the fluctuations in the cascade and the non-local wave-particle interactions associated with kinetic micro-instabilities driven by anisotropy in the thermal pressure (namely, firehose, mirror, and ion-cyclotron). We present theoretical estimates for, and calculate directly from the simulations, the effective collisionality and plasma viscosity in pressure-anisotropic high-beta turbulence, demonstrating that, for strong Alfvénic turbulence, the effective parallel-viscous scale is comparable to the driving scale of the cascade. Below this scale, the kinetic-energy spectrum indicates an Alfvénic cascade with a slope steeper than $-5/3$ due to the anisotropic viscous stress. The magnetic-energy spectrum is shallower than $-5/3$ near the ion-Larmor scale due to fluctuations produced by the firehose instability. Most of the cascade energy (80-90%) is dissipated as ion heating through a combination of Landau damping and anisotropic viscous heating. Our results have implications for models of particle heating in low-luminosity accretion onto supermassive black holes, the effective viscosity of the intracluster medium, and the interpretation of near-Earth solar-wind observations.

34 pages, 19 figures, accepted to PRX

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