Kinetic Turbulence in Collisionless High-Beta Plasmas

Understand

We present results from three-dimensional hybrid-kinetic simulations of Alfv\'enic 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 Alfv\'enic turbulence, the effective parallel-viscous scale is comparable to the driving scale of the cascade.
• Below this scale, the kinetic-energy spectrum indicates an Alfv\'enic cascade with a slope steeper than $-5/3$ due to the anisotropic viscous stress.