Wave Interactions in High-β Collisionless Plasmas

Coupling numerical simulations with analytic theory, we investigate the nature of hydromagnetic wave interactions and their effects on magnetized turbulence within high-β, collisionless plasmas. Specifically, the interactions of parallel-propagating Alfvén waves with acoustic and other Alfvén waves are studied using a CGL-MHD framework with Landau-fluid heat fluxes. We find that strong modifications can occur to Alfvén-wave packets on timescales comparable with their linear propagation in response to the spatially varying pressure anisotropy generated by other modes. These interactions have consequences for our understanding of magnetized turbulence in environments such as the intracluster medium of galaxy clusters and black-hole accretion flows. With this physics borne in mind, we reexamine the interaction between compressive and Alfvénic fluctuations in a turbulent cascade, as well as the ability of Alfvén waves to establish a weak cascade parallel to the guide magnetic field (otherwise forbidden within standard MHD). Our conclusions are further tested using hybrid-kinetic particle-in-cell simulations, which self-consistently capture feedback from kinetic microinstabilities such as firehose and mirror.