Self-defeating Alfvén waves and self-sustaining sound in a collisionless, high-beta plasma

Matthew Kunz
Princeton University, Department of Astrophysical Sciences, 4 Ivy Lane, Princeton, NJ 08544 USA
Many space and astrophysical plasmas are so hot and dilute that they cannot be rigorously described as fluids. These include the solar wind, low-luminosity black-hole accretion flows, and the intracluster medium of galaxy clusters. We present theory and hybrid-kinetic simulations of the propagation of shear-Alfvén and ion-acoustic waves in such weakly collisional, magnetized, high-beta plasmas. Following Squire et al. (2016), we demonstrate that shear-Alfvén waves ``interrupt'' at sufficiently large amplitudes by adiabatically driving a field-biased pressure anisotropy that both nullifies the restoring tension force and excites a sea of ion-Larmor-scale instabilities (viz., firehose) that pitch-angle scatter particles. This physics places a beta-dependent limit on the amplitude of shear-Alfvén waves, above which they do not propagate effectively. We also demonstrate that similar physics afflicts compressive fluctuations, except that it is the collisionless damping of such waves that is interrupted. Above a beta-dependent amplitude, compressive fluctuations excite ion-Larmor-scale mirror and firehose fluctuations, which trap and scatter particles, thereby impeding phase mixing of the distribution function and yielding MHD-like dynamics. Implications for magnetokinetic turbulence and transport will be discussed.