kinetic

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.

A Hamilitonian 2-field reduced gyrofluid model for kinetic Alfvén waves taking into account ion FLR corrections, parallel magnetic field fluctuations and electron inertia, is used to study turbulent cascades, from the MHD to the electron ranges, in the case of imbalance between waves propagating along or opposite to the direction of the ambient magnetic field. The weak turbulence formalism in the absence of electron inertia leads to kinetic equations for the spectral densities of total energy and generalized cross-helicity, which reduce to those of RMHD at large scales, and REMHD at small scales. Leith-type nonlinear diffusion equations are derived in the limit of ultra-local interactions and a phenomenological formulation is obtained for the strong turbulence regime. These equations are studied analytically and integrated numerically. For a given level of imbalance in the MHD range, the flux of cross-helicity is much smaller when a dispersive range is present before dissipation scales are reached. Large imbalance leads to steeper sub-ion range spectra.