Selfsustaining sound in collisionless, highβ plasma
Abstract
Using analytical theory and hybridkinetic numerical simulations, we demonstrate that, in a collisionless plasma, longwavelength ionacoustic waves (IAWs) with amplitudes $δ n/n_0 \gtrsim 2/β$ (where $β ≫ {1}$ is the ratio of thermal to magnetic pressure) generate sufficient pressure anisotropy to destabilize the plasma to firehose and mirror instabilities. These kinetic instabilities grow rapidly to reduce the pressure anisotropy by pitchangle scattering and trapping particles, respectively, thereby impeding the maintenance of Landau resonances that enable such waves' otherwise potent collisionless damping. The result is wave dynamics that evince a weakly collisional plasma: the ion distribution function is nearMaxwellian, the fieldparallel flow of heat resembles its Braginskii form (except in regions where largeamplitude magnetic mirrors strongly suppress particle transport), and the relations between various thermodynamic quantities are more `fluidlike' than kinetic. A nonlinear fluctuationdissipation relation for selfsustaining IAWs is obtained by solving a plasmakinetic Langevin problem, which demonstrates suppressed damping, enhanced fluctuation levels and weakly collisional thermodynamics when IAWs with $δ n/n_0 \gtrsim 2/β$ are stochastically driven. We investigate how our results depend upon the scale separation between the wavelength of the IAW and the Larmor radius of the ions, and discuss briefly their implications for our understanding of turbulence and transport in the solar wind and the intracluster medium of galaxy clusters.
 Publication:

Journal of Plasma Physics
 Pub Date:
 November 2020
 DOI:
 10.1017/S0022377820001312
 arXiv:
 arXiv:2006.08940
 Bibcode:
 2020JPlPh..86f9003K
 Keywords:

 astrophysical plasmas;
 Astrophysics  High Energy Astrophysical Phenomena;
 Physics  Plasma Physics;
 Physics  Space Physics
 EPrint:
 25 pages, 10 figures, Journal of Plasma Physics, in press