Self-sustaining sound in collisionless, high-beta plasma

Abstract

Using analytical theory and hybrid-kinetic numerical simulations, we demonstrate that, in a collisionless plasma, long-wavelength ion-acoustic waves (IAWs) with amplitudes $\delta n/n_0\gtrsim 2/\beta$ (where $\beta \gg 1$ 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 pitch-angle 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 near-Maxwellian, the field-parallel flow of heat resembles its Braginskii form (except in regions where large-amplitude magnetic mirrors strongly suppress particle transport), and the relations between various thermodynamic quantities are more `fluid-like' than kinetic. A nonlinear fluctuation-dissipation relation for self-sustaining IAWs is obtained by solving a plasma-kinetic Langevin problem, which demonstrates suppressed damping, enhanced fluctuation levels, and weakly collisional thermodynamics when IAWs with $\delta n/n_0\gtrsim 2/\beta$ 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.

25 pages, 10 figures, Journal of Plasma Physics, in press

References in corpus (9)

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• Nonlinear growth of firehose and mirror fluctuations in turbulent galaxy-cluster plasmas
• Do sound waves transport the AGN energy in the Perseus Cluster?
• Electron Heating by the Ion Cyclotron Instability in Collisionless Accretion Flows. I. Compression-Driven Instabilities and the Electron Heating Mechanism
• Suppressed effective viscosity in the bulk intergalactic plasma
• Nonlinear mirror instability
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• Proton fire hose instabilities in the expanding solar wind
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Cited by in corpus (8)

• Kinetic Simulations of Cosmic-Ray-Modified Shocks II: Particle Spectra
• Whistler-regulated MHD: Transport equations for electron thermal conduction in the high $\beta$ intracluster medium of galaxy clusters
• Adaptive critical balance and firehose instability in an expanding, turbulent, collisionless plasma
• Particle energization in relativistic plasma turbulence: solenoidal versus compressive driving
• AGN Feedback in Groups and Clusters of Galaxies
• A Measurement of the Effective Mean-Free-Path of Solar Wind Protons
• Electron Re-acceleration via Ion Cyclotron Waves in the Intracluster Medium
• Acoustic waves and g-mode turbulence as energy carriers in a viscous intracluster medium