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Self-sustaining sound in collisionless, high-β plasma

Published online by Cambridge University Press:  09 November 2020

M. W. Kunz*
Affiliation:
Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ08544, USA Princeton Plasma Physics Laboratory, PO Box 451, Princeton, NJ08543, USA
J. Squire
Affiliation:
Department of Physics, University of Otago, 730 Cumberland St, North Dunedin, Dunedin9016, New Zealand
A. A. Schekochihin
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, Clarendon Laboratory, University of Oxford, Parks Road, OxfordOX1 3PU, UK Merton College, Merton Street, OxfordOX1 4JD, UK
E. Quataert
Affiliation:
Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ08544, USA Department of Astronomy and Theoretical Astrophysics Center, University of California, Berkeley, CA94720, USA
*
Email address for correspondence: [email protected]

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.

Type
Research Article
Copyright
Copyright © The Author(s), 2020. Published by Cambridge University Press

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