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Abstract Supersonic flows are ubiquitous in warm and cool media; their dissipation leads to heating, generation of nonthermal particles, and amplification of background magnetic fields. We present 2D hybrid (kinetic ions–fluid electrons) simulations of decaying shear flows across the subsonic-to-supersonic transition, finding that the canonical Kelvin–Helmholtz instability in subsonic cases gives way to the formation of shocklets in supersonic shears, where dissipation is faster and nonthermal particles are produced. We discuss the dependence on the flow Mach number of particle acceleration, the viscosity induced by kinetic effects, and the production of magnetic turbulence. We outline the potential impact of these findings for turbulence in the warm interstellar medium, for molecular clouds, and for accretion disks, leaving to a companion paper the discussion of the effects on the shear of self-generated and preexisting energetic particles.
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This content will become publicly available on July 15, 2027
Hybrid Simulations of Supersonic Shear Flows. II. Cosmic-Ray Viscosity
Abstract In this second paper in a series dedicated to characterizing shear layers via 2D hybrid (kinetic ions–fluid electrons) simulations, we study the dynamical role of nonthermal particles (cosmic rays, CRs), either spontaneously generated or preexisting. We initialize Kolmogorov-type sinusoidal velocity shear flows unstable to the Kelvin–Helmholtz instability, which evolve nonlinearly into turbulence. Particles with large gyroradii act as long-range messengers that promote momentum exchange between layers, hence introducing a form of CR viscosity. Even when not energetically dominant, increasing the CR energy density generally enhances momentum transfer, provided that their gyroradii are smaller than the shear length scale. We consider flows ranging from subsonic to supersonic and assess the rate of shear dissipation, the partition of the initial kinetic energy among heating, thermal ion acceleration, CR reacceleration, magnetic-field amplification, and the maximum energy attained by accelerated particles.
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- PAR ID:
- 10700684
- Publisher / Repository:
- IOP
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 1006
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 68
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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