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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. 

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. 

ABSTRACT In classical diffusion, particle step-sizes have a Gaussian distribution. However, in superdiffusion, they have power-law tails, with transport dominated by rare, long ‘Lévy flights’. Similarly, if the time interval between scattering events has power-law tails, subdiffusion occurs. Both forms of anomalous diffusion are seen in cosmic ray (CR) particle tracking simulations in turbulent magnetic fields. They also likely occur if CRs are scattered by discrete intermittent structures, as recently suggested. Anomalous diffusion mimics a scale-dependent diffusion coefficient, with potentially wide-ranging consequences. However, the finite size of galaxies implies an upper bound on step-sizes before CRs escape. This truncation results in eventual convergence to Gaussian statistics by the central limit theorem. Using Monte-Carlo simulations, we show that this occurs in both standard finite-thickness halo models, or when CR diffusion transitions to advection or streaming-dominated regimes. While optically thick intermittent structures produce power-law trapping times and thus subdiffusion, ‘Gaussianization’ also eventually occurs on time-scales longer than the maximum trapping time. Anomalous diffusion is a transient, and converges to standard diffusion on the (usually short) time-scale of particle escape, either from confining structures (subdiffusion), or the system as a whole (superdiffusion). Thus, standard assumptions of classical diffusion are physically justified in most applications, despite growing simulation evidence for anomalous diffusion. However, if escape times are long, this is no longer true. For instance, anomalous diffusion in the CGM or ICM would change CR pressure profiles. Finally, we show the standard diagnostic for anomalous diffusion, $$\langle d^2 \rangle \propto t^{\alpha }$$ with $$\alpha \ne 1$$, is not justified for truncated Lévy flights, and propose an alternative robust measure.