Hybrid Simulations of Supersonic Shear Flows: I) Particle Acceleration

TL;DR

This work demonstrates, via 2D hybrid simulations with kinetic ions and fluid electrons, that decaying shear flows transition from Kelvin-Helmholtz–dominated dynamics at subsonic speeds to shocklet-dominated dissipation at supersonic speeds, with substantial energy transferred to nonthermal ions and magnetic turbulence. The authors quantify dissipation and particle-acceleration metrics across a range of Alfvénic and sonic Mach numbers, revealing that higher sonic Mach numbers produce stronger nonthermal tails (up to ~10% of energy) and more rapid magnetic-field amplification, though the maximum field energy does not saturate at equipartition. Box size and plasma beta modulate dissipation timescales and the relative importance of shocklets versus kinetic transport, highlighting finite-Larmor-radius effects and CR-viscosity–like behavior that are missing from fluid models. The results have implications for pressure support and heating in molecular clouds and the ISM/ICM, and for angular-momentum transport in accretion disks, with a companion paper addressing the role of preexisting energetic particles and self-generated CR viscosity.

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 pre-existing energetic particles.

Figures (12)

• Figure 1: Initial setup for the trans-Alfvénic and subsonic shear Run $\mathcal{B}$. The velocity shear is set by Equation \ref{['eq:shear']} and the initial magnetic field is mostly out-of-plane along $z$, with a small component along $x$ such that $B_{0,z}=20B_{0,x}$, following henri+13. The layers at $50d_{\text{i}}$ and $150d_{\text{i}}$ have a width of 3$d_i$ and $\mathbf{B}\cdot (\nabla \times\mathbf{U})<0$ and $>0$, respectively.
• Figure 2: Kinetic relaxation of the number density $n/n_{0}$ for the sub-Alfvénic and subsonic Run $\mathcal{B}$ in the first $125 \omega_{c}^{-1}$, before KHI kicks in. Disturbances in density propagate through the simulation domain at a constant speed.
• Figure 3: Time evolution of out-of-plane magnetic field $B_{z}$ and density $n/n_{0}$ fore the subsonic simulation Run $\mathcal{B}$, showing the linear to the nonlinear evolution of the KHI, which for the chosen parameters has a growth time of $\sim 50 \omega_c$.
• Figure 4: Evolution of $\Delta$ (Equation \ref{['eqn:Delta']}) for Run $\mathcal{B}$. $\tau_X$ is the time for $\Delta$ to reduce to $X\%$, so that $\tau_{90}$ measured the KHI onset timescales, $\tau_{50}$ gives a general estimate for the shear dissipation, and the viscosity timescale $\tau_{\nu}\equiv \tau_{20}-\tau_{90}$ determines to the duration of the nonlinear stage.
• Figure 5: Out-of-plane magnetic field $B_{z}$ over time for Run $\mathcal{D}1-3$ with $M_s=1.1, 2.2, 4.4$ as labeled, from transonic to supersonic. At higher $M_{\rm s}$ local shocklets grows to distort the initial velocity structure and evolved in turbulence.