Particle Acceleration in Magnetized Shear-Driven Turbulence

TL;DR

This study uses advanced simulations to show that non-relativistic, magnetized shear-driven turbulence can efficiently accelerate particles through a process similar to second-order Fermi acceleration, producing non-thermal energy distributions.

Contribution

First numerical demonstration of particle acceleration in non-relativistic, magnetized shear-driven turbulence including full particle backreaction.

Findings

Sustained turbulence is necessary for continuous acceleration.

Energy gain scales quadratically with shear velocity.

Particles develop non-thermal tails and pitch-angle anisotropy.

Abstract

Shear flows, ubiquitous in space and astrophysical plasmas, can accelerate particles through turbulence excited by the Kelvin-Helmholtz instability. We present the first numerical study of particle acceleration in non-relativistic, magnetized, and purely shear-driven turbulence that includes full particle backreaction. Using two-dimensional MHD-PIC simulations with an initially uniform flow-aligned magnetic field and external stirring force, we demonstrate that sustained particle acceleration requires continuously driven turbulence, whereas freely decaying turbulence rapidly depletes its energy reservoirs and halts the acceleration. The acceleration mechanism operates through the systematic distortion of gyro-orbits by turbulent electric fields: acceleration phases extend the particle trajectory along the electric force, increasing the energy gain, while deceleration phases shorten the trajectory, reducing the energy loss. This asymmetry produces net energy gain despite stochastic fluctuations, with the mean energy change scaling quadratically with shear velocity, characteristic of second-order Fermi acceleration. Initially monoenergetic particles develop substantial non-thermal tails after the turbulence onset. For particles repeatedly crossing shear layers, their energization follows geometric Brownian motion with weak systematic drift, yielding a log-normal distribution. High-energy particles exhibit pitch-angle anisotropy, becoming preferentially perpendicular to the flow-aligned magnetic field as their gyroradii exceed the turbulent layer width. These results establish shear-driven turbulence as a viable particle acceleration mechanism, providing a general model for particle energization in shear flows.