Particle Acceleration in Magnetized Shear-driven Turbulence

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 sustained, subsonic, nonrelativistic, and magnetized turbulence driven purely by velocity shear, including full particle backreaction. Using 2D MHD-PIC simulations with an initially uniform flow-aligned magnetic field and external stirring force, we demonstrate that, in this subsonic regime, 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 nonthermal tails after the turbulence onset. The energization of particles that repeatedly cross shear layers follows geometric Brownian motion, 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.

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BibTeX RIS

BibTeX

@article{liu2026particle,
  title = {{Particle Acceleration in Magnetized Shear-driven Turbulence}},
  author = {Mingxuan Liu and Mateusz Ruszkowski and Ellen Zweibel and Xiaochen Sun and Damiano Caprioli and Naixin Liang and S. Peng Oh and Anatoly Spitkovsky},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = sep,
  doi = {10.3847/1538-4357/aea409},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea409},
  note = {CC BY 4.0},
}

RIS

TY  - JOUR
TI  - Particle Acceleration in Magnetized Shear-driven Turbulence
AU  - Mingxuan Liu
AU  - Mateusz Ruszkowski
AU  - Ellen Zweibel
AU  - Xiaochen Sun
AU  - Damiano Caprioli
AU  - Naixin Liang
AU  - S. Peng Oh
AU  - Anatoly Spitkovsky
PY  - 2026
DA  - 2026/09/30
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea409
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea409
AB  - 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 sustained, subsonic, nonrelativistic, and magnetized turbulence driven purely by velocity shear, including full particle backreaction. Using 2D MHD-PIC simulations with an initially uniform flow-aligned magnetic field and external stirring force, we demonstrate that, in this subsonic regime, 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 nonthermal tails after the turbulence onset. The energization of particles that repeatedly cross shear layers follows geometric Brownian motion, 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.
SN  - 0004-637X
SN  - 1538-4357
ER  -

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