Year: 2026
Type: article
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.
Source: The Astrophysical Journal
Authors: Mingxuan Liu, Mateusz Ruszkowski, Ellen Zweibel, Xiaochen Sun, Damiano Caprioli, Naixin Liang, S. Peng Oh, Anatoly Spitkovsky
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Awards: G1453181110, G1587926220, G1588448908, Track 4: Advanced CI Coordination Ecosystem: Monitoring and Measurement Services, G1941825669, G2154383771, G2452989760, G2595738051, Collaborative Research: WoU-MMA: Multimessenger Plasma Physics Center (MPPC), G3037948166, G3245429106, G3398237449, G3503245242, G3562386289, Track 1: ACCESS Resource Allocations Marketplace and Platform Services (RAMPS), G4010157413, Collaborative Research: Microscale Plasma Processes at High Beta in Shock and Turbulent Environments, G4437173897, ONE OF THE MOST IMPORTANT SCIENTIFIC CONTRIBUTION OF THE FERMI SATELLITE HAS BEEN TO SHOW THAT THE GAMMA-RAY EMISSION FROM SEVERAL SUPERNOVA REMNANTS (SNRS) IS OF HADRONIC ORIGIN THEREBY SUPPORTING THE IDEA THAT SNRS ARE THE SOURCES OF GALACTIC COSMIC RAYS (CRS). ALMOST ALL GEV--BRIGHT SNRS EXPAND INTO PARTIALLY-NEUTRAL MEDIA AND/OR ARE ASSOCIATED WITH DENSE MOLECULAR CLOUDS (MCS). IN THE PRESENCE OF NEUTRAL HYDROGEN THE SHOCK DYNAMICS THE SPECTRUM OF ACCELERATED PARTICLES AND THE CR--MAGNETIC FIELD COUPLING ARE RADICALLY DIFFERENT THAN IN FULLY-IONIZED PLASMAS. HOWEVER THE EFFECTS OF NEUTRALS ARE USUALLY NEGLECTED IN CALCULATIONS OF CR ACCELERATION AND PROPAGATION. WE WILL USE SELF-CONSISTENT KINETIC SIMULATIONS OF COLLISIONLESS SHOCKS PROPAGATING IN PARTIALLY-IONIZED PLASMAS TO ASSESS THE IMPACT OF PHENOMENA SUCH AS CHARGE EXCHANGE IONIZATION AND ION--NEUTRAL DAMPING ON SHOCK DYNAMICS CR ACCELERATION AND MAGNETIC FIELD AMPLIFICATION WHICH ARE NEEDED TO CHARACTERIZE THE HADRONIC GAMMA-RAY EMISSION OBSERVED IN SNR/MC SYSTEMS., ACO: An Open CI Ecosystem to Advance Scientific Discovery (OpenCI), G4689434848, G4774908964, G478808451, Category I: Crossing the Divide Between Today's Practice and Tomorrow's Science, Track 3: COre National Ecosystem for CyberinfrasTructure (CONECT), G5367127336, Collaborative Research: Multidimensional Kinetic Simulations of Particle Acceleration in Astrophysical Collisionless Shocks, G5464919783, Collaborative Research: Cosmic Ray Feedback from Plasma to Circumgalactic Scales, G6316823789, WoU-MMA: Multi-messenger Signatures of Ion Acceleration in Active Galactic Nuclei, Kavli Institute for Theoretical Physics, G7377509079, Category I: Stampede3 -- Modernizing and Evolving The Largest ACCESS Compute Resource, Programs on Critical Problems in Physics, Astrophysics and Biophysics at the Aspen Center for Physics, G7976526739, G8250941178, Track 2: Customized Multi-tier Assistance, Training, and Computational Help (MATCH) for End User ACCESS to CI
OpenAlex ID: W4417449984