Plasma mixing driven by the collisionless Kelvin–Helmholtz instability

Insights from a fully kinetic simulation and density-based diagnostics

¹ Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium
² Royal Belgian Institute for Space Aeronomy, Solar-Terrestrial Centre of Excellence, Ringlaan 3, 1180 Uccle, Belgium
³ Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA
⁴ Institute for Plasma Science and Technology, National Research Council, CNR-ISTP, Bari, Italy
⁵ Laboratoire Lagrange, Observatoire Côte d’Azur, Université Côte d’Azur, CNRS, Nice, France
⁶ LPC2E, CNRS, Université d’Orléans, CNES, Orléans, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 February 2026
Accepted: 20 March 2026

Abstract

Context. Simulations and observations of the low-latitude magnetosphere–magnetosheath boundary layer indicate that the Kelvin–Helmholtz instability (KHI) drives vortex structures that enhance plasma mixing and magnetic reconnection, thereby influencing transport and particle acceleration.

Aims. We investigated the spatial localization, species dependence, and physical mechanisms of plasma mixing driven by the nonlinear evolution of the KHI.

Methods. We performed high-resolution 2D particle-in-cell simulations using a finite-Larmor-radius shear-flow initial configuration. Plasma mixing was quantified using particle labeling, a complementary density-based mixing tracer, and diagnostics of magnetic reconnection.

Results. Mixing across the shear layer is present but localized, occurring mainly in narrow interface regions and plasma structures. Ions mix more effectively than electrons, which remain largely frozen to field lines. Enhanced mixing spatially and temporally correlates with localized magnetic reconnection within and between Kelvin–Helmholtz vortices.

Conclusions. The cross-boundary transport driven by the kinetic KHI remains intrinsically localized and is mediated by vortex advection and magnetic reconnection. Electron mixing is strongly constrained, indicating that kinetic-scale transport across collisionless shear layers remains limited.