Plasma mixing driven by the collisionless Kelvin–Helmholtz instability. Insights from a fully kinetic simulation and density-based diagnostics

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