Plasma Mixing in Collisionless Magnetized Plasmas Driven by the Kelvin–Helmholtz Instability

Plasma mixing across velocity shear layers is a key process controlling mass and momentum transport at planetary magnetospheric boundaries. At the Earth’s magnetopause, the Kelvin–Helmholtz instability (KHI) is expected to facilitate such transport by generating large-scale vortices and turbulence. However, in collisionless and magnetized plasmas, the efficiency of KHI-driven mixing remains an open question, particularly in the presence of a magnetic field component aligned with the shear flow.We investigate plasma mixing driven by the KHI using high-resolution, two-dimensional, fully kinetic particle-in-cell simulations of magnetized shear layers. We consider configurations with opposite orientations of vorticity relative to the flow-aligned magnetic field and analyze the nonlinear evolution of KHI vortices and the resulting turbulent boundary layer. Plasma mixing is quantified through particle tracing, allowing us to assess the degree of interpenetration between initially distinct plasma populations. Our results show that, despite the development of fully nonlinear KHI vortices that merge and evolve into complex dynamics, plasma mixing across the shear layer can remain strongly inhibited when even a modest magnetic field component is aligned with the flow. In this regime, magnetospheric and magnetosheath plasmas preserve partially distinct topologies within the turbulent layer, highlighting the stabilizing role of magnetic tension at kinetic scales. These findings demonstrate that KHI-driven turbulence does not necessarily imply efficient plasma mixing in collisionless magnetized environments and have important implications for solar wind–magnetosphere coupling and plasma transport at planetary boundaries.