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Description
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, magnetized plasmas, the efficiency of KHI-driven mixing remains an open question, particularly in the presence of a magnetic field aligned with the shear flow.
We investigate plasma mixing driven by the KHI using high-resolution, two-dimensional, fully kinetic particle-in-cell (PIC) simulations performed with the iPic3D and ECsim codes. Both codes retain full ion and electron phase-space dynamics, in contrast to hybrid or fluid models, enabling an accurate description of collisionless plasma processes. In particular, ECsim employs an energy-conserving semi-implicit formulation that preserves total energy to machine precision, providing robust control of numerical dissipation and long-term nonlinear evolution. We consider shear-layer configurations with opposite orientations of vorticity relative to the flow-aligned magnetic field and analyze the nonlinear development of KHI vortices and the resulting turbulent boundary layer.
Despite the formation of fully nonlinear KHI vortices and turbulence, we find that plasma mixing across the shear layer remains strongly inhibited even when a modest magnetic field component is aligned with the flow. In this regime, magnetospheric and magnetosheath plasmas retain partially distinct topologies within the turbulent layer, highlighting the stabilizing role of magnetic tension at kinetic scales.