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, influencing transport and particle acceleration. We investigate the efficiency and physical mechanisms of plasma mixing driven by the nonlinear evolution of the KHI. We perform high-resolution two-dimensional Particle-In-Cell (PIC) simulations using a finite-Larmor-radius shear-flow initial configuration. Plasma mixing is quantified using particle tracking, passive tracers, 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 correlates with localized reconnection within and between KH vortices. Cross-boundary transport driven by the kinetic KHI is highly localized and mediated by vortex advection and reconnection. Electron mixing is strongly constrained, providing an upper bound on kinetic-scale transport across collisionless shear layers.
The Kelvin-Helmholtz instability (KHI) develops when a plasma shear flow becomes super-Alfvénic with respect to the magnetic-field component parallel to the shear (Chandrasekhar 1961). At Earth's low-latitude magnetopause, this condition is frequently met during intervals of strongly northward or southward interplanetary magnetic field (IMF), and signatures of surface waves and rolled-up vortices have been widely observed (Hasegawa et al. 2004;Eriksson et al. 2016;Stawarz et al. 2016;Settino et al. 2024). KH activity has also been reported at other planetary magnetospheres, including Mercury (Aizawa et al. 2020), Mars (Koh et al. 2025), Saturn (Dialynas 2018), and Jupiter (Montgomery et al. 2023), raising the question of how magnetospheric size influences the multi-scale development of the instability. After saturation, KH vortices generate strong gradients in velocity, density, and magnetic field down to ion and electron scales, triggering secondary instabilities that broaden the mixing layer and enhance plasma transport across the solar wind-magnetosphere boundary (Nykyri & Otto 2001;Hasegawa et al. 2004;Nakamura et al. 2017). Kinetic studies and in-situ observations show that this transport is mediated by processes such as reconnection, wave-particle interactions, and vortex-induced reconnection (VIR), enabling solar-wind plasma entry even under northward IMF conditions (Nykyri et al. 2006;Stawarz et al. 2016;Nakamura et al. 2017;Nakamura et al. 2020). While magnetohydrodynamic (MHD) simulations cap-⋆ silvia.ferro@kuleuven.be ture the global KH dynamics and reconnection-driven transport (Faganello et al. 2012;Ma et al. 2017;Ferro et al. 2024), they do not resolve the kinetic mechanisms governing particle diffusion and heating. Despite these advances, quantitatively characterizing plasma mixing in fully kinetic KHI remains challenging, particularly in distinguishing genuine plasma interpenetration from large-scale advection (Nakamura et al. 2013;Karimabadi et al. 2013).
In this Letter, we investigate plasma mixing driven by the nonlinear evolution of the collisionless KHI using fully kinetic Particle-in-Cell (PIC) simulations of an electron-ion plasma. We quantify mixing efficiency based on particle origin and kinetic diagnostics, and discuss the implications for solar windmagnetosphere interactions.
We perform 2D (PIC) simulations using semi-implicit methods (the Implicit Moment Method and the Energy Conserving Semi-Implicit Method) implemented in the iPIC3D and ECsim codes (Markidis et al. 2010;Lapenta 2017;Bacchini 2023;Croonen et al. 2024). Details of the numerical methods are given in Appendix A.
We model a collisionless magnetized plasma with uniform per-species density n 0 and an initial magnetic field composed of an in-plane component along x and a guide field B z,0 = 10B x,0 . All quantities are normalized using ion reference scales: the ion gyro-frequency Ω c,i = q i B tot,0 /(m i c), where B tot,0 = B 2
x,0 + B 2 z,0
Article number, page 1 arXiv:2602.17404v1 [physics.plasm-ph] 19 Feb 2026
A&A proofs: manuscript no. arxiv_version1 is the total magnetic field magnitude at initialization; the ion plasma frequency ω p,i = (4πn 0 q 2 i /m i ) 1/2 ; the ion inertial length d i = c/ω p,i ; and the Alfvén speed v A = B tot,0 / √ 4πn 0 m i . At initialization, the ion cyclotron-to-plasma frequency ratio is Ω c,i /ω p,i = 0.05.
The initial equilibrium configuration is implemented following the approach of Cerri et al. (2013), ensuring a selfconsistent shear-velocity profile within a finite-Larmor-radius (FLR) framework. Although this is not an exact kinetic equilibrium, it retains first-order FLR corrections, which significantly reduce the spurious fluctuations that arise from pressuretensor responses at ion kinetic scales when using fluid-like MHD initial conditions, thereby yielding a more accurate representation of shear layers only a few ion Larmor-radii thick (Henri et al. 2013). Ions and electrons are initialized with drifting Maxwellian distributions with an initial drift velocity V drift = 0.28 c. Our boundary conditions are periodic in all directions. We impose an initial velocity profile with a double shear
where δ = 3d i represents the initial shear-layer thickness, and the shears are located at y sh,1 = L y /4 and y sh,2 = 3L y /4. The shear layers are chosen to study configurations where vorticity and guide magnetic field are parallel (at y sh,1 ) or antiparallel (at y sh,2 ). The computational domain is a 2D box of size 150d i × 400d i with 2304 × 6144 grid points, giving a spatial resolution ∆x = 0.0651 d i = 0.5208 d e = 5.21 λ D with λ D = v th,e /ω p,e the Debye length. The time step is ∆t = 0.1ω -1 p,i . This choice resolves the electron gyration with approximately 17.6 time steps per cyclotron rotation. The ion-to-electron mass ratio is m i /m e = 64, with 640 particles per cell. The temperature ratio is T e /T i = 0.2, and the plasma-β = n 0 kT/(B 2 0 /(8π)) for ions and elec
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