Plasma Mixing in Collisionless Kelvin-Helmholtz Dynamics

Abstract

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.

Figures (4)

• Figure 1: From left to right: electron mixing fraction $F_e$ in the lower shear layer during the nonlinear phase. Shown are the rolled-up vortices at $t = 389\,\Omega_{c,i}^{-1}$, their merging at $t = 501\,\Omega_{c,i}^{-1}$, and the late nonlinear stage at $t = 946\,\Omega_{c,i}^{-1}$. White lines trace the in-plane magnetic field; in the right panel, two ejected plasma parcels (P1: red; P2:blue) are highlighted.
• Figure 2: Morphological evolution of the lower ($y=y_{sh,1}$) and upper ($y=y_{sh,2}$) shear layers at $t=445,501,723,946 \, \Omega_{c,i}^{-1}$ during the nonlinear stage of the KHI. Each panel shows a region around each shear that is $100 \times 150 \, d_i^2$. The colorbar shows $\tilde{n}/n_0$ in percentage, which describes the mixing of the two plasma regions normalized to the initial value (see text). The top two rows show the ion mixing (a--d upper shear, e--h lower shear), and the bottom two rows show the electron mixing (i--l upper shear, m--p lower shear).
• Figure 3: Temporal evolution of plasma mixing, current density, and X-points. Bottom and top shear layers ($y_{sh,1}$, solid; $y_{sh,2}$, dashed) are indicated in all panels. Left panel: percentage of mixed plasma $\tilde{n}/n_0$ for electrons (blue) and ions (red). Middle panel: maximum absolute out-of-plane current density $|J_z/(n_0 q_i c)|$ for electrons (blue) and ions (red). Right panel: number of X-points in each shear layer.
• Figure 4: Panels (a) and (c) show the percentage of mixed plasma, $\tilde{n}/n_0$, for ions and electrons in the lower shear layer with a finite $B_x$ at $t = 334\,\Omega_{c,i}^{-1}$. Panels (b) and (d) show the corresponding ion and electron distributions in the lower shear layer without $B_x$ at the same time. Each panel shows a region around the shear layer of size $100 \times 150 \, d_i^2$. A power-law normalization is used to enhance the visibility of low-amplitude structures.