Plasmoid formation via competing lower-hybrid drift and Kelvin-Helmholtz instabilities: A hybrid kinetic-gyrokinetic simulation study

TL;DR

This study addresses how microinstabilities shape large-scale reconnection by examining the interaction between lower-hybrid drift instability (LHDI) and Kelvin--Helmholtz instability (KHI) in 2D, low-$\beta_e$ current sheets using a hybrid kinetic--gyrokinetic model with fully kinetic ions and drift-kinetic electrons. The approach resolves ion-scale dynamics and LHDI while allowing long-time evolution of macroscopic structures, revealing two distinct plasmoid formation pathways: an LHDI-driven inverse cascade that seeds multiple plasmoids and can suppress KHI, and a KHI-dominated route with large rolled-up vortices and internal reconnection. When LHDI and KHI coexist, LHDI tends to appear along KH vortex edges, creating a mixed turbulent state that modifies reconnection patterns and energy transfer across scales. These findings imply that microturbulence can govern large-scale magnetic topology in space plasmas, with implications for boundary-layer dynamics at the solar wind–magnetosphere interface and similar collisionless reconnection environments. Future work should extend to three dimensions, incorporate fully kinetic electrons, and compare with in-situ observations to validate the proposed LHDI-induced plasmoid formation mechanism.

Abstract

We investigate the nonlinear formation of plasmoids in 2D low-beta current sheets through the interplay between the Kelvin-Helmholtz instability (KHI) and the lower-hybrid drift instability (LHDI). Using a hybrid kinetic-gyrokinetic model-based Super Simple Vlasov (ssV) code with fully kinetic ions and drift-kinetic electrons, we simulate Harris-type current sheets and velocity shear layers with strong cross-field density gradients. Our central hypothesis is that steep density gradients drive LHDI, which can grow faster than KHI and initiate an inverse cascade from kinetic to fluid scales, potentially suppressing KHI. Our simulations confirm that, in thin current sheets, LHDI develops rapidly at the sheet edges and nonlinearly merges into larger-scale magnetic islands before KHI can evolve. These LHDI-driven structures distort the velocity shear and suppress classical KH vortices. In contrast, for thicker current sheets or weaker density gradients, KHI dominates and produces the expected rolled-up vortices and associated plasmoids. These findings demonstrate that LHDI-induced turbulence can act as both a seed and a regulator of plasmoid-generating instabilities, mediating cross-scale energy transfer. This mechanism is relevant to thin boundary layers in space plasmas, such as the solar wind magnetosphere interface, and suggests that microturbulence can govern large-scale magnetic topology during collisionless reconnection.

Figures (11)

• Figure 1: Initial equilibrium profiles for (left) the Harris-type current sheet and (right) the shear-layer configuration, showing $n(x)$, $B_y(x)$, and $V_y(x)$ where applicable.
• Figure 2: Qualitative parameter–space sketch showing the LHDI, mixed, and KHI regimes associated with the shear–layer runs.
• Figure 3: Linear evolution of the lower–hybrid drift instability in the Harris current sheet for $m_i/m_e=36$, $T_i/T_e=10$, and $L=1\rho_i$: (a) electric–field spectral energy showing a dominant peak near $k_y\rho_i\approx 6$, and (b) corresponding temporal growth rate.
• Figure 4: Electric–field spectral energy $E_{E_y}(k_y,t)$ for the three shear–layer configurations. (a) LHDI-dominated case showing early high-$k_y$ fluctuations. (b) Mixed case where both instability families contribute. (c) KHI-dominated case showing the gradual emergence of low-$k_y$ KH modes.
• Figure 5: Two–dimensional $E_y(x,y)$ contours during the early phase ($t \lesssim 40$-$80\,\Omega_{ci}^{-1}$) for all four configurations. (a) Harris sheet showing LHDI at the density–gradient edges. (b) Shear layer with LHDI-dominated behavior. (c) Mixed case with coexisting LHDI and early KH distortions. (d) KHI-dominated case displaying clear Kelvin–Helmholtz roll-up.