Investigating the Lower Hybrid Drift Instability in Reconnecting Current Sheets Using a Hybrid Kinetic Model (ssV Code)

TL;DR

This work addresses how the lower hybrid drift instability (LHDI) operates in collisionless reconnection by employing a hybrid kinetic model with fully kinetic ions and drift-kinetic electrons in the ssV code. By performing a systematic parameter scan over $m_i/m_e$, $T_i/T_e$, $\beta_e$, and current-sheet half-thickness $L$, and by contrasting electrostatic (ES) and electromagnetic (EM) runs, the study separates edge-localized electrostatic LHDI from a more disruptive electromagnetic LHDI that can thin and bifurcate the current sheet. Key findings show that ES-LHDI saturates through edge density/pressure flattening with negligible impact on reconnection, whereas EM-LHDI can drive central sheet perturbations, bifurcation, and even island formation, especially for thin sheets and higher $\beta_e$; the growth and nonlinear outcomes scale strongly with $m_i/m_e$ and $T_i/T_e$, revealing a threshold around ion-scale sheet thickness $L \sim \rho_i$. These results imply that LHDI can either enhance anomalous transport or actively trigger reconnection under suitable conditions, highlighting the kinetic microphysics that govern energy dissipation and reconnection onset in space and laboratory plasmas.

Abstract

We investigate the nonlinear evolution of the lower hybrid drift instability (LHDI) in reconnecting current sheets using a hybrid kinetic simulation model implemented in the Super Simple Vlasov (ssV) code. The model treats ions kinetically and electrons with a drift-kinetic approximation, solving self-consistent coupled electrostatic and electromagnetic fields. A parametric study explores the effects of mass ratio, temperature ratio, plasma beta, and sheet thickness. In electrostatic cases, LHDI remains localized at the sheet edges, flattening density gradients. In electromagnetic regimes, turbulence induced by LHDI generates magnetic perturbations that kink the current sheet and enhance anomalous resistivity. These dynamics may facilitate fast magnetic reconnection under certain conditions. Our results bridge prior theoretical predictions and simulations, emphasizing the importance of kinetic instabilities in reconnection physics.

Figures (10)

• Figure 1: Initial Harris equilibrium profiles for the magnetic field $B_y(z)$ and plasma density $n(z)$, with half-thickness $L = 1\,\rho_i$. The density peaks at the current sheet center ($z = 0$), while the magnetic field reverses direction across the sheet.
• Figure 2: Frequency-wave number spectrum of electrostatic field (Ey) for mass ratio 36 (color coded), LHDW can be observed at $\omega_{LH} = 6 \Omega_{ci}$ and analytical dispersion relation for the High Frequency Waves (dashed line).
• Figure 3: Electrostatic LHDI signatures in the electric field $E_z$ for $m_i/m_e = 36$, $T_i/T_e = 10$, and $L = 1\,\rho_i$ at $t \Omega_{ci}^{-1}=125$. Flute-like structures localized at the sheet (dotted black line) edges ($z \approx \pm L$) are evident, consistent with the electrostatic nature of the instability at low $\beta$.
• Figure 4: Electromagnetic LHDI signatures in the magnetic field $B_y$ for $m_i/m_e = 36$, $T_i/T_e = 10$, $L = 1\,\rho_i$, and plasma $\beta = 0.01$ at $t \Omega_{ci}^{-1}=125$. Long-wavelength undulations extending into the sheet(dotted black line) center are visible, characteristic of the electromagnetic branch of the LHDI.
• Figure 5: Temporal evolution of the LHDI in a simulation run with ion-to-electron mass ratio $m_i/m_e = 36$, temperature ratio $T_i/T_e = 10$, and current sheet half-thickness $L = 1\,\rho_i$, as diagnosed by the amplitude of the electrostatic potential $\phi$