Influence of the Lower-Hybrid Drift Instability on Magnetic Reconnection in Asymmetric Configurations

Using fully kinetic 3D simulations of magnetic reconnection in asymmetric antiparallel configurations, we demonstrate that an electromagnetic lower-hybrid drift instability (LHDI) localized near the $X$ line can substantially modify the reconnection mechanism in the regimes with large asymmetry, a moderate ratio of electron to ion temperature, and low plasma $\beta$. However, the mode saturates at a small amplitude in the regimes typical of Earth’s magnetopause. In these cases, LHDI-driven turbulence is predominantly localized along the separatrices on the low-$\beta$ side of the current sheet, in agreement with spacecraft observations.

Figure 1

Power of ${\tilde{E}}_y$ (top) and ${\tilde{B}}_x$ (bottom) fluctuations in the range $|\omega |<3{\omega }_{\mathrm{LH}}$ and $|k_y{\rho }_e|<2$. The solid lines represent the field lines of the $y$-averaged in-plane magnetic field, with the thick lines marking the separatrix.

Figure 2

Panels (a), (b), and (c): spectrum of ${\tilde{E}}_y$ fluctuations at positions marked 1, 2, and 3, respectively, in Fig. 1. Panels (d), (e), and (f): the same for ${\tilde{B}}_x$. The solid line in panel (c) corresponds to the local dispersion relation for LHDI [13]. The arrow in panels (a) and (d) marks $k_y=({\rho }_e{\rho }_i{)}^{-1/2}$. The values of ${\omega }_{\mathrm{LH}}$ and ${\rho }_s$ are computed with the shoulder value of the magnetic field and temperatures at the center of the layer.

Figure 3

Panels (a) and (b) show current density $j_y$ in 2D and 3D simulations, respectively, with $T_i^0/T_e^0=0.3$ at $t{\omega }_{ci}^0=42$. Here $j_y$ is normalized to its peak value in the 2D case; (c) profiles of $j_y$ across the layer at $x$ positions marked by dashed lines in panels (a) and (b); Panel (d) shows a profile of $j_y$ in a $y\mathrm{\text{−}}z$ plane at $x/d_e^0=140$ in the 3D simulation.

Figure 4

Spatial profiles of various quantities in 3D simulation with $T_i^0/T_e^0=0.3$ across the reconnection layer (left column, $x/d_e^0=140$) and across the separatrix on the low-$\beta$ side (right column, $x/d_e^0=200$). Top row: profiles of fluctuation amplitude. Middle row: profiles of average density, magnetic field, and current. Bottom row: contribution of fluctuations to the average force balance (1) : nonideal term $(⟨\mathit{E}⟩+(⟨\mathit{V}/c⟩\times ⟨\mathit{B}⟩{)}_y$; (2): $\widetilde{⟨n{E}_y⟩}$; (3): $\widetilde{⟨(n\mathit{V}\times \mathit{B}{)}_y⟩}$; (4): $⟨(\nabla ·\mathit{P}{)}_y⟩$. All terms are normalized to local $⟨nE⟩$.

Figure 5

A subsolar magnetopause crossing by the THEMIS satellite. Panel (a) shows the reconnection magnetic field, which changed from the negative value in the magnetosheath to the positive value in the magnetosphere. Panel (b) is the plasma density. Panel (c) shows the spectrum of $E_y$ fluctuations, which are localized near the region of large density gradients.