Capturing Secondary Kinetic Instabilities in Three-Dimensional Dayside Reconnection Using an Improved Gradient-Based Closure

Abstract

Magnetic reconnection is a highly dynamic process that excites a wide variety of kinetic waves and instabilities. Transverse current sheet instabilities such as the lower-hybrid drift and secondary drift-kink instabilities in particular have been shown by kinetic simulations to modify the reconnection and introduce significant turbulence and mixing to the reconnection layer. Past studies using the ten-moment fluid model to capture important kinetic physics such as the electron inertia and full representation of the pressure tensor proved advantageous to a two-fluid representation of reconnection, but the model struggled when using a local relaxation closure for the heat flux to replicate the current sheet instabilities and subsequent mixing seen in kinetic simulations. This work uses the \texttt{Gkeyll} software framework to perform simulations of asymmetric reconnection based on the 16 October 2015 MMS crossing of a diffusion region, the Burch event. An improved gradient-based heat flux closure is implemented, showing significant improvement in secondary kinetic instabilities that grow in the current sheet. These instabilities generate turbulence which leads to growth of secondary magnetic islands and flux ropes.

Figures (12)

• Figure 1: Electron density snapshots at $z=L_z/2$. The electrons are significantly more diffuse in the gradient closure case (right), and the reconnection layer itself has an island with a density plateau, while in the local closure simulation (left) the layer has a smooth density gradient across the island.
• Figure 2: Slices of the electron density taken at the X-line across various times. No significant mode growth occurs for the simulation run using the local closure (left), while the gradient closure (right) allows LHDI growth at the edge of the current sheet. During nonlinear saturation, the LHDI triggers a shearing instability.
• Figure 3: Magnetic reconnection rates (left), and density gradients (right) across the reconnection layer and separatrices at $x=20d_i$ and averaged over the $z$-direction. The gradient closure experiences a steady decline in the reconnection rate around the saturation time of the LHDI. The decreasing density profile across the separatrices in the local closure results demonstrates significant diffusion perpendicular to the field lines, while the plateaus and sharper drops in the gradient closure suggest not much diffusion is occurring cross-field.
• Figure 4: Transverse current $J_z$ (color), and in-plane magnetic field lines (black lines) in the reconnection plane of the gradient closure simulation. Significant distortion of the flux tube in the transverse direction occurs as the kink instability develops.
• Figure 5: 3D visualization of the magnetic field topology with field lines traced through regions of high magnetic flux for the gradient closure. The primary bundle of field lines (red, $+B_z$) passes directly through the layer, veering to the side and ultimately reversing direction near the edge of the domain. A secondary tube of field lines (blue, $-B_z$) extends the opposite direction through the island that forms.