The Spreading of Magnetic Reconnection X-Line in Particle-In-Cell Simulations– Mechanism and the Effect of Drift-Kink Instability

Using three-dimensional particle-in-cell (PIC) simulations, we study the spread of magnetic reconnection X-line. We show that structural asymmetries along the X-line direction develop during its spread. On the plane of the current sheet (i.e., corresponding to the equatorial plane of the magnetotail), sharp cusp-shaped signatures develop along the ion-drifting direction, capturing the spread of the X-line. The spreading is catalyzed by the lower ion pressure from the active diffusion region, and the X-line spreads at the ion-drifting speed of the non-reconnecting current sheet. Along the electron-drifting direction, the X-line barely spreads even though the electron-drifting speed is high within the electron diffusion region, and reconnected flux is transported toward this direction by the Hall effect. This preferential spread in the ion-drifting direction provides an additional explanation for the higher occurrence rate of reconnection events on the dusk side in Earth's magnetotail. In contrast to the laminar X-line, in a companion run, we demonstrate that the fluctuation and turbulence caused by drift-kink instability only suppress the X-line spreading. Even though the fluctuation breaks the frozen-in condition, it does not lead to the continuous onset of reconnection adjacent to the active region.