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

Abstract 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.