Collisionless reconnection in a quasi‐neutral sheet near marginal stability

Particle simulations are used to investigate the process of collisionless reconnection in a magnetotail configuration which includes a pressure gradient along the tail axis and tail flaring. In the absence of electron stabilization effects, the tearing mode is stabilized when the ion gyrofrequency in the normal field exceeds the growth rate in the corresponding one‐dimensional current sheet. The presence of a low‐frequency electromagnetic perturbation in the lobes can serve to destabilize a marginally stable current sheet by producing an extended neutral‐sheet region which can then undergo reconnection. These results help to explain how X ‐type neutral lines, such as those associated with the onset of magnetospheric substorms, can be formed in the near‐earth plasma sheet.