On the Magnetic Dip Ahead of the Dipolarization Fronts

Dipolarization front (DF), a transient structure that is associated with the earthward plasma flow, is widely considered as a result of magnetic reconnection. It is often characterized with a sharp increase in the northward magnetic field B-z preceded by a minor decrease of it. However, the small magnetic dip ahead of the DF is not always present, the reason of which is not well known. By analyzing in site Magnetospheric MultiScale spacecraft measurements at the magnetotail, we present two events of DF with and without a dip ahead of it. It is found that the magnetic dip ahead of DF is accompanied by a guide field B-y. To investigate the physical mechanism that determines the appearance of the dip, we perform kinetic simulations of symmetric single X line magnetic reconnection under various guide fields. The simulation results show that the dip is generated due to the separation of the gyro-motion between ions and electrons under the influence of the guide field. Ions are reflected at the DF and further deviated by the guide field away from the neutral sheet, while more electrons remain in the neutral sheet due to their small gyroradius. In the absence of a compensating ion current, a dawnward current thus is formed, carried by the electrons, and generates the dip. Our results bring in a new perspective on the formation of the dip in single X line reconnection.