Dipolarization fronts as earthward propagating flux ropes: A three-dimensional global hybrid simulation

Dipolarization fronts (DFs) as earthward propagating flux ropes (FRs) in the Earth's magnetotail are presented and investigated with a three-dimensional (3-D) global hybrid simulation for the first time. In the simulation, several small-scale earthward propagating FRs are found to be formed by multiple X line reconnection in the near tail. During their earthward propagation, the magnetic field B_z of the FRs becomes highly asymmetric due to the imbalance of the reconnection rates between the multiple X lines. At the later stage, when the FRs approach the near-Earth dipole-like region, the antireconnection between the southward/negative B_z of the FRs and the northward geomagnetic field leads to the erosion of the southward magnetic flux of the FRs, which further aggravates the B_z asymmetry. Eventually, the FRs merge into the near-Earth region through the antireconnection. These earthward propagating FRs can fully reproduce the observational features of the DFs, e.g., a sharp enhancement of B_z preceded by a smaller amplitude B_z dip, an earthward flow enhancement, the presence of the electric field components in the normal and dawn-dusk directions, and ion energization. Our results show that the earthward propagating FRs can be used to explain the DFs observed in the magnetotail. The thickness of the DFs is on the order of several ion inertial lengths, and the electric field normal to the front is found to be dominated by the Hall physics. During the earthward propagation from the near-tail to the near-Earth region, the speed of the FR/DFs increases from ~150 km/s to ~1000 km/s. The FR/DFs can be tilted in the GSM (x, y) plane with respect to the y (dawn-dusk) axis and only extend several Earth radii in this direction. Moreover, the structure and evolution of the FRs/DFs are nonuniform in the dawn-dusk direction, which indicates that the DFs are essentially 3-D.