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 magnetotailare presented and investigated with a three-dimensional (3-D) global hybrid simulation for the first time. In thesimulation, several small-scale earthward propagating FRs are found to be formed by multiple X line reconnectionin the near tail. During their earthward propagation, the magnetic field Bz of the FRs becomes highly asymmetricdue to the imbalance of the reconnection rates between the multiple X lines. At the later stage, when the FRsapproach the near-Earth dipole-like region, the antireconnection between the southward/negative Bz ofthe FRs and the northward geomagnetic field leads to the erosion of the southward magnetic flux of theFRs, which further aggravates the Bz asymmetry. Eventually, the FRs merge into the near-Earth regionthrough the antireconnection. These earthward propagating FRs can fully reproduce the observationalfeatures of the DFs, e.g., a sharp enhancement of Bz preceded by a smaller amplitude Bz dip, an earthwardflow 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 DFsobserved in the magnetotail. The thickness of the DFs is on the order of several ion inertial lengths, and theelectric field normal to the front is found to be dominated by the Hall physics. During the earthward propagationfrom the near-tail to the near-Earth region, the speed of the FR/DFs increases from 150km/s to 1000 km/s. TheFR/DFs can be tilted in the GSM (x, y) plane with respect to the y (dawn-dusk) axis and only extend several Earthradii in this direction. Moreover, the structure and evolution of the FRs/DFs are nonuniform in the dawn-duskdirection, which indicates that the DFs are essentially 3-D.