LAUSR

Using high‐resolution data of Magnetospheric Multiscale mission, we observed a gradual but periodical dipolarization process (DPs) in the Earth's magnetotail for the first time. These sub‐DPs, characterized by increase of Bz component and magnetic magnitude Bt, have a period of ~16 s. The electrons have a cigar distribution in the first three sub‐DPs but have a pancake distribution in the last sub‐DP, which excites two oppositely propagating whistler waves. At the trailing boundary of the last sub‐DP, Bz decreases sharply. Meanwhile, strong tailward and duskward plasma flow, intense current, ion and electron demagnetization, and positive energy conversion from the fields to the plasmas are observed at this boundary. By comparing the motion of this boundary with the plasma flow, we infer that the flow rebounds in longer period of the last DP and causes perpendicular electron acceleration. The periodical sub‐DPs and kinetic effects on the DP could improve our understanding of substorm and the magnetotail dynamics.