Observation of large-amplitude magnetosonic waves at dipolarization fronts

Various plasma waves have been observed in the vicinity of dipolarization fronts (DFs) and the rarefaction regions behind them. It was suggested that these waves not only play crucial roles in regulating particle kinetics at the DFs but also may potentially affect the large‐scale dynamics of the magnetotail. In this paper, we present the observations of large‐amplitude electromagnetic waves at DFs that occurred during magnetospheric substorms. The DFs were embedded in either the tailward or earthward flows in the near‐Earth magnetotail. The wave frequencies were near the local proton cyclotron frequency. The waves propagated at highly oblique angles with respect to the ambient magnetic field (~80°–100°). Their corresponding wavelengths were on the order of the local ion gyroradii. The major magnetic field fluctuations were along the background magnetic field, while the electric field fluctuations were predominantly perpendicular to the background magnetic field. The waves were compressional waves as there was an anticorrelation between the plasma density and the wave magnetic field strength. The electric potential associated with the waves reached to more than half of the electron temperature, indicating the waves are nonlinear. We suggest that the waves were magnetosonic or ion Bernstein mode waves driven by the ion ring distribution. The waves were able to provide significant anomalous resistivity at the front, with major contributions from the electric field fluctuations. The effects of these waves on the electron pitch angle scattering and energy diffusion are also discussed. Key Points The first report of magnetosonic wave or ion Bernstein wave at the DF They are nonlinear waves The waves are probably excited by the ion ring distribution formed at the front