Electron dynamics and wave activities associated with mirror mode structures in the near-Earth magnetotail

Abstract

We report the observation of mirror mode structures by Cluster spacecraft at around X∼-16 RE in the Earth’s magnetotail. The wavelength of the mirror structure is larger than 7000 km, corresponding to tens of ion gyroradii. Features of the mirror structures are similar to those detected in the magnetosheath: the anti-correlation between the magnetic field strength and plasma density, zero phase velocity in the plasma rest frame and linear polarization. The structures were observed in a region bounded by two dipolarizations during a substorm intensification. Thus, the dipolarization process may provide a plasma condition facilitating the growth of the mirror mode structures. Another interesting feature is the electron dynamics within the mirror structures. Thermal electron energy flux has an enhancement at 0° and 180° pitch angles inside the magnetic dips of the first three mirror structures and an enhancement at 90° pitch angle inside the magnetic dip of the last structure. The different electron distribution inside the mirror structures might be a result of different evolution stages of the mirror wave. The last structure may be in the nonlinear stage of the mirror instability, whereas the three others with quasi-sinusoidal waveforms may be in the linear stage. In addition, we found that intense whistler waves were confined within the magnetic dips. We conjecture that whistler waves observed in the first three dips were generated in a remote region, then they were trapped in the mirror mode troughs and transported toward the spacecraft; while the whistler wave detected in the last dip was excited locally by the electron anisotropy instability.