Statistical investigation of electric field fluctuations around the lower-hybrid frequency range at dipolarization fronts in the near-earth magnetotail

Abstract

Dipolarization fronts (DFs) are thin magnetic boundary structures, embedded in short-duration, fast earthward-moving plasma flows, so-called bursty bulk flows. Previous case studies have shown that the density gradient at DFs can excite the lower-hybrid drift instability (LHDI) and resulting kinetic-scale waves. These waves feature strong electric field fluctuations perpendicular to the ambient magnetic field and associated effective particle acceleration/heating. In this study, we statistically examine electric field fluctuations in the lower-hybrid (LH) frequency range of 61 DF events, using data from the Magnetospheric Multiscale (MMS) Mission between 2017 and 2018 when the MMS apogee was in the magnetotail. We observed that all DF events exhibit enhanced power in the electric field fluctuations in the LH frequency range. Among the observed events, the power can vary by several orders of magnitude. The waves are detected for both high and low values of the perpendicular electron density or pressure gradient. In addition, the peak wave power within the DF is often observed at the time of steepest density gradient within the DF. The results also reveal that the wave power correlates with the magnetic flux transport rate of the DFs. These findings suggest that enhanced density and pressure gradients, which can be formed by large-scale flux transport at DFs, lead to LHDI-related kinetic-scale wave signatures at the DFs, and this may modify the original gradient layer profile of the DF in the course of its propagation from the source to the observation point.