Electron‐Scale Measurements of Antidipolarization Front

Antidipolarization fronts (ADFs), usually formed at the leading edge of the tailward reconnection jets in the Earth's magnetotail, are characterized by sharp enhancements of southward magnetic field (negative Bz). The fine structures at electron scale of the ADFs have not been studied so far, due to instrumental limitations and large separation of previous spacecraft. Here, we use the Magnetospheric Multiscale mission to investigate the structure and dynamics of an ADF at electron scale. We find an intense current at the ADF, with the parallel current carried by a fast electron jet and the perpendicular current contributed by ion flow. Strong electric fields are found at the front and balanced by Hall term. The ion current leads to strong energy conversion, transforming the electromagnetic energy into particle energy. Our study makes essential steps toward understanding ADF dynamics in the magnetotail. Plain Language Summary Reconnection fronts, the leading edge of the reconfigured magnetic field after reconnection, expand both earthward and tailward from the reconnection site. They are also called dipolarization fronts (DFs) and antidipolarization fronts (ADFs). DFs and ADFs play crucial roles in global flux transport and energy conversion during substorm. The characteristics of DFs at different scales (MHD, ion, and electron scale) have been widely studied by spacecraft observations and simulations. However, the fine structures at electron scale of the ADFs have not been studied so far. Here, for the first time, we present electron‐scale measurements of an ADF in the Earth's magnetotail using high‐resolution data from the Magnetospheric Multiscale mission. We investigate the detailed features of the particle properties, current system, electric field structures, and energy conversion at the ADF. This observation presents typical features of the ADF at electron scale. Key Points Structure and dynamics of antidipolarization front (ADF) at electron scale are investigated for the first time The ADF hosts intense electron parallel current, ion perpendicular current, and strong electric fields balanced by Hall term Strong energy conversion to ions is occurring at the ADF