Strong Field‐Aligned Current and Its Driven Energy Conversion at Anti‐Dipolarization Front.

Anti‐dipolarization fronts (ADFs), characterized by the rapid increase of the negative magnetic field Bz component, are typically formed at the leading edge of the tailward reconnection jets in the Earth's magnetotail. To date, the electron‐scale current structures, which govern the energy conversion at ADFs, are still barely understood due to the lack of high‐resolution measurements. Here, using Magnetospheric Multiscale mission, we for the first time report a tailward ADF associated with strong field‐aligned current (FAC). The FAC appears at the leading part of the ADF and its densities can reach about 200 nA/m2, which is significantly larger than those reported before. Such current is primarily contributed by the electron flow, which also forms electron beam distribution in the anti‐parallel direction. Significant energy conversion (E⋅ J, E is electric field and J is current density) is also observed at the ADF, which is mainly contributed by the FAC and the fluctuating electric fields. This study makes essential steps toward understanding the current system and the energy conversion at the ADF in the Earth's magnetotail. Plain Language Summary: Dipolarization fronts (DFs) and anti‐dipolarization fronts (ADFs) are the leading edge of the reconfigured magnetic field after reconnection, expanding earthward and tailward from the reconnection site respectively. The field‐aligned current (FAC) at DFs play crucial roles in global mass transport and energy conversion during substorm and they have been widely studied by in situ observation and simulation. However, the FAC at ADFs and the corresponding energy conversion have not been studied so for. Here, for the first time, we report an ADF associated with strong FAC using high‐resolution data from the MMS mission. We investigate the detailed features of the currents, electron distribution functions, electric fields and energy conversion at the ADF. This study shed light on how strong FAC drive energy conversion at ADFs. Key Points: We for the first time report a tailward anti‐dipolarization front (ADF) associated with strong field‐aligned current (FAC)The FAC is primarily carried by electrons and can reach ∼200 nA/m2, which is even stronger than the FACs reported in earthward Dipolarization frontsThe strong FAC and fluctuating electric fields can drive significant energy conversion at the ADF [ABSTRACT FROM AUTHOR]