Electron Sublayers and the Associated Magnetic Topologies in the Inner Low-Latitude Boundary Layer

The low-latitude boundary layer (LLBL) plays an important role as a transition layer in coupling the magnetosheath and magnetosphere. Using high-resolution Magnetospheric Multiscale data, we analyze the electron distributions in the inner region of the LLBL, during an active period of magnetic reconnection under southward interplanetary magnetic field. According to the measured electron energy anisotropy, we suggest that this inner LLBL can be divided into six sublayers corresponding to three types of magnetic field-line topologies: (1) open magnetic field line topology from magnetosheath to southern magnetosphere, (2) open magnetic field line topology from magnetosheath to northern magnetosphere, and (3) reclosed magnetic field line topology. These different scenarios indicate that magnetic reconnection occurs at both northern and southern locations of the spacecraft and thus suggest that magnetic reconnection was active simultaneously at high and low latitude on the magnetopause, equatorward of the cusps. These results provide evidence within the LLBL for such multiple X-line formation.