Observation of Three-Dimensional Magnetic Reconnection in the Terrestrial Magnetotail

Study of magnetic reconnection has been focused on two-dimensional geometry in the past decades, whereas three-dimensional structures and dynamics of reconnection X line are poorly understood. In this paper, we report Cluster multispacecraft observations of a three-dimensional magnetic reconnection X line with a weak guide field (~25% of the upstream magnetic field) in the Earth's magnetotail. We find that the X line not only retreated tailward but also expanded across the tail following the electron flow direction with a maximum average speed of (0.04–0.15) V_A,up, where V_A,up is the upstream Alfvén speed, or (0.14–0.57) V_de, where V_de is the electron flow speed in the out-of-plane direction. An ion diffusion region was observed by two spacecraft that were separated about 10 ion inertial lengths along the out-of-plane direction; however, these two spacecraft observed distinct magnetic structures associated with reconnection: one spacecraft observed dipolarization fronts, while the other one observed flux ropes. This indicates that reconnection proceeds in drastically different ways in different segments along the X line only a few ion inertial lengths apart.