Observation of Three-Dimensional Magnetic Reconnection in the Terrestrial Magnetotail

摘要

abstract_textpStudy 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 (similar to 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,V-up, where V-A,V-up is the upstream Alfven 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./ppPlain Language Summary Two-dimensional reconnection model has been employed for decades to interpret the observations in space. However, reconnection sometimes happens in a three-dimensional manner that is dramatically different from the 2-D model. The 3-D dynamics of reconnection is poorly known. By using observations from multiple points in Earth's magnetosphere, we are able to reveal some important aspects of the three-dimensional dynamics of magnetic reconnection X line, which is characterized by expansion and large gradient in the out-of-plane direction. For the first time, we estimate the X line expansion speed using the in situ data. We found that the X line expanded along the electron out-of-plane flow direction with a speed much less than the upstream Alfven speed and a small fraction of the electron flow speed in the out-of-plane direction. An ion diffusion region was observed by two spacecraft that were separated about one Earth radius along the out-of-plane direction; however, these two spacecraft observed distinct magnetic structures associated with reconnection. This indicates that the characteristic length of X line is on the order of the ion scale. Reconnection produces 3-D structures on the ion scale in Earth's magnetosphere./p/abstract_text