Cluster Measurements of Fast Magnetic Reconnection in Earth's Magnetotail

Fast magnetic reconnection is crucial to explosive processes in magnetospheres, solar and laboratory plasmas, and other astrophysical objects where magnetic energy is rapidly converted to kinetic and thermal energies. To be responsible for energy conversion in dynamic events such as solar flares and Earth's substorms, the reconnection rate Rrec has to be as high as ~ 0.1, in Alfvénic scale. Though reconnection has been studied in many theoretical models and numerical simulations, ultimate confirmation of the fast process must be achieved by detecting the reconnection rate itself and associated dimensions of the diffusion region with multi-spacecraft measurements. We report here a fortuitous multiple traversing of Cluster constellation of four-spacecraft through a diffusion region in the Earth's magnetotail on 15 September, 2001, as satellites were meandering around the X-line for a short period. Based on observed magnetic field and plasma data, we directly obtain a fast reconnection rate of Rrec=Vin/V{A}~ 0.1. The result is the first reliable evidence of fast reconnection in magnetospheric plasmas and provides a solid observation basis for theories and simulations of collisionless fast reconnection. The characteristic sizes of the diffusion region Lz ~ 0.9 di(= 460 km), and Lx ~ Lx~(3.3-5.1)di are also measured. The length of the diffusion region is determined for the first time based on observations, and further supports the fast reconnection rate measurement and casts light onto the physics inside the diffusion region. Furthermore, other features detected during the event all match the previous observations and simulations very well. These outcomes establish a comprehensive overview of fast reconnection in the ion inertial scale.