For Case 0. (a) Evolution of in-plane magnetic field lines and out-of-plane magnetic field ﹩{B}_{y}/{B}_{0}﹩ during the onset process. (b) Time histories of reconnected magnetic flux ﹩\bigtriangleup \psi ﹩ and reconnection rate ﹩d\,\bigtriangleup \psi /{dt}﹩. Here the reconnected magnetic flux ﹩\bigtriangleup \psi ﹩ is in units of ﹩{B}_{0}{d}_{i}﹩, and it is defined as the difference between the maximum and minimum of ψ at z = 0. The reconnection rate ﹩d\,\bigtriangleup \,\psi /{dt}﹩ is in units of ﹩{B}_{0}{V}_{{\rm{A}}}﹩, and it is equivalent to the magnitude of reconnection electric field E_y at the reconnection site. In this study, we focus on the onset period (gray) covering the transition from the collisionless tearing mode instability (Ω_it = 13) to fast growth of reconnection (Ω_it = 33).