Fast magnetic reconnection in free space: Self-similar evolution process

We present a new model for the time evolution of fast magnetic reconnection in free space, which is characterized by self-similarity. Reconnection tri ggered by locally enhanced resistivity assumed at the center of the current sheet can self-similarly and unlimitedly evolve until external factors aff ect the evolution. The possibility and stability of this type of evolution are verified by numerical simulations in a very wide spatial dynamic range. Actual astrophysical reconnection in solar flares and geomagnetospheric su bstorms can be treated as an evolutionary process in free space, because th e resultant scale is much larger than the initial scale. In spite of this f act, most of the previous numerical works focused on the evolutionary chara cters strongly affected by artificial boundary conditions on the simulation boundary. Our new model clarifies a realistic evolution for such cases. Th e characteristic structure around the diffusion region is quite similar to the Petschek model, which is characterized by a pair of slow-mode shocks an d the fast-mode rarefaction-dominated inflow. However, in the outer region, a vortex-like return flow driven by the fast-mode compression caused by th e piston effect of the plasmoid takes place. The entire reconnection system expands self-similarly.