  We present a new model for time evolution of fast magnetic reconnection in
free space, which is characterized by self-similarity. Reconnection triggered
by locally enhanced resistivity assumed at the center of the current sheet can
self-similarly and unlimitedly evolve until external factors affect 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 substorms can
be treated as an evolutionary process in free space, because the resultant
scale is much larger than the initial scale. In spite of this fact, most of the
previous numerical works focused on the evolutionary characters strongly
affected by artificial boundary conditions on the simulation boundary. Our new
model clarifies a realistic evolution for such cases. The characteristic
structure around the diffusion region is quite similar to the Petschek model
which is characterized by a pair of slow-mode shocks and the fast-mode
rarefaction-dominated inflow. However, in the outer region, a vortex-like
return flow driven by the fast-mode compression caused by the piston effect of
the plasmoid takes place. The entire reconnection system expands
self-similarly.
