In order to understand the nature of magnetic reconnection in “ free space ” which is free from any influence of external circumstances , I have studied the structure of spontaneous reconnection outflow using a shock tube approximation .
The reconnection system of this case continues to expand self-similarly .
This work aims 1 ) to solve the structure of reconnection outflow and 2 ) to clarify the determination mechanism of reconnection rate of the “ self-similar evolution model ” of fast reconnection .

Many cases of reconnection in astrophysical phenomena are characterized by a huge dynamic range of expansion in size ( \sim 10 ^ { 7 } for typical solar flares ) .
Although such reconnection is intrinsically time dependent , a specialized model underlying the situation has not been established yet .

The theoretical contribution of this paper is in obtaining a solution for outflow structure which is absent in our previous papers proposing the above new model .
The outflow has a shock tube-like structure , i.e. , forward slow shock , reverse fast shock and contact discontinuity between them .
By solving the structure in a sufficiently wide range of plasma- \beta : 0.001 \leq \beta \leq 100 , we obtain an almost constant reconnection rate ( \sim 0.05 : this value is the maximum for spontaneous reconnection and is consistent with previous models ) and a boundary value along the edge of the outflow ( good agreement with our simulation result ) which is important to solve the inflow region .
Note that everything , including the reconnection rate , is spontaneously determined by the reconnection system itself in our model .