Adiabatic expansion acceleration mechanism of superfast jets in the spontaneous fast magnetic reconnection model

In contrast to the Petschek reconnection model, the plasma outflow jet in f ront of the plasmoid associated with the spontaneous fast reconnection mode l is found to exceed steadily the Alfven velocity measured in the upstream magnetic field region. According to two-dimensional magnetohydrodynamic sim ulations, the final velocity of the plasma jet is observed to be superfast and can reach 1.4 times of the Alfven velocity, which is maintained until t he jet encounters a fast shock generated in front of the plasmoid. On the b asis of the Rankine Hugoniot relation and the Bernoulli equation, it is the oretically found that the superfast plasma jet generated by slow shocks ass ociated with the reconnection process is effectively accelerated beyond the Alfven velocity by the adiabatic expansion of the plasma jet without any m agnetic effect. In the plasma accelerations, the initial plasma acceleratio n caused in the slow shock is consistent with that of the Petschek reconnec tion model, but the subsequent plasma acceleration caused by the adiabatic expansion is not considered in his model. In association with the new accel eration mechanism, one pair of low-pressure regions emerges in the upstream magnetic field region. The generation of the low-pressure regions indicate s that the significant adiabatic expansion results from the distortion of t he surrounding magnetic field lines associated with the swelling plasmoid. (C) 2000 American Institute of Physics. [S1070-664X(00)04306-8].