Shock wave phenomena in collisions between a current loop and a plasmoid

We present the results of 3-D MHD simulations of collisions between an equi librium current and a plasmoid. Three typical equilibrium configurations we re analyzed. Our simulation results show that when a plasmoid approaches a current loop, an active region is created in front of the plasmoid bounded on the front side by a bow shock wave and on the back side by a reverse sho ck wave. The collision process modifies the current system and a strong ele ctric field is also induced in the active region. An additional magnetic fi eld generated by the induced current upsets the initial equilibrium conditi on. As a result, the whole loop is compressed and heated. We found that whe n the plasmoid approaches the loop, before reaching it the induced electric field amounts to its maximum value. The current loop is curved under the c ollision. The core of the plasmoid can not drive into the loop, it is sprun g back by a magnetic counterpressure. This collision process between a plas moid and a current loop may be responsible for the triggering of a solar fl are observed by Yohkoh.