Computer studies on the spontaneous fast reconnection evolution in a force-free current sheet system

Abstract

Two-dimensional magnetohydrodynamic simulations study the spontaneous fast reconnection evolution in a force-free current sheet system where the magnetic field simply rotates by 180 deg across the central current sheet without changing its magnitude. It is demonstrated that, as in the conventional coplanar case, the fast reconnection mechanism drastically evolves because of the positive feedback between (current-driven) anomalous resistivity and global reconnection flow; also, the fast reconnection evolution becomes more drastic for the lower plasma β. Once an anomalous resistivity is ignited and a sufficient amount of the sheared field component Bz is ejected from near the X reconnection point, the ambient magnetic field collapses into the X point, giving rise to the drastic buildup of the fast reconnection mechanism. On the nonlinear saturation phase, the Bz field is completely swept away from the reconnection region, so that coplanar slow shocks extend outward, and a large-scale plasmoid swells and propagates. The resulting plasmoid has a double structure that is quite different from the well-known coplanar one or from the so-called flux rope. In the backward half of the plasmoid, the plasma pressure is enhanced in the butterfly-shaped region, and (coplanar) slow shocks stand along the plasmoid boundary. On the other hand, in the forward half of the plasmoid a finite-amplitude intermediate wave stands along the plasmoid boundary; hence, across the plasmoid boundary, the magnetic field simply rotates without changing plasma quantities nor magnetic field magnitude.