The dynamics of plasmoid instability in the presence of asymmetric parallel shear flow.
The nonlinear evolution of the magnetic reconnection and onset of the plasmoid instability are investigated by using 2.5-dimensional MHD simulations when the sheared plasma flow is anti-symmetric on either side of the boundary layer. In particular, we considered a wide range of velocity amplitude of shear flow (V0) (from sub-Alfvénic to level of super-Alfvénic) and the shear flow scale length (av) compared to equilibrium magnetic field scale length (aB). We found that sub-Alfvénic shear flows (here V0<0.6VA) can change the O-point position of magnetic islands. The plasmoid instability is suppressed with increasing shear flow velocity, and the Kelvin-Helmholtz instability appears instead of the plasmoid instability when the shear flow is of the order of the Alfvénic or larger. Thus, at the limit of Alfvénic velocity, the magnetic field lines twist near the magnetic reconnection site due to the presence of asymmetric shear flows. The shear flow scale length (or shear flow thickness) can have either stabilization or destabilization effects on the current sheet development. For sub-Alfvénic shear flow (here V0=0.8VA), avaB has a boosting effect on the plasmoid instability. Therefore, we found a critical value for the shear flow thickness that magnetic reconnection has the maximum value. The boosting effect of the shear flow on the current sheet becomes strongest at avc=1.2aB.