On the effect of parallel shear flow on the plasmoid instability

The effect of symmetric sheared plasma flow parallel to the equilibrium magnetic field on the evolution of plasmoid instability is investigated by using two-dimensional MHD simulations. It is shown that as the velocity amplitude of a shear flow increases from sub-Alfvénic to the level of super-Alfvénic, the current sheet is stabilized against the plasmoid instability. Suppression of plasmoid instability also takes place for a sub-Alfvénic shear flow, provided that the shear length is sufficiently small compared to the magnetic shear. Therefore, an initially elongated thin current sheet with a Lundquist number greater than the critical value does not necessarily guarantee the development of the plasmoid instability. As a result, the Kelvin-Helmholtz (KH) instability is triggered. Although the rate of reconnection is quite fast in the nonlinear regime of the plasmoid instability, the reconnection of twisted field lines during the KH instability is very slow.