The Influence of Sheared Parallel Flows on the Onset of Magnetic Reconnection in Collisionless Regimes

The influence of sheared parallel flows on the onset of magnetic reconnection has been the subject of many investigations, majority of which employed various MHD models. At the same time, relatively little is known about the influence of parallel flows on magnetic reconnection in collisionless plasmas. As a first step in a broader program aimed at tackling this complicated problem, the linear stability of a newly developed exact kinetic equilibrium configuration is investigated. This model incorporates sheared parallel flows into the well-known Harris current sheet configuration, and allows considerable flexibility in the choice of the flow profile. The results from fully kinetic 2D PIC simulations are compared against numerical solution of a linearized complete Vlasov-Maxwell system, and against predictions of a simple semi-analytical model. MHD analysis typically predicts existence of an instability driven by the flow shear when the magnitude of the flow exceeds a critical speed which is of the order of Alfvén speed. In the kinetic regime the dispersion relation of the tearing instability is seen to be continuously modified by the flow. For some parameters the growth rate of the unstable reconnecting mode in the presence of a sheared flow may become larger than the growth rate of the tearing instability in the absence of flow, indicating that the mode has an additional driving mechanism. However, for the range of parameters considered in the present study, the growth rate of this "hybrid" instability is comparable to that of the tearing mode. When the magnitude of the flow is further increased, the growth rate significantly decreases. Consistent with these results, preliminary nonlinear PIC simulations demonstrate that if it proceeds, the onset of fast magnetic reconnection develops in the manner that is very similar to the situation when the flow is absent. In particular, no simulation has produced highly wrapped vortices that are observed in MHD simulations.