Hybrid Simulations of Current Sheet Thinning

It is well established that the thinning of current sheets leads to rapid magnetic reconnection and the conversion of magnetic energy to particle energy. Two-dimensional numerical simulations performed with different physical and numerical models of an initial planar current sheet (so-call GEM challenge problem) have demonstrated the importance of the Hall term for fast reconnection. A more recent combined study, the Newton Challenge (Birn et al., GRL, 32, L06105, 2005), involves similar calculations initialized with a thicker current sheet that is thinned by the presence of plasma inflow from the exterior regions. Again, the various simulation models produced similar final states and the different types of particle simulations showed similar fast reconnection rates as well as a slower secondary growth phase. In this paper we discuss in detail simulations of this problem carried out with a hybrid (particle ion, massless fluid electron) code that was part of the Challenge study. The electron model contains off-diagonal electron pressure (gyro-viscous) terms rather than a simple scalar pressure. We show how the system evolves in time, the resulting reconnection rate, as well as where in the thinning current sheet are the gyro-viscous terms become important. We show the dependence of the results on parameters of the model and discuss how the reconnection rate varies with the physical conditions.