Diamagnetic Effects on Asymmetric Reconnection: A Comparative PIC and Hall MHD Study

We present a comparative study of reconnection in a Harris current sheet with a guide field, modified by the addition of an equilibrium pressure gradient at the reconnection layer to introduce diamagnetic drifts, using Hall MHD and particle-in-cell (PIC) simulations. Previous kinetic studies of a similar configuration in magnetopause conditions showed significant decoupling of X-point and island drift speeds as the pressure gradient was suppressed across the growing island while steepening near the X-point, resulting in reduced reconnection rates. One goal of our comparative study is to investigate how much of the relevant kinetic physics is captured by two-fluid simulations, and differences that occur as the magnitude of the guide field is varied. We also extend the parameter space to stronger guide fields and higher $\beta$, relevant to fusion plasmas. Our Hall MHD simulations utilize the Magnetic Reconnection Code (MRC), which features a Generalized Ohm's Law including the Hall term and electron pressure gradient. It supports non-uniform grids and implicit time-stepping. PIC results are provided by the Particle Simulation Code (PSC). We focus on the nonlinear evolution of reconnection rates and the asymmetric structure of the X-point, and test the validity of recent expressions for the asymmetric reconnection rate recently proposed in the literature.