Particle-in-Cell Simulation of Collisionless Reconnection with Open Outflow Boundaries

A new method for applying open boundary conditions in particle-in-cell (PIC) simulations has been utilized to study magnetic reconnection. For this method, particle distributions are assumed to have zero normal derivatives at the boundaries. Advantages and possible limitations of this method for PIC simulations will be discussed. Results from a reconnection simulation study will be presented. A 2 ½-dimensional electromagnetic PIC simulation using open conditions at the outflow boundaries and simple reflecting boundaries to the inflow regions will be discussed. We define the electron diffusion region as that region where the out-of-plane electron inertial electric field is positive indicating acceleration and flux transfer; the evolution of this region has been analyzed. We have found that this region varies in the range 2.5-4 local electron inertial lengths in total width and in the range 10-15 local electron inertial lengths in total length for the mass ratio 25. The reconnection rate has been investigated in terms of the aspect ratio of this electron diffusion region plus inflow and outflow measures at its boundaries. We will show that a properly measured aspect ratio predicts the flux transfer rate, scaled to account for the decline in field strength and electron density at the inflow boundaries to the electron diffusion region. We conclude that this electron diffusion region either adjusts its aspect ratio for compatibility with the flux transfer rate that is set elsewhere, as in the Hall reconnection model, or that it is this region that controls the reconnection flux transfer rate.