Three-Dimensional Collisionless Magnetic Reconnection: Simulation With a Particle Electromagnetic Code

We have investigated the onset of reconnection in thin current sheets by means of three-dimensional full particle (PIC) simulations. Instead of imposing reconnection ab initio, reconnection is allowed to develop out of the numerical noise. We do not impose symmetry about the midplane, so that the drift kink instability, the Kelvin-Helmholtz instability, and the sausage instability are allowed for, and we use a high mass ratio of m_{i/m_e=160}. The system is double periodic with two current sheets, which limits the time reconnection can proceed, but simplifies the boundary conditions. Two cases are investigated: (1) a thin current sheet with exactly antiparallel fields, and (2) a thin current sheet with a guide field of the same order as the antiparallel field. In case (1) the lower hybrid drift instability (LHDI) is excited and leads to current sheet thinning. Subsequently, patchy reconnection sets in, and arranges itself within a few ion times into a single neutral line. In case (2) the onset of reconnection is delayed, but eventually a single neutral line emerges. No sausage mode or kink mode, respectively, preceed in either case the onset of reconnection. After a single neutral line has evolved it kinks in the current direction.