Onset and non-steady state evolutions of collisionless magnetic reconnection

We study the evolutions of collisionless magnetic reconnection with full kinetic particle-in-cell simulations. Two scenarios of forced reconnection, one for steady state and the other for non steady state, are setup with open boundary conditions. The fast reconnection onset is discovered to be an nonlinear electron self- reinforcing process. Accelerated by the reconnection electric field (Ey), the small portion of energetic electrons in the vicinity of the X point enhance the reconnection rate through the off-diagonal terms of the electron pressure tensor. For the overall evolution of reconnection, there are different stages: the onset or early growing stage when the Ey structure is a monopole at the X point, the bipolar stage when the Ey structure is bipolar and the outer electron diffusion region (EDR) is being elongated over time, and the possible final steady state stage when Ey is uniform in the reconnection plane. We find the change of reconnection rate is not empowered or dependent on the length of the EDR. During the early growing stage, the EDR is elongated while the reconnection rate is growing. During the later stage, the reconnection rate may significantly decrease but the length of the inner EDR is largely stable. The results indicate that reconnection is not controlled by the downstream physics, but rather by the availability of plasma inflows from upstream. The Hall current induced by the quadrupole magnetic field is discovered to play an important role in the elongation of the EDR. The electron super-Alfvénic outflow jet structure could be elongated when the structure of Ey is bipolar, and remains stable during steady state.