Driven magnetic reconnection near the Dreicer limit

The influence of Coulomb collisions on the dynamics of driven magnetic reconnection in geometry mimicking the Magnetic Reconnection eXperiment (MRX) [M. Yamada, Phys. Plasmas 4, 1936 (1997)] is investigated using two-dimensional (2D) fully kinetic simulations with a Monte Carlo treatment of the collision operator. For values of collisionality typical of MRX, the reconnection mechanism is shown to be a combination of collisionless effects, represented by off-diagonal terms in the electron stress tensor, and collisional momentum exchange between electrons and ions. The ratio of the reconnection electric field E_R to the critical runaway field Ecrit provides a convenient measure of the relative importance of these two mechanisms. The structure of electron-scale reconnection layers in the presence of collisions is investigated in light of the previously reported [S. Dorfman, Phys. Plasmas 15, 102107 (2008)] discrepancy in the width of the electron reconnection layers between collisionless simulations and experimental observations. It is demonstrated that the width of the layer increases in the presence of collisions, but does not substantially deviate from its collisionless values, given by the electron crossing orbit width, unless E_R Ecrit. Comparison with MRX observations demonstrates that the layer width in 2D simulations with Coulomb collisions is substantially smaller than the value observed in the low-density experiments with E_R Ecrit, indicating that physical mechanisms beyond those included in the simulations control the structure of the electron layers in these experiments.