Three-dimensional collisionless magnetic reconnection in the presence of a guide field

Previous investigations of collisionless magnetic reconnection in a standard Harris neutral sheet configuration have demonstrated the importance of the Hall term for producing near-Alfvenic rates of reconnection and the existence of a very thin (similar toc/omega(pe)) electron current layer and sharp density/pressure gradients on c/omega(pi) scales. The present work uses three-dimensional (3-D) particle-in-cell simulations with an open geometry to investigate the changes in the reconnection physics produced by a "guide field" component B-0y of the magnetic field. With B-0y less than or similar to B-0, the nonlinear reconnection rate is not substantially modified from that for the Harris case. The properties of the reconnection fields and particle dynamics, however, are strongly altered. The familiar quadrupole B-y pattern is replaced by an enhancement of \B-y\ between the separatrices. The enhanced parallel electric field and parallel electron velocity are confined to one pair of separatrix arms ( which are positively charged), while the electron current peaks on the other pair ( which are negatively charged). The ion outflow along the current sheet polarizes the separatrices, thereby creating large components of the in-plane electric field. The electrons are accelerated to form a beam structure with parallel speed limited by the electron Alfven speed. The beam-dominated electron distribution produces some y-dependent structures in E-parallel to. For B-0y >> B-0, the reconnection rate is reduced by a factor of 2-3, and the parallel fields and velocities are somewhat smaller; the Hall current produced perturbations in B-y are considerably reduced.