Effect of nonzero dawn-dusk magnetic field component on particle acceleration in three-dimensional geomagnetic reconnection

The acceleration of protons is investigated by tracing their trajectories i n the electric and magnetic fields obtained from a three-dimensional magnet ohydrodynamic simulation of local magnetic reconnection. The magnetic recon nection is induced by imposing a localized anomalous resistivity in two dif ferent initial configurations, the first one without a B-y component and th e second with a positive B-y superimposed in the two-dimensional Harris fie ld lying in the xz plane. Applying the trajectory calculations to the geota il condition, the previously published results have been mostly recovered f or the B-y = 0 case. For the nonzero B-y case, remarkable asymmetry about t he z = 0 plane appears in the trajectories, and the pitch angle and spatial dependencies of energy gain. For the particles found above the z = 0 plane at the earthward edge from the X-line, results are similar to those in the B-y = 0 case, while they are quite different for the particles found below the z = 0 plane. The trajectories of the particles exiting the simulation domain below the z=0 plane are much more complicated than those of the part icles exiting above the z = 0 plane. Consequently, the duskward skewness of the energy gain, which is well defined above the z = 0 plane, is no longer a general and robust feature below the z = 0 plane. It is expected that th is up-down asymmetry appears not only in the geotail condition but also in other systems as long as the symmetry about the reconnection current sheet plane breaks down by the presence of B-y, i.e., the magnetic field componen t aligned with the X-line. (C) 2001 American Institute of Physics.