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 in the electric and magnetic fields obtained from a three-dimensional magnetohydrodynamic simulation of local magnetic reconnection. The magnetic reconnection is induced by imposing a localized anomalous resistivity in two different initial configurations, the first one without a B[sub y] component and the second with a positive B[sub y] superimposed in the two-dimensional Harris field lying in the xz plane. Applying the trajectory calculations to the geotail condition, the previously published results have been mostly recovered for the B[sub y]=0 case. For the nonzero B[sub y] case, remarkable asymmetry about the 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[sub 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 particles 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 this 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[sub y], i.e., the magnetic field component aligned with the X-line. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]