We present the results of a series of test particle simulations that model particle acceleration both in self-consistent magnetospheric fields obtained by magnetohydrodynamic (MHD) simulations and in simple x line fields. In particular, we investigate the physical mechanisms responsible for the formation of beam-like features (beamlets) in the accelerated outgoing particle distributions reported by previous authors. We find that in our simulations these effects arise from the x line, where the field gradients are strong; despite this finding they can be readily explained by using a simple analysis based on the phase space partitioning and resonances in one-dimensional current sheets, consistent with the conclusions of previous authors. We confirm the expected scaling using both two- and one-dimensional models of the x line with imposed uniform electric field. However, we find that the formation of these features is critically dependent on both the field geometry and the electric field imposed at the x line. In particular, use of an MHD-generated nonuniform electric field model eliminates the energized particle population, while use of current sheets with a self-consistent width blurs the features until they are no longer easily discernible. This finding calls into question the degree to which the beamlet structures (those that depend on magnetic field gradients along the tail with a uniform electric field) can be observed in physical systems. ¿ 1998 American Geophysical Union