RESONANCE-GENERATED PARTICLE DISTRIBUTIONS IN MAGNETOTAIL X-LINE CONFIGURATIONS

We present the results of a series of test particle simulations that m odel particle acceleration both in self-consistent magnetospheric fiel ds obtained by magnetohydrodynamic (MHD) simulations and in simple x l ine fields. In particular, we investigate the physical mechanisms resp onsible for the formation of beam-like features (''beamlets'') in the accelerated outgoing particle distributions reported by previous autho rs. We find that in our simulations these effects arise from the x lin e, where the field gradients are strong; despite this finding they can be readily explained by using a simple analysis based on the phase sp ace partitioning and resonances in one-dimensional current sheets, con sistent with the conclusions of previous authors. We confirm the expec ted scaling using both two-and one-dimensional models of the x line wi th 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 energize d particle population, while use of current sheets with a self-consist ent width blurs the features until they are no longer easily discernib le. This finding calls into question the degree to which the beamlet s tructures (those that depend on magnetic field gradients along the tai l with a uniform electric field) can be observed in physical systems.