This animation shows the surface-of-section (phase space structure) plot for plasma particles in the tail current sheet, as the energy H is varied. The yellow region represents chaotic orbits, the green and blue regions are transient orbits, and the red region represents trapped orbits. The changes in the transient regions lead to the well-known energy resonances. If you concentrate on the trapped (red) regions, you can see that they get a very complex structure for some energies. We are currently exploring this effect to see how it might be related to the resonances and to see how it might affect the real system. The Existence of trapped populations has been shown to be necessary for some tail equilibria [e.g. Sitnov, et al, 2000], so understanding the fundamental dynamics will be useful in determining the conditions for equilibria.