NONGUIDING CENTER MOTION AND SUBSTORM EFFECTS IN THE MAGNETOTAIL

Thick and thin models of the middle magnetotail were developed using a consistent orbit tracing technique, It was found that currents carrie d near the equator by groups of ions with anisotropic distribution fun ctions are not well approximated by the guiding center expressions, Th e guiding center equations fail primarily because the calculated press ure tensor is not magnetic held aligned, The pressure tensor becomes f ield aligned as one moves away from the equator, but here there is a s mall region in which the guiding center equations remain inadequate be cause the two perpendicular components of the pressure tensor are uneq ual, The significance of nonguiding center motion to substorm processe s then was examined. One mechanism that may disrupt a thin cross-tail current sheet involves field changes that cause ions to begin followin g chaotic orbits, The lowest-altitude chaotic region, characterized by an adiabaticity parameter kappa approximate to 0.8, is especially imp ortant. The average cross-tail particle drift is slow, and we were una ble to generate a thin current sheet using such ions, Therefore any pr ocess that tends to create a thin current sheet in a region with kappa approaching 0.8 may cause the cross-tail current to get so low that i t becomes insufficient to support the lobes, A different limit may be important in resonant orbit regions of a thin current sheet because pa rticles reach a maximum cross-tail drift velocity, If the number of io ns per unit length decreases as the tail is stretched, this part of th e plasma sheet also may become unable to carry the cross-tail current needed to support the lobes. Thin sheets are needed for both resonant and chaotic orbit mechanisms because the distribution function must be highly structured, A description of current continuity is included to show how field aligned currents can evolve during the transition from a two-dimensional (2-D) to a 3-D configuration.