Nonguiding center motion and substorm effects in the magnetotail

Abstract

Thick and thin models of the middle magnetotail were developed using a consistent orbit tracing technique. It was found that currents carried near the equator by groups of ions with anisotropic distribution functions are not well approximated by the guiding center expressions. The guiding center equations fail primarily because the calculated pressure tensor is not magnetic field aligned. The pressure tensor becomes field aligned as one moves away from the equator, but here there is a small region in which the guiding center equations remain inadequate because the two perpendicular components of the pressure tensor are unequal. The significance of nonguiding center motion to substorm processes then was examined. One mechanism that may disrupt a thin cross‐tail current sheet involves field changes that cause ions to begin following chaotic orbits. The lowest‐altitude chaotic region, characterized by an adiabaticity parameter κ ≈ 0.8, is especially important. The average cross‐tail particle drift is slow, and we were unable to generate a thin current sheet using such ions. Therefore any process that tends to create a thin current sheet in a region with κ approaching 0.8 may cause the cross‐tail current to get so low that it becomes insufficient to support the lobes. A different limit may be important in resonant orbit regions of a thin current sheet because particles reach a maximum cross‐tail drift velocity. If the number of ions per unit length decreases as the tail is stretched, this part of the 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.