PARTICLE DRIFT IN THE EARTHS PLASMA SHEET

We generalize the derivation of the average gradient/curvature-drift f or a flux tube filled with an isotropic distribution of particles at s pecified kinetic energy. The present treatment is restricted to a two- dimensional magnetic field with zero electric field, but it includes a ll chaotic and Speiser orbits, which do not correspond to the simple p icture of gradient/curvature drift. We assume that particles are evenl y distributed throughout the regions of phase space allowed by their e nergy and canonical momentum. This assumption is closely related but n ot exactly equivalent to the assumption of isotropic pitch-angle distr ibution. Our derivation assumes that the maximum Larmor radius is smal l compared to the scale length for equatorial variations in the flux t ube volume, but it does not involve any restrictions on the curvature of the field line. The resulting expression for the drift rate is vali d for situations where the particle drift velocity is comparable to th e thermal speed in some regions. The apparent implication of this gene ralized treatment is that the existence of very complex non-adiabatic particle trajectories in the plasma sheet may not invalidate previous estimates of the average rate of particle drift out the sides of the t ail, estimates that were made under the assumption of simple guiding-c enter drifts.