DOES CHAOS ALTER THE ENSEMBLE-AVERAGED DRIFT EQUATIONS

Can the average drift of an ensemble of charged particles in Earth's p lasma sheet still be described by adiabatic theory even if many partic les in the ensemble execute non adiabatic motion? This is part of a br oader spectrum of questions which asks if chaotic microscopic processe s can be parameterized as macroscopic ones when ensemble averaged. Thr ough a comparison of numerical test particle simulations with adiabati c theory we show that the average particle drift speed of an appropria tely chosen ensemble of nonrelativistic particles, including those exe cuting chaotic and Speiser [1965, 1967] motion, is given correctly by the simple adiabatic guiding-center drift formula. We further show tha t the dispersion of particles about the mean drift speed tends to decr ease due to the presence of chaotic particle scattering. These conclus ions are demonstrated for tail-like magnetic fields B(x, z), including a non zero y component. The presence of an electric field does not ch ange the conclusions provided that E/B is small compared to the total velocity of an individual particle. Thus we have shown that a standard way of representing particle transport in the middle magnetosphere, n amely, the formalism describing average drift in a flux tube filled wi th an isotropic particle distribution, remains a useful theoretical de scription for the central plasma sheet, despite the presence of non ad iabatic particle motion.