Diffusion of Equatorial Particles in the Outer Radiation Zone

Expansions and contractions of the permanently compressed magnetosphere lead to the diffusion of equatorially trapped particles across drift shells. A general technique for obtaining the electric fields induced by these expansions and contractions is described and applied to the Mead geomagnetic field model. The resulting electric drifts are calculated and are superimposed upon the gradient drift executed by a particle that conserves its first (μ) and second (J = 0) adiabatic invariants. The noon-midnight asymmetry of the unperturbed drift trajectory (resulting from gradient drift alone) is approximated by means of a simple model. In this model the angular drift frequency is found to be the geometric mean of a particle\textquoterights angular drift velocities at noon and midnight. The radial diffusion coefficient D = (\textonehalf) (ΔL)\texttwosuperior/time is calculated as a function of the McIlwain parameter L and in terms of the spectral density of fluctuations in the stand-off distance of the magnetosphere boundary. Because the unperturbed drift trajectories are asymmetric, drift-resonant diffusion of particles is produced by spectral components at all harmonics of the drift frequency, although the first (fundamental) harmonic is the major contributor.