Enhanced Radial Diffusion of Outer Zone Electrons in an Asymmetric Geomagnetic Field

Temporal variations in the electric and magnetic fields guiding the drift of radiation belt particles can lead to a radial diffusion of particles across field lines. This occurs through violation of the third adiabatic invariant when the characteristic time scales are commensurate with the particle drift period. Analyses by Fälthammar[JGR 70, 2503, 1965] and others, examining the effect of time varying electric potential fields acting on particles drifting in a dipole magnetic field, suggested that radial diffusion in such cases would go as D_LL ~ L⁶ ∑ {P_m}, where P_m is the power spectral density of electric field variations at the mth harmonic of the particle drift frequency. This diffusion is essentially a result of a resonant interaction between particles drifting in the dipole field and the global electric fields perturbing their drift, the resonance condition satisfying ω -mω _d=0. However, in the outer zone radiation belts the magnetic field is often markedly distorted from a purely dipolar geometry by the combined action of the solar wind and various internal magnetospheric current systems. In this work we examine radial diffusion in an asymmetric, rather than dipolar, magnetic field geometry, and find that new modes of radial diffusion (corresponding to new drift resonance conditions) are induced by the asymmetry. We examine the character of these new modes of diffusion, and the combined effect of symmetric and asymmetric resonances on diffusion rates expected in the outer radiation belts.