Derivation of Pitch-Angle-Dependent Radial Diffusion Coefficients of off-Equatorial Relativistic Electrons in the Radiation Belts

Radial diffusion of energetic electrons by Ultra-low frequency (ULF) waves is a key mechanism for the acceleration and radial transport of hundreds-keV to few-MeV electrons in the radiation belts, via their drift-resonant interactions. Until recently, estimates of the radial diffusion rates have focused on the equatorial plane and have been derived for equatorially mirroring electrons. Recent statistical in situ observations based on THEMIS, Arase and Cluster have shown that the wave power of broadband magnetic and electric field ULF fluctuations is significantly enhanced away from the magnetic equator. Using 3D particle tracing under broadband ULF waves that are guided by these observations, we show that there is a significant dependence of the radial transport of relativistic electrons on their pitch angle, and that the diffusion coefficients of off-equatorial electrons can be up to an order of magnitude higher than that of equatorially-mirroring electrons. These findings point to the need for incorporating new radial diffusion coefficients in global radiation belt models that are pitch angle-dependent, together with magnetic latitude-dependent ULF wave power.