Neoclassical and anomalous radial diffusion of trapped electrons in the inner belt

Abstract:

The combination of pitch-angle scattering and drift-shell splitting can cause trapped particles to diffuse in radial position due to the product of the pitch-angle diffusion coefficient and the square of the derivative of the Roederer L* with respect to equatorial pitch-angle. When the pitch-angle scattering is caused by electromagnetic waves generated by plasma turbulence we call this type of diffusion ‘anomalous’ and when the pitch-angle scattering is caused by Coulomb collisions we call the diffusion ‘neoclassical’ in accordance with the terms used in the fusion community. O’Brien has studied anomalous diffusion due to pitch-angle scattering from chorus waves at geosynchronous orbit (L=6.62) and found that it can be larger than radial diffusion due to drift-resonant interactions for 100 keV electrons during times of high geomagnetic activity. Cunningham et al studied neoclassical diffusion at very low L-shells (1.15-1.21) and found that it explains observations in the 1960’s that the lifetime of relativistic electrons produced by the Starfish high-altitude nuclear explosion at these L-shells and at low altitude (<300 km) is 10-100x longer than expected from Coulomb collisions alone when computed over a 3-year time window. In Cunningham et al’s study the neoclassical diffusion acts as a source of electrons from larger L that increases the apparent lifetimes at lower L by a factor of 10-100. Finally, Selesnick found that measurements made aboard DEMETER (700 km altitude) of 300 keV electrons in the inner belt (1.2<L<1.6) also have longer apparent lifetimes than expected from Coulomb collisions. Selesnick computed the radial diffusion coefficient needed so that electrons from larger L would be brought to lower L in sufficient quantity to increase the apparent lifetimes to those observed, but the explanation for what causes the radial diffusion is still in doubt. Here we report on recent work to investigate whether the radial diffusion needed by Selesnick to reproduce the DEMETER observations might be due to the combined effects of anomalous and neoclassical diffusion, i.e. the combination of drift-shell splitting in the IGRF magnetic field and pitch-angle scattering caused by Coulomb collisions and very-low frequency waves.