Breaking all the invariants: Anomalous electron radiation belt diffusion by pitch angle scattering in the presence of split magnetic drift shells

Relativistic electron observations near geostationary orbit routinely show pitch angle distributions peaked away from 90 degrees. These butterfly distributions are consistent with magnetic drift shell splitting combined with a radial flux gradient. During magnetic storms, nature adds pitch angle scattering to split drift shells, breaking all three adiabatic invariants of the particle's motion. Therefore, some degree of anomalous radial diffusion is likely, and cross terms between the gyration and drift invariants and between the bounce and drift invariants arise. Using typical assumptions about the pitch angle scattering and the magnetic field topology, we calculate these anomalous diffusion coefficients near geostationary orbit. We show that the anomalous radial diffusion can exceed that due to more traditional drift-resonant wave-particle interactions. We also show that the neglected cross terms, particularly the bounce-drift cross term, can be significant. These results suggest necessary additions to some global electron radiation belt simulations. Key Points <list list-type="bulleted" id="grl51286-list-0001"> <list-item id="grl51286-li-0001">Pitch angle scattering breaks all 3 adiabatic invariants on split drift shells <list-item id="grl51286-li-0002">Resulting anomalous radial diffusion is significant compared to drift resonance <list-item id="grl51286-li-0003">Anomalous diffusion adds cross terms to the typical Fokker-Planck formulation