Coherent and Stochastic Transport of Trapped Energetic Particles in Earth's Radiation Belts

Radial transport of trapped energetic particles in the inner magnetosphere is a fundamental process affecting the energy spectrum and morphology of the radiation belts. Transporting particles to regions of stronger magnetic field while conserving the particles' first adiabatic invariant, M, will increase the energy of the particles undergoing transport. Conversely, transport to regions of lower magnetic field strength will reduce the energy of the particle population undergoing transport, and, if transported outward to drift shells intersecting the magnetopause, will cause particles to be lost from the system. Broadly, radial transport may be categorized as either stochastic (diffusive), comprising a random walk of individual particles in the radial coordinate L*; or coherent (advective), whereby a population of particles is moved in aggregate to higher or lower drift shells.

In this work we examine diffusive and advective transport of radiation belt electrons by combining global MHD/test particle simulations with efficient Fokker-Planck modeling methods. In particular, we examine the characteristics of injection fronts that contribute to advective transport of electrons, including the disturbance amplitude, propagation speed, and morphology. We address the differences that arise when translating radial motion from the momentum and configuration space easily obtained from MHD/test particle methods to the (M, K, L) invariant space intrinsic to the Fokker-Planck formalism. An ultimate product of this research will be an understanding that allows us to generate empirical advection and diffusion coefficients that can be applied in a computationally-efficient Fokker-Planck framework.