Analytical and Observational Study of MLT-Localised Energetic Particle Injections in the Radiation Belts

Rapid injections of energetic and relativistic particles into the Earth’s radiation belts have been observed to produce multi-energy enhancements on timescales of tens of minutes, with effects that can persist for hours. Motivated by recent observations demonstrating that such injections occur nearly simultaneously across a broad energy range and exhibit sharp, step-like signatures (e.g. Kim et al., GRL, 48, 2021), we investigate the subsequent evolution of magnetically localised particle populations. We first develop an analytical description of the ballistic evolution of MLT-localised injections confined to a given L-shell, using drift-kinetic theory to quantify curvature and gradient drift trajectories as functions of pitch angle, first adiabatic invariant, energy, and time. This framework provides explicit predictions for the azimuthal phase mixing and phase-space evolution expected under purely adiabatic transport. We then examine energetic particle injections using GPS observations, cross-calibrated with Van Allen Probes measurements, to track their global evolution and assess deviations from ballistic drift behaviour. Together, the combined theoretical and observational approach constrains the extent to which rapid radiation belt enhancements can be explained by adiabatic drift physics alone and identifies signatures of non-ballistic transport processes.