Collisionless Phase Mixing Mimics Diffusive Transport in Radiation Belt Observations

Original Abstract

Since the dawn of the space age, observations of energetic particles in planetary radiation belts have been interpreted within a diffusive transport framework, even though the processes that populate and deplete these belts produce highly structured and spatially localized distributions. This exposes a fundamental problem: how can coherent phase-space structures evolving under collisionless dynamics give rise to observational signatures that appear consistent with diffusion-based transport? Here we show that diffusion-like behaviour can arise from an observational phase-mixing effect, independent of stochastic wave-particle transport. As spacecraft sample neighbouring drift shells while particles undergo electromagnetic drifts, spatially localized drift-phase structures are converted into rapidly decorrelating temporal signals, making them observationally indistinguishable from stochastic processes. We show that the effective lifetime of these structures is only a few drift periods, preventing the resolution of fine-scale structure. These results demonstrate that collisionless dynamics can mimic diffusive transport on short timescales, limiting the inference of particle acceleration processes and biasing transport estimates. This calls for a reassessment of diffusion-based interpretations of radiation belts at Earth, across the solar system, and in the radiation belts of ultra-cool brown dwarfs.