A new theoretical framework to model radial transport of energetic particles by ULF waves in the Earth's magnetosphere.

Radial diffusion in planetary radiation belts is a dominant transport mechanism resulting in the energisation and loss processes of charged particles by ultra-low frequency (ULF) fluctuations in the Pc4-Pc5 range. The theoretical framework upon which radial diffusion coefficients have been analytically derived in the past 60 years belongs to various types of quasi-linear theories. In quasi-linear theories, the evolution equation for the distribution function experiencing radial diffusion is only valid on slow timescales longer than the characteristic period of the ULF waves and the azimuthal drift period of the particles, ranging from tens of minutes to a few hours for electrons with energies between tens of keV to several hundreds of keV. Therefore, radiation belts’ dynamical processes occurring on fast timescales comparable to ULF wave periods or azimuthal drift periods, such as fast magnetopause losses localised in magnetic local time (MLT), cannot self-consistently be quantified in terms of radial diffusion models. In this communication, we present a new theoretical framework based on drift kinetic (Hazeltine, 1973) to distinguish between the fast and slow response of energetic electrons to ULF waves. We conclude our talk with two examples to demonstrate the benefits of the drift kinetics approach: 1) fast electron losses due to MLT localised compression of the magnetopause, and 2) non-diffusive acceleration associated with symmetric ULF fluctuation.