Minima in phase space density and how they relate to the multi-MeV electron radiation belt depletions

The Earth's radiation belts include electrons over a wide energy range. The dynamics of electrons can differ significantly, depending on the energy. In comparison to ~MeV energies, multi-MeV electrons are less predictable during geomagnetic storms [1], as their population can be depleted, enhanced, or remain unchanged, with nearly equal probability [2]. The depletion of electrons can be reversible (adiabatic) or irreversible, due to wave-particle interactions and loss at the outer boundary. Nonadiabatic changes can be identified by analyzing phase space density (PSD) as a function of the three adiabatic invariants. Fast-localized losses, such as interaction with electromagnetic ion cyclotron (EMIC) waves, can produce deepening PSD minima [3]. The EMIC waves are very effective in scattering multi-MeV electrons and can create sharp gradients in pitch angle distributions, although they do not resonate with nearly equatorial mirroring electrons. The depletion of electrons in a wide range of pitch angles occurs with assistance of the hiss and chorus waves [4]. However, the local minimum in PSD may be also observed due to outward radial diffusion with either subsequent refilling of the radiation belts or local acceleration. In this case, the formation of the minima will not result in continued deepening [5].