Measurements from the Van Allen Probes mission clearly demonstrated that the radiation belts cannot be considered as a bulk population above approximately electron rest mass. Ultra-relativistic electrons (~>4 MeV) form a new population that shows a very different morphology (e.g. very narrow remnant belts) and sporadic acceleration and loss of electrons.
Novel analysis of phase space densities at multiple energies allows to differentiate between various acceleration mechanisms at ultra-relativistic energies. This method allows us to trace how particles are being accelerated at different energies and shows how long it takes for acceleration to reach a particular energy. This method clearly demonstrates the importance of local acceleration and also demonstrates the importance of the outward radial diffusion in transporting electrons to GEO. Acceleration to such high energies occurs only when cold plasma in the trough region is extremely depleted, down to the values typical for the plasma sheet. VERB-2D simulations are then used to explain these observations. There is also a clear difference between the loss mechanisms at MeV and multi-MeV energies. There is also a clear difference between the loss mechanisms at MeV and multi-MeV energies due to EMIC waves, that can very efficiently scatter ultra-relativistic electrons but leave MeV electrons unaffected.