Unusual Stable Trapping of the Ultra-Relativistic Electrons in the Van Allen Radiation Belts

Recent observations by NASA's Van Allen Probes showed an event, where three radiation zones were observed at ultra-relativistic energies. The additional middle, and unusually narrow, belt (long-lived ring) persisted for approximately 4 weeks, and resulted from a combination of electron losses to the interplanetary medium and scattering into the Earth's atmosphere by Electromagnetic Ion Cyclotron(EMIC) waves. We present the detailed comparison of the 3D VERB simulations with Van Allen Probes observations. It is shown in the current study that ultra-relativistic electrons are not directly accelerated or scattered by the VLF waves and can stay trapped in the outer zone for a very long time due to the lack of scattering into the atmosphere. The lack of scattering is explained as a result of ultra-relativistic particles being too energetic to resonantly interact with waves at low latitudes. This study shows that a completely different set of physical processes determines the evolution of ultra-relativistic electron radiation belts above ~2 MeV as resonant scattering by VLF waves becomes inefficient, while EMIC waves become most efficient, which explains the formation of the long-lived narrow structures observed in the outer belt by the Van Allen Probes.