Local Acceleration of Radiation Belt Electrons to Ultra-relativistic Energies

Observations show that electrons in the Van Allen radiation belts can have energies in excess of 7 MeV. Whether electrons at these ultra-relativistic energies are locally accelerated, arise from betatron and Fermi acceleration due to transport across the magnetic field, or if a combination of both mechanisms is required, has remained an unanswered question in radiation belt physics. We present a unique way of analyzing phase space density data which demonstrates that local acceleration is capable of heating electrons up to 7 MeV. Our results show signatures of local acceleration and the subsequent outward radial diffusion of ultra-relativistic electron populations. The Van Allen Probes mission not only provided unique measurements of ultra-relativistic radiation belt electrons, but also simultaneous observations of plasma waves that allowed for the routine inference of total plasma number density. Based on long term observations in 2015, we show that the underlying plasma density has a controlling effect over local acceleration to ultra-relativistic energies, which occurs only when the plasma number density drops down to very low values (~10 cm^-3). Such low density creates preferential conditions for local diffusive acceleration of electrons from 100s of keV up to >7 MeV. We complement these observations with a numerical model to show that the conditions of extreme cold plasma depletion result in acceleration up to >7 MeV.