Highly Relativistic Electrons: Van Allen Probes Observations of the Earth's Radiation Belts

The dual-spacecraft Van Allen Probes mission has already provided a new window into megaelectron Volt (MeV) particle dynamics in the Earth’s radiation belts. Observations (up to E _˜10 MeV) show clearly the behavior of the outer electron radiation belt at different time scales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. Analysis of multi-MeV electron flux and phase space density (PSD) changes are presented in the context of the first two years of Van Allen Probes operation. The Relativistic Electron-Proton Telescope (REPT) designed and built at the University of Colorado demonstrates through new data the classic signatures both of inward radial diffusive energization as well as abrupt localized acceleration deep within the outer Van Allen zone. The remarkable results reveal graphically that different mechanisms of multi-MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession. This also shows in graphic ways how the coldest plasmas in the magnetosphere intimately control the most energetic particles.