Subcritical Growth of Electron Phase-space Holes in Planetary Radiation Belts

The discovery of long-lived electrostatic coherent structures with large-amplitude electric fields (1≤slant E ≤slant 500 mV/m) by the Van Allen Probes has revealed alternative routes through which planetary radiation belts’ acceleration can take place. Following previous reports showing that small phase-space holes, with qφ /{T}_e^c≃ {10}^-2{--}{10}^-3, could result from electron interaction with large-amplitude whistlers, we demonstrate one possible mechanism through which holes can grow nonlinearly (I.e., γ \propto \sqrt{φ }) and subcritically as a result of momentum exchange between hot and cold electron populations. Our results provide an explanation for the common occurrence and fast growth of large-amplitude electron phase-space holes in the Earth’s radiation belts.