Plasmaspheric hiss waves generate a reversed energy spectrum of radiation belt electrons

Highly energetic electrons are trapped in the magnetic field of Earth's radiation belts. The physical mechanisms driving the dynamics of the Van Allen belts can be understood from the electron's energy spectrum, which is believed to be steeply falling with increasing energy. This view has been prevalent for the past 60 years since the energy spectra were first measured. Here, we report the observation of a reversed energy spectrum with abundant high-energy and fewer low-energy electrons spanning from hundreds of keV to around two MeV in electron energy in data collected with NASA's Van Allen Probes. We find that this spectrum dominates inside the plasmasphere-a dense cold plasma region co-rotating with the Earth. Using two-dimensional Fokker-Planck diffusion simulations with a time-dependent, data-driven model of hiss waves in the plasmasphere, we demonstrate that the formation of the reversed spectrum is explained by the scattering of hiss waves. The results have important implications for understanding the distributions of charged particles and wave-particle interactions in magnetized plasmas throughout the solar system and beyond.