Van Allen radiation belts are typically two zones of energetic particles encircling the Earth separated by the slot region. How the outer radiation belt electrons are accelerated to relativistic energies remains an unanswered question. Recent studies have presented compelling evidence for the local acceleration by very-low-frequency (VLF) chorus waves. However, there has been a competing theory to the local acceleration, radial diffusion by ultra-low-frequency (ULF) waves, whose importance has not yet been determined definitively. Here we report a unique radiation belt event with intense ULF waves but no detectable VLF chorus waves. Our results demonstrate that the ULF waves moved the inner edge of the outer radiation belt earthward 0.3 Earth radii and enhanced the relativistic electron fluxes by up to one order of magnitude near the slot region within about 10 h, providing strong evidence for the radial diffusion of radiation belt relativistic electrons.
Bibliographical noteFunding Information:
This work was supported by the National Natural Science Foundation of China grants 41422405, 41274169 and 41274174, the Chinese Academy of Sciences grants KZCX2-EW-QN510 and KZZD-EW-01-4, the National Key Basic Research Special Foundation of China grant 2011CB811403, the Fundamental Research Funds for the Central Universities WK2080000077. We acknowledge the University of Iowa as the source for the EMFISIS data in this study (this acknowledgement does not imply endorsement of the publication by the University of Iowa or its researchers), acknowledge J.H. King, N. Papatashvilli and CDAWeb for providing interplanetary parameters and magnetospheric indices, and acknowledge C. Torrence and G. Compo for providing the wavelet analysis software.