Abstract
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.
Highlights
Van Allen radiation belts are typically two zones of energetic particles encircling the Earth separated by the slot region
By analysing the high-resolution data and performing the detailed simulation, we demonstrate that the radial diffusion by intense ULF waves was responsible for the radiation belt relativistic electron evolution in this event
The radiation belt event of interest occurred on 15 February 2014, with the required particle/field data collected by the Relativistic Electron–Proton Telescope (REPT)[23] and the Magnetic Electron Ion Spectrometer (MagEIS)[24] of the Energetic particle, Composition and Thermal plasma (ECT) instrument suite[25], the Electric Fields and Waves (EFW) instruments[26] and the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument suite[27] on board the Van Allen Probes
Summary
Van Allen radiation belts are typically two zones of energetic particles encircling the Earth separated by the slot region. The electrons in drift resonance with broadband ULF waves move stochastically along the radial direction, which is described by the radial diffusion theory[5,7,8] This theory was indirectly supported by the strong correlation between ULF wave power and radiation belt electron flux enhancement[14,15,16,17,18], but due to the limitations in previous particle/field observations, the in situ wave–particle interaction characteristics were lacking to clarify the associated physical process. Previous works[7,14,15,16,19,20] often concentrated on the radiation belt reformation events during the geomagnetic storms These geomagnetic storms did cause marked dynamics of energetic electrons, but the strongly disturbed magnetosphere very likely allowed the concurrence[20,21,22] of ULF wave-driven radial diffusion and VLF chorus wave-driven local acceleration. By analysing the high-resolution data and performing the detailed simulation, we demonstrate that the radial diffusion by intense ULF waves was responsible for the radiation belt relativistic electron evolution in this event
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