Relative contribution of ULF waves and whistler-mode chorus to the radiation belt variation during May 2017 storm

The purpose of the present study is to understand when, where, and how ultra-low frequency (ULF) waves and whistler-mode chorus contribute to the Earth's radiation belt dynamics. We first investigate the temporal contribution of both plasma waves to the relativistic electron flux enhancement during the 27 May 2017 storm. Both Arase (post-midnight) and Van Allen Probe (RBSP)-B (dusk) show the global enhancement of ULF waves during the early recovery phase, which corresponds to the global increase of relativistic electron fluxes. On the other hand, the whistler-mode chorus is enhanced mainly during the late recovery phase even at the RBSP-B location in the dusk sector, which is far from the ordinary location of wave generation. Relativistic electron fluxes significantly increase around L~4 during the late recovery phase. We also investigate the spatial contribution of waves using the Comprehensive Ring Current Model (CRCM) coupled with Block-Adaptive-Tree Solar-Wind Roe-Type Upwind Scheme (BATS-R-US) simulation. The simulation qualitatively reproduces the global evolution of externally-driven ULF waves. The estimated region where the anisotropy of thermal energy electrons (~20-60 keV) is large shifts toward dusk during the recovery phase. We also find the large magnetic field curvature at noon and dusk sectors during the recovery phase. Estimated spatial distributions of thermal electron anisotropy and magnetic field curvature give explanation of the observational result that enhanced whistler-mode chorus exists in the dusk sector.