Cluster observations of simultaneous resonant interactions of ULF waves with energetic electrons and thermal ion species in the inner magnetosphere

摘要

In this study, we report in situ observations on the simultaneous periodic modulationsin the drifting energetic electrons (- 100 keV) and in the bouncing thermal ion species(O~+ at ~4.5 keV and H~+at ~280 eV) with the same frequency of 3.3 mHz during the stormrecovery phase on 21 October 2001. The Cluster fleet was traveling outbound in theinner magnetosphere from the Southern to Northern Hemisphere on the morning sector(0900 MLT). The ultra-low-frequency (ULF) waves from the magnetic field and electricfield measurements show a mixture of several dominant wave components in thetransverse modes. The poloidal mode at the modulation frequency of 3.3 mHz appears tobe a standing wave with an odd harmonic, although other wave components revealpropagating features. The radial extent of this standing wave is around 0.58 RE. Theoscillation periods of the energetic electron fluxes (~100 keV) and the thermal O~+(~4.5 keV) and H~+(~280 eV) fluxes are observed the same as the period of the poloidalstanding wave, indicating that the energetic electrons and the thermal ion species aremodulating by the same wave. Further, we suggest the simultaneous drift resonances ofthe energetic electrons around 94 keV and the bounce resonances of the thermal O~+around4.5 keV and H~+around 280 eV with the same poloidal standing wave. In addition,the electron energy spectra variations reveal the accelerations of the electrons in the energyrange of 50-110 keV, which are most likely due to the drift resonances. This is thefirst study to show both energetic particles (radiation belt population, ~ a few hundredkeV) and thermal ions (background plasma population, ~ a few keV) can be affected bythe same ULF wave simultaneously. Furthermore, this study implies that the superdenseionospheric origin O~+ ions in the inner magnetosphere during storm times can modifythe local field line eigenfrequency and result in the energetic electron accelerations bythe ULF waves in the deep region of the radiation belt.