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

Abstract

In this study, we report in situ observations on the simultaneous periodic modulations in 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 storm recovery phase on 21 October 2001. The Cluster fleet was traveling outbound in the inner magnetosphere from the Southern to Northern Hemisphere on the morning sector (0900 MLT). The ultra‐low‐frequency (ULF) waves from the magnetic field and electric field measurements show a mixture of several dominant wave components in the transverse modes. The poloidal mode at the modulation frequency of 3.3 mHz appears to be a standing wave with an odd harmonic, although other wave components reveal propagating features. The radial extent of this standing wave is around 0.58 RE. The oscillation 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 poloidal standing wave, indicating that the energetic electrons and the thermal ion species are modulating by the same wave. Further, we suggest the simultaneous drift resonances of the energetic electrons around 94 keV and the bounce resonances of the thermal O+ around 4.5 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 energy range of 50∼110 keV, which are most likely due to the drift resonances. This is the first study to show both energetic particles (radiation belt population, ∼ a few hundred keV) and thermal ions (background plasma population, ∼ a few keV) can be affected by the same ULF wave simultaneously. Furthermore, this study implies that the superdense ionospheric origin O+ ions in the inner magnetosphere during storm times can modify the local field line eigenfrequency and result in the energetic electron accelerations by the ULF waves in the deep region of the radiation belt.