Pc4‐5 Poloidal ULF Wave Observed in the Dawnside Plasmaspheric Plume

摘要

A localized Pc4‐5 ultralow‐frequency (ULF) wave event associated with a plasmaspheric plume was observed by THEMIS‐E on the dawnside near L = 6, which was identified as a second harmonic poloidal wave. The plume was identified as a sudden density enhancement during an outbound pass. The charged particle populations in the plume have a variety of periodic modulation characteristics at different energies. First, there is an antiphase relationship between magnetic field Br and particle flux across a wide energy range both for ions and electrons (~50 keV to 1 MeV). Second, there is a 180° phase shift in the modulated ion flux within an energy range of ~2-6 keV, with negative slope dispersions of ion pitch angle distributions at ~2-6 keV and ~50-75 keV, which are characteristic of drift‐bounce resonances. Third, the lower‐energy (<32 eV) ion flux is modulated at double the wave frequency, which are the result of E × B effect. Considering the generation mechanism of this poloidal mode wave within the plume, we find that it is likely generated by drift‐bounce resonance from an unstable population of ions, due to an inward radial phase space density gradient. We suggest that the localization of waves to the plume is because the high plasma density reduces the local poloidal mode eigenfrequency, enabling a match to the drift‐bounce frequencies of these ions, and resonant energy transfer from these particles to the eigenfunction at this location. This generates the Pc4‐5 second harmonic poloidal waves at a much lower L region than would otherwise be expected.