Cluster Observations of Energetic O+ Ions Correlated with Ulf Waves in the Night-Side High-Latitude Magnetosheath during a Storm Initial Phase

Measurements from Cluster spacecraft C4 are used to investigate the energization of singly charged oxygen ions, O⁺, by Ultra-Low Frequency (ULF) waves in the night-side high-latitude magnetosheath during the initial phase of an intense storm on 24 October 2011. The number density of O⁺ ions was ~0.1 cm^-3 detected by Cluster C4 within the night-side high-latitude magnetosheath. The observed energetic O⁺ ion flux was periodic in the ULF wave Pc 5 range, ~ 3 min. The fluctuations of O⁺ number density, dN_O+ were closely correlated with the pulsations of magnetic field, dB_x. The phase difference between the dN_O+ and dB_x is ~ 180⁰ or ~ 90⁰, which corresponding to the O⁺ pitch angle distribution mainly in the direction parallel or perpendicular to the magnetic field, respectively. These results indicate that the O⁺ ions can be energized by Pc 5 waves both in the parallel and perpendicular to the magnetic field in the night-side high-latitude magnetosheath during the storm initial phase. Our observation investigation presents a new source of the abundant energetic O⁺ in the plasma sheet during the storm main phase, which is from the night-side high-latitude magnetosheath throughout the magnetopause/mantle/lobe region. These energetic ions are likely one of the major sources of O⁺ filling into the near-Earth plasma sheet and ring current during the storm main phase.