A case study of EMIC wave-associated He+ energization in the inner magnetosphere

On 30 March 2002, the COmposition and DIstribution Function (CODIF) Analyzer on board the Cluster spacecraft observed a He+ energization event from 0755 to 0823 UT. In the event, He+ ions were energized up to 0.4 keV and the heating was perpendicular to magnetic field. We perform a case study of the event by using in situ plasma and magnetic field measurements from the Cluster CIS-CODIF, WHISPER, EFW, and FGM instruments and ground-based wave observations. During the event, the four Cluster spacecraft, passing through perigee, were located at L = ~4.4 and MLT = ~22.3 near the equatorial plane. Cluster sampled two types of ion populations: 1) cold, dense ions originating from the plasmasphere and 2) hot, anisotropic ions (mostly H+) from the ring current. Significant electromagnetic ion cyclotron (EMIC) wave activity (PTot ≥ 1.0 nT2/Hz) was found near fHe+, eq (= ~1.0 Hz) in the wavelet transform spectrograms of the high-resolution FGM magnetic field measurements. The He+ energization event was thus caused through resonant interactions between cold He+ ions and the EMIC wave, which had obtained free energies from hot (2 - 40 keV) H+ with temperature anisotropy as high as 2.