A case study of EMIC wave-associated He plus energization in the outer magnetosphere: Cluster and Double Star 1 observations

On December 4, 2004, the COmposition and DIstribution Function (CODIF) Analyzer on board the Cluster spacecraft observed a prolonged He+ energization event from 06:00 to 10:30 UT. In this paper, we perform a case study of the event by using in situ plasma and magnetic field measurements from the Cluster and Double Star Program (DSP) Tan Ce 1 (TC1) spacecraft. In the event, the He+ ions were energized up to 1 keV. The He+ ion heating was associated with three consecutive EMIC waves, which occurred near the dusk-side magnetospheric flank, L = 13.1-14.5, a region where EMIC wave activity has not been reported before. As a result, the observed wave frequencies, as low as 0.03 Hz, are far lower than those of typical EMIC waves, i.e., 0.1-5 Hz. It is found that the first wave had already propagated for some time and exhibited a large spatial extent, while the latter two were newly generated at the expense of anisotropic, energetic (> 1 keV) protons and had sharper spatial boundaries. Unlike the patchy wave activity, the He+ energization region displayed a continuous spatial distribution and corresponded to a region of enhanced cold ion density. The reason for the presence of the anisotropic protons and cold density in this region is most likely the quiet geomagnetic conditions, which allow the proton anisotropy to develop and the plasmaspheric plume to expand into such high L-values. A discrepancy, found in the comparison of particle and wave observations with linear theory, suggests that the theory should include He+ and/or nonlinear effects.