Wave‐particle interactions near Ω_He+ observed on GEOS 1 and 2 1. Propagation of ion cyclotron waves in He⁺‐rich plasma

The GEOS 1 and 2 spacecraft contain a set of particle and wave detectors which allow for a very comprehensive study of wave‐particle interactions occurring within the equatorial region of the magnetosphere. This paper is devoted to interactions involving protons in the energy range 20 keV to 300 keV and ULF waves with frequencies below the proton gyrofrequency. It is shown that most of the ion cyclotron waves (ICW's) detected in this frequency range have spectra whose characteristic frequencies are organized in the vicinity of the He + gyrofrequency. Simultaneous measurements of the ion composition in the thermal energy range ( E ≲110 eV) show these waves to be clearly associated with the abundance of cold He + as well as the anisotropy of ions above 20 keV. The general characteristics of these helium‐associated ULF events are presented in case studies of four events. The interpretation of this phenomenon is given in the present paper in terms of the propagation of ICW's in a He + ‐rich plasma. It is shown that the shape of the cold plasma dispersion curve (for both parallel and non‐parallel propagation) can adequately explain the main characteristics of the observed waves (frequency spectrum, polarization) as well as the differences between observations made onboard GEOS 1 and GEOS 2. The generation conditions of ion cyclotron waves in such a multi‐component plasma, as well as their quasi‐linear effects on both the cold He + ions and the hot protons, are discussed in a companion paper.