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 • 1 l0 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. lation termed 'energetic,protons.(The discovery that the magnetosphere contains large numbers of heavy ions with energies of tens of keV, makes the traditional identification of all energetic positively charged particles as 'protons' doubtful to say the least. Unfortunately the solid state detectors used for energetic particle measurements on GEOS do not provide ion mass identification (section 3b). We are able, however, to infer from the Ion Composition Experiment that the heavy ion contribution up to •20 keV is <•10% for the events presented in this study. In any case, heavy ions have a lower velocity than do protons for the same mean energy and thus would contribute proportionately less to the instability. Also the fact that L mode waves are observed at f > file+ indicates the wave energy source cannot be primarily energetic He + or 2 Centre de Recherches en Physique de l'Environnement Terrestre O +. Thus we will continue to make use of the assumption that et Planitaire/Centre National d'Etudes des Tiltcommunications, all energetic ions are 'protons' throughout this and paper 2. 92131 Iss¾-les-Moulineaux, Frange. Our purpose in this paper is to report on measurements 3 Max-Planck-Institut far Aeronomie, 3411 Katlenburg-Lindau 3, made with the ESA/GEOS 1 and 2 spacecraft which give an Federal Republic of Germany.

A detailed analysis of the ULF noise observed on the GEeS magnetic antennas in the frequency range -0.2-12 Hz has revealed the properties of structured emissions occurring just above the proton gyrofrequency whose existence was reported by Russell et al. (1970) and Gurnett (1976). These waves are observed in the vicinity of the geomagnetic equator at all L values between -4 and -8. They propagate in a direction perpendicular to the dc magnetic field. The waves consist of harmonically related monochromatic emissions. The fundamental frequency is generally of the order of the local proton gyrofrequency. Sometimes the fundamental and first harmonics are missing. If there is more than one fundamental frequency present, nonlinear coupling often occurs between the different emissions. The amplitudes of individual events vary from some tens of milligammas to some hundreds. Their duration ranges from some tens of minutes to some hours. Within the range of sensitivity of the detectors (10 -2 3/Hz -1/2 at 1 Hz, 10 -3 3/Hz -1/2 at 8 Hz) the average probability of emission occurrence during a given hour is 12%, this number increases to ---30% during the afternoon and in the premidnight sectors. Simultaneous observations of proton fluxes, as obtained from the two GEeS particle experiments show that these waves are often associated with distribution functions peaking at some energy (5 •< E •< 30 keV) for 90 ø pitch angle particles. This ring-like distribution provides the energy source for the excitation of non-resonant (k•} = 0) instabilities near nF.+ (n = running number). A theoretical model is presented that qualitatively explains the main characteristics of the observed waves.

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