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.

This work is a continuation of paper 1 (Young et al., 1981) and is devoted to the generation process of ion cyclotron waves (ICWs) and the acceleration of He+ ions up to suprathermal energies. Simultaneous measurements are used from the ion composition experiment (0 < E < 16 keV), the energetic particle experiment (24 < E < 3 300 keV), and the ULF wave experiment (0.2–10 Hz) on board the GEOS 1 and GEOS 2 spacecraft. General characteristics of the local time distribution of ICWs will be presented and compared with those of the thermal anisotropy of energetic protons and the He+ abundance. Further calculations of the convective growth rate are conducted by applying two different methods, both of which are based upon the measured proton fluxes. The generation conditions of the ICWs in the presence of He+ ions will be investigated and three possible explanations will be discussed: (1) enhanced convection growth rates, (2) lowering of the threshold for absolute instabilities, and (3) change of the ICWs ray path (laser‐like effect). Finally, it is shown that the flux of suprathermal He+ ions is modulated at the ICW frequency. Owing to nonlinear effects, part of the energy of the energetic protons is transfered via the ICWs to the He+ ions that are essentially accelerated in the direction perpendicular to the static magnetic field. Then in the otherwise collisionless plasma the friction between energetic anisotropic protons and thermal He+ ions is achieved through the ICWs.

Abstract. The Spatio Temporal Analysis of Field Fluctuations (STAFF) experiment is one of the five experiments, which constitute the Cluster Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, a waveform unit (up to either 10 Hz or 180 Hz) and a Spectrum Analyser (up to 4 kHz). The Spectrum Analyser combines the 3 magnetic components of the waves with the two electric components measured by the Electric Fields and Waves experiment (EFW) to calculate in real time the 5×5 Hermitian cross-spectral matrix at 27 frequencies distributed logarithmically in the frequency range 8 Hz to 4 kHz. The time resolution varies between 0.125 s and 4 s. The first results show the capabilities of the experiment, with examples in different regions of the magnetosphere-solar wind system that were encountered by Cluster at the beginning of its operational phase. First results obtained by the use of some of the tools that have been prepared specifically for the Cluster mission are described. The characterisation of the motion of the bow shock between successive crossings, using the reciprocal vector method, is given. The full characterisation of the waves analysed by the Spectrum Analyser, thanks to a dedicated program called PRASSADCO, is applied to some events; in particular a case of very confined electromagnetic waves in the vicinity of the equatorial region is presented and discussed.

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