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.

The detailed analysis of an isolated dispersionless substorm is performed on the basis of field and particle data collected in situ by the geostationary satellite GEOS 2 and of data from ground-based instruments installed close to the GEOS 2 magnetic footprint. These data give evidence for (1) quasi-periodic variations of the magnetic field configuration, which is alternatively taillike and dipolelike, (2) in-phase oscillations of the flux of energetic electrons, which is high when the configuration is dipolelike and vice versa, (3) a gradient in the flux of energetic ions, which is, on the average, earthward but undergoes large fluctuations around this average direction, and (4) large transient fluctuations of the quasi-dc electric field, which reverses its direction from eastward to westward. It is shown that these results are consistent with the development of an instability which leads to a westward propagating "wave." The source of the instability is the differential drift of energetic electrons and ions in a highly stressed magnetic field configuration (in a high /3 plasma). Evidence is given for a system of localized field-aligned currents flowing alternately earthward and equatorward at the leading and trailing edges of the westward propagating wave. This current system resulting from the temporal development of the instability produces the so-called Pi 2 pulsations, at the ionospheric level. The closure of this current system in the equatorial region leads to a current antiparallel to the tail current, and therefore to its reduction or cancellation. This reduction/ cancellation of the tail current restores the dipole magnetic field (dipolarization) and generates a large westward directed induced electric field (injection). Hence, dipolarization and injection are the consequences of the instability. Finally, it is suggested that the westward traveling surge observed simultaneously by all-sky cameras, close to the magnetic field of GEOS 2, is the image of the instability in the equatorial region transmitted to the upper atmosphere by precipitating electrons. 12 18 06 AFTER EXPANSION 12 Fig. 1. Schematics adapted from Akasofu [1977]. (a) The auroral arcs as they appear at breakup, at the northern boundary of the diffuse auroras, at a magnetic latitude typically between 65 ø and 70 ø. The shaded areas indicate the regions where diffuse auroras are observed. (b) After breakup, the discrete auroral forms expand to the west, to the east, and to the north. At the end of the expansion phase, the discrete arcs cover a region which typically extends from 650-70 ø to 750-80 ø magnetic latitude. diffuse reverse current has been identified west and more clearly east of the surge [e.g., Kozelova and Lyatskiy, 1984]. According to Baumjohann et al. [1981], the ratio between the height-integrated Hall and Pedersen conductivities increases in the westward traveling surge region, due to precipitation of relatively high energy electrons (10-20 keV). This kind of precipitation was directly observed, for example, by Kremser et...

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.

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.