Abstract. On board the four Cluster spacecraft, the Cluster Ion Spectrometry (CIS) experiment measures the full, threedimensional ion distribution of the major magnetospheric ions (H + , He + , He ++ , and O + ) from the thermal energies to about 40 keV/e. The experiment consists of two different instruments: a COmposition and DIstribution Function analyser (CIS1/CODIF), giving the mass per charge composition with medium (22.5 • ) angular resolution, and a Hot Ion AnalCorrespondence to: H. Rème (Henri.Reme@cesr.fr) yser (CIS2/HIA), which does not offer mass resolution but has a better angular resolution (5.6 • ) that is adequate for ion beam and solar wind measurements. Each analyser has two different sensitivities in order to increase the dynamic range.

Abstract. We report observations of "fast solitary waves" that are ubiquitous in downward current regions of the mid-altitude auroral zone. The single-period structures have large amplitudes (up to 2.5 V/m), travel much faster than the ion acoustic speed, carry substantial potentials (up to ~100 Volts), and are associated with strong modulations of energetic electron fluxes. The amplitude and speed of the structures distinguishes them from ion-acoustic solitary waves or weak double layers. The electromagnetic signature appears to be that of an positive charge (electron hole) traveling anti-earthward. We present evidence that the structures are in or near regions of magnetic-field-aligned electric fields and propose that these nonlinear structures play a key role in supporting parallel electric fields in the downward current region of the auroral zone.

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Large fluxes of energetic heavy ions (M / q ≈ 16) were observed in the inner magnetosphere during the geomagnetic storm of December 17, 1971. The observations were made by a set of energetic‐ion mass spectrometers covering the energy range 0.7–12 kev on board the polar‐orbiting satellite 1971‐089A (800‐km altitude, ≈0300 LT). Significant heavy‐ion fluxes were observed for a period of approximately 48 hours during the main phase of the storm. The heavy‐ion fluxes frequently exceeded the proton fluxes in the 0.7‐ to 12‐kev energy range. The heavy‐ion spectrums were highly variable and frequently contained a peak of several kev. The heavy ions were observed over a wide latitudinal range (2.4 ≲ L ≲ 9) and generally extended to somewhat lower latitudes than the protons. The peak energy flux of these ions was approximately 0.4 erg/cm² sec ster, which is substantial in terms of expected observable ionospheric effects. They may also contribute significantly to the storm‐time magnetic‐field depression (Dst), since at the same flux they represent an energy density greater by a factor of 4 than that of the protons.

A large statistical survey of the 0.1‐ to 16‐keV/e plasma sheet ion composition has been carried out using data obtained by the Plasma Composition Experiment on ISEE 1 between 10 and 23 RE during 1978 and 1979. This survey includes more than 10 times the quantity of data used in earlier studies of the same topic and makes it possible to investigate in finer detail the relationship between the ion composition and the substorm activity. The larger data base also makes it possible for the first time to study the spatial distribution of the principal ion species. As found in previous studies, the ion composition has a large variance at any given value of the AE index, but a number of distinct trends emerge when the data are averaged at each activity level. During quiet conditions the plasma sheet is dominated by ions of solar origin (H+ and He++), as found in earlier studies, and these ions are most numerous during extended periods of very low activity (AE ≲ 30 γ). The quiet time density of these ions is particularly large in the flanks of the plasma sheet (GSM Y ∼ ± 10 RE), where it is about twice as large as it is near the central axis of the plasma sheet (Y = Z = 0). In contrast, the energy of these ions peaks near the central axis. When the AE index approaches zero for extended periods (several hours), the energy of the solar ions approach values that are similar to solar wind kinetic energies (∼1 keV/nucleon). Conversely, as the AE index increases, the solar ion energy increases. When a correction is made for the finite instrumental energy window, the data indicate that the solar H+ and He++, on the average, retain more nearly equal energy/nucleon than equal energy/charge. With increasing AE index the solar ion density decreases at all GSM Z, on the average, and the solar ions are partially replaced by ions of terrestrial origin. The most conspicuous of the terrestrial ions is the O+, which has an average energy of about 3–4 keV/ion at all activity levels. The increase in the O+ density is strongest around local midnight (GSM |Y| ≲ 5 RE), where the O+ often becomes the most numerous ion during strongly disturbed conditions (AE ∼ 1000 γ). At each level of substorm activity the average O+ density has a long‐term variability, increasing by a factor of 3 between early 1978 and early 1979, possibly in response to changing solar EUV radiation.