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.

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.

The plasma sheet boundary layer is a temporally variable transition region located between the magnetotail lobes and the central plasma sheet. We have made a survey of these regions by using particle spectra and three‐dimensional velocity‐space distributions sampled by the ISEE 1 LEPEDEA. Ion composition measurements obtained by the Lockheed ion mass spectrometers indicate that ionospheric ions play a crucial role in magnetotail dynamics. Eleven crossings from the lobes to the central plasma sheet taken at various local times and levels of geomagnetic activity are analyzed in detail. The average ratios of He+/H+, He++/H+, and O+/H+ are not significantly different between the plasma sheet boundary layer and central plasma sheet. Densities and temperatures intermediate between the central plasma sheet and lobes are observed in the plasma sheet boundary layer although bulk flow speeds there are typically enhanced. Counter‐streaming ion beams are often observed in the plasma sheet boundary layer at energies of ∼1 keV/q to >45 keV/q. Intense antisunward‐flowing beams of ionospheric origin at E/q of <1 kV are often seen in the tail lobes, the plasma sheet boundary layer, and, infrequently, in the central plasma sheet. Such beams are not commonly observed in the central plasma sheet, which is characterized by hotter and more isotropic ion and electron distributions. Our samples of ion distributions in the plasma sheet boundary layer frequently show an evolution of distribution functions from highly anisotropic single beams or counter‐streaming beams toward the more isotropic distributions typical of the hot component of the central plasma sheet. Provided that the acceleration process for these beams can be identified, we can then account for the transport and injection of hot plasma into the central plasma sheet. We conclude that the plasma sheet boundary layer is a primary transport region of the magnetotail.

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Data obtained in the near‐equatorial magnetosphere, between L = 3 and R = 23 RE, by the plasma composition experiment on ISEE‐1 are examined for possible effects of varying solar activity, as measured by the daily F10.7 index. The data consist of velocity moments for H+, He++, He+, and O+ ions, primarily number densities and mean energies, integrated over the 0.1‐ to 16‐keV/e energy range. These are grouped into four ranges of F10.7, less than 100, 100 to 150, 150 to 200, and greater than 200, using two methods. In one the data are averaged over geomagnetic activity, in the other the data are restricted to times of AE<200 γ. In both cases, the strongest effect is found in the number densities of the He+ and the O+, which increase by factors of about 3–5 and 5–10, respectively, over the full range of the F10.7, the rate of increase varying somewhat with location. The peak density of the O+ is about 20 times that of the He+ and is the highest at L ∼ 3–5 (>1 cm−3). Both species show a decreasing energy with increasing F10.7 at R<10 RE, from about 4–5 keV at low F10.7 to about 2–3 keV at high F10.7. The O+ shows a similar but weaker trend at greater distances as well. These effects are presumably all caused by the increase in the solar EUV irradiation of the Earth' atmosphere over the rising phase of the solar cycle (cycle 21), in ways that have been discussed elsewhere in the literature in conjunction with similar effects in other data sets. A third species that shows an increasing density over the time period of these data is the He++ (by about a factor of 3). It is argued, based on published solar wind data, that this effect is due to a variation of the helium abundance in the solar wind. The fourth species, the H+, is generally the dominant one at L>7 and appears to have both solar and terrestrial origins. It does not show a strong increase in density anywhere, however. In the plasma sheet it actually decreases slightly with increasing F10.7 (about 25% over the full range of the F10.7), an effect that is ascribed in part to a known variation of the solar wind proton density and in part to the apparent tendency of the plasma sheet to maintain a constant total pressure (including all ion species). At low L the H+ density appears to be dominated by a terrestrial source, but shows little or no increase with the F10.7, suggesting that this source is essentially saturated with respect to the EUV (F10.7>80).