Survey of 0.1‐ to 16‐keV/<i>e</i> plasma sheet ion composition

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 R E 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 R E ), 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 R E ), 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.