Correlations of magnetospheric ion composition with geomagnetic and solar activity

A large ion composition data set consisting of 1‐month averages has been assembled for the energy per charge range 0.9–15.9 keV/e. It includes 48 months of data taken by the Ion Composition Experiments on the ESA/GEOS 1 and 2 satellites at or near geostationary orbit. Data were obtained during the rising and maximum phases of the current solar cycle from May 1977 through November 1981 inclusive. Five ion species are routinely identifiable: H + , He ++ , He + , O ++ , and O + , above a limiting density ∼10 −3 ions cm −3 . Ion densities exhibit a number of very striking statistical correlations with one another and with both Kp and solar EUV as measured by F 10.7 . One principal result is that increases in the densities of magnetospheric He + , O ++ , and O + are observed that are apparently due entirely to increased solar EUV fluxes associated with the rising phase of the current solar cycle. There is a marked rise in O + density by a factor of ∼8 with increasing geomagnetic activity, but no corresponding increase in either He + or O ++ and only a small increase in H + . The He ++ /H + ratio is found to be remarkably constant at ≈0.01. Contrary to ion density results, little or no variation is found in mean energy. These observations are interpreted in terms of the composition and dynamics of two sources of magnetospheric plasma: the solar wind and the high‐latitude topside ionosphere.