Ion composition and energy distribution during 10 magnetic storms

Data from the plasma composition experiment on ISEE 1 were used to investigate the relative quantities and energy characteristics of H + , He ++ , He + , and O + in the near‐equatorial magnetosphere at R ≤15 R E during magnetic storms, principally during the early main phase. The ions included in this study had energies in the range of 0.1 ≤ E/Q ≤17 keV/ e . The number densities were characterized by a large to dominant fraction of terrestrial ions throughout this energy window. Terrestrial O + ions were most clearly identified, but strong evidence for a significant contribution of terrestrial H + ions was also found. On occasions, the O + alone contributed 50% or more of the integral number density, as well as the energy density, over distances of several earth radii along the orbit. The largest fractions of O + (≲75%) and He + (≲25%) were found at R <3 R E ( L <5). In general, the He + only represented a few percent, however. Small fractions of O + (<10%) and He + (<1%) were mostly found in the 0100‐0600 LT sector, at R ≳7 R E . The He ++ was often obscured by background and rarely exceeded 2%, except in the 0100–0600 LT sector, at R ≳7 R E , where it reached several percent relatively frequently, suggesting a larger solar wind component here. It is argued, based on certain signatures in the energy spectra, that solar wind ions may enter the inner magnetosphere through this region and thereby contribute a larger portion of the high‐energy ring current population (50–100 keV). The data do not suggest, however, that the solar wind is always the dominant source of ions for the high‐energy ring current.