In the previous quadrennium, 1971–1974, energetic ion composition measurements provided the first compelling evidence that the ionosphere was contributing large quantities of ions to the energetic ion fluxes in the earth's magnetosphere. With an energetic ion mass spectrometer aboard a low altitude polar orbiting satellite (1971‐089A) relatively large fluxes of energetic (0.7–12 keV) O+ and He+ ions were discovered precipitating from the magnetosphere (Shelley et al., 1972; Sharp et al., 1974; and Johnson et al., 1974). Because of the instrumentation orientation and the low altitude of the satellite, it was difficult to obtain specific information on the important basic questions relating to these ions: What were the relative contributions of the solar wind and the ionosphere to the energetic ion populations? Where was the ionospheric source region located in latitude, altitude, and local time? What physical processes were responsible for the acceleration, transport, and loss of the ionospheric ions? The presence of large fluxes of heavy (O+ and He+) energetic ionospheric ions also made it appear likely that some of the energetic H+ ions in the magnetosphere were also of ionospheric origin. This further emphasized the need for ion mass and charge identification for investigations of the entry of solar wind ions into the magnetosphere and of their subsequent acceleration, transport, and loss processes since the solar wind H+ ions could no longer always be assumed to be dominant over other sources of H+ ions in the magnetosphere. Copyright 1979 by the American Geophysical Union.
