The composition, temperature, and density structure of cold ions in the quiet terrestrial plasmasphere: GEOS 1 results

The authors present a comprehensive study of the composition, and the density and temperature distributions of the thermal (energy ≤ 110 eV) ion population in the terrestrial plasmasphere under quiet geomagnetic conditions. The data were collected by the Ion Composition Experiment (ICE) on board the European Space Agency's GEOS 1 satellite and cover the period from June 1977 to May 1978. They find typical quiet time proton densities to vary smoothly between ∼ 102 cm-3 (L ∼ 6) and 2 x 103 cm-3 (L ∼ 3). In the local time sector 1,700-2,200 the equatorial proton concentration obeys an inverse fourth power dependence with dipole L. He+ is a major ionic component in all L ranges investigated. Its concentration relative to H+ is highly variable, ranging from ∼ 1% to, on occasion, over 100%. The averaged variation of He+ with dipole L in the 1,700-2,200 local time range shows a somewhat more rapid decrease with increasing L than does H+. The temperature in the quiet plasmasphere are between 4 x 103 K and 1.5 x 104 K and generally exhibit a slow increase with L value. The average radial temperature gradient near the equator is ∼ 0.15 K/km. The main ionic constituents are usually in thermal equilibrium throughout the plasmasphere. Using alternate passes in the same region, they discuss the poststorm recovery of the plasmasphere on L shells > 4 for both H+ and He+. Data are also presented on the minor ions O+, O++, D+, and He++