We 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. For the data reduction we employ a method based on the modulation of the detector count rates by the rotation of the spacecraft. We find typical quiet time proton densities to vary smoothly between ~102 cm-3 (L≈6) and 2¿103 cm-3 (L≈3). In the local time sector 1700--2200 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 most frequent values we obtain lie in the range ~2--6%. The averaged variation of He+ with dipole L in the 1700--2200 local time range shows a somewhat more rapid decrease with increasing L than does H+. The temperatures in the quiet plasmasphere are between 4¿103 K and 1.5¿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.

There are indications that the ionic component is in a state of thermal equilibration with the electronic component in the outer plasmasphere. Using alternate passes in the same region, we 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++. The O+ densities around L≈3 are ≲1 cm-3, and the densities of O++ are comparable. Cases are shown where the mass/charge=2 ion is predominantly D+ and others where it is mainly He++. Typical densities of this ion in the L range 2.6--3.6 are a few tenths per cubic centimeter. With one exception we found these minor ions to share a common temperature with the main constituents of the plasmasphere. The results of our survey are compared with previous studies and with theoretical modeling. In particular, the GEOS 1 H+ temperature structure is in substantial agreement with those from the Plasma Composition Experiment (PCE) on ISEE 1 and the retarding ion mass spectrometer (RIMS) on DE 1. We confirm the enhanced O++ to O+ density ratio in the equatorial plasmasphere with respect to values in the mid-latitude ionosphere. Our observations on this ratio are compared with the predictions of three theoretical works. ¿ American Geophysical Union 1989