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

Abstract

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 x 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 ofHe + 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 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. 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. 1. equatorial proton temperatures and densities for the period March-April 1968. Density measurements by the light ion mass spectrometer, also on OGO 5, were given in a series of papers by Chappell and coworkers [Chappell et al., 1970a, b, 1971; Harris et al., 1970; Chappell, 1972] spanning the periods March-April 1968 (for the nightside magnetosphere), June-No...

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