OGO 4 observations of ion composition and temperatures in the topside ionosphere

Direct measurement of the densities of ionic constituents (H+, He+, and O+) and the temperatures of ions and electrons have been obtained from the Ogo 4 planar retarding potential analyzer in the altitude range 400–900 km. Results are presented from day and night passes in the middle and low latitudes near the 1967 fall equinox. The passes are selected to empasize the latitudinal rather than the height dependence of the measurements. The main results can be summarized as follows: (1) Above 800 km at night, there is a deep equatorial trough in He+ and a corresponding rise in O+, suggesting a charge exchange between He+ and O as an important loss mechanism for He+. (2) The dominant ion in the night at these altitudes between ±40° geomagnetic latitudes is H+ followed generally by O+ and He+. Outside this latitude region O+ becomes the dominant constituent, increasing continuously toward the pole. (3) The major ionic constituent in the daytime is O+ throughout the altitude and latitude range of observations. In the height range 400–500 km, the latitudinal variation in O+ shows the well-known feature of the geomagnetic anomaly. (4) Both electron and ion temperatures generally increase poleward from their low latitude values, attaining maxima between 40 and 50° geomagnetic latitude.