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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.

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The possibility of using airglow techniques for estimating the F layer height and electron density is discussed by deriving a simple theoretical relationship between the height of the F2 peak and the column emission rates of the O I 6300‐Å and O I 1356‐Å lines. The feasibility of this method is demonstrated by presenting numerical calculations of the F2 peak heights and electron densities from the simultaneous measurement of O I 1356 Å and O I 6300 Å obtained from Ogo 4. The heights of the F2 peak estimated from this method are in good agreement with the values estimated from top side and bottom side ionosondes and with ion densities obtained from the retarding potential analyzer on the same spacecraft. The monitoring of these airglow emissions from a satellite therefore offers the possibility of mapping the height and density of the F2 peak over an extended area, a possibility that has been difficult to realize from top side sounders. Since the height of the F2 peak is a very sensitive indicator of the ionospheric motion, such methods can be used in the study of the global wind system.

A modified Chapman function with a variable scale height gradient has been found to be in good agreement with the electron density distribution obtained experimentally within the height range from about 100 km below the F2 peak to an altitude of about 700 km. The scale height distribution derived from this model is also consistent with the neutral gas scale height of the 1961 Cospar international reference atmosphere.