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.

Intercomparison measurements of the major ionospheric parameters of ion and electron density and temperature and ionic species made by direct measurement probes on the Explorer XXXl satellite are presented. Plasma density results are compared with simultaneous data from the Alouette II satellite. Probe results are from the following experiments: planar ion trap, planar electron trap, cylindrical electrostatic probes, high resolution magnetic ion mass spectrometer, planar Langmuir plate, and spherical ion probe.The plasma densities measured by the various probes generally agree with simultaneous Alouette II sounder values to within 20 percent. Electron temperatures measured by three different types of probes generalty agree within 10 percent. Ion composition measurements by the planar ion trap and spherical probe show good agreement with the high resolution magnetic mass spectrometer. Ion temperature measurements from the ion trap are consistently higher than spherical ion probe results.

The planar thermal ion and electron trap experiments on the Explorer XXXI satellite are described. These are capable of measuring ion and electron density and temperature and ionic composition at the satellite. Instrumentation, principles of operation, data analysis procedures, and sources,of error are discussed.

Values of positive‐ion concentration (N+ = 1.3 ± 0.1 × 104 ions/cm3), electron temperature (Te = 1750° ± 200°K), and the ratio of atomic helium to oxygen ions (He+/O+ = 1.3 ± 0.3), measured by three separate experiments at an altitude of 1630 km on the Explorer VIII satellite, are presented. These together with ionosonde data are shown to be consistent with a model of an isothermal upper ionosphere in diffusive equilibrium. The model infers that hydrogen ions are less important than either helium or oxygen ions at altitudes below 1600 km. The measured ratio of helium to oxygen ions is consistent with that postulated by Nicolet.

Two Nike‐Apache rockets were launched in 1964 to measure: (a) positive ion density (N+) with an altitude resolution of approximately 10 meters by use of a modified Gerdien condenser, (b) electron density by radio‐propagation techniques, and (c) the optical depth of solar radiation absorbed in the 60–120 km region with a photoelectron retarding potential analyzer. The flights took place at a time when the intensities of important portions of the solar spectrum were being measured simultaneously from a satellite. The simultaneity of all these data and the high altitude resolution of the charged particle density profiles permits us to identify several regions between 65 and 120 km and to associate them with different portions of the solar spectrum and with different loss mechanisms. The average N+ in the D region (65–83 km) is found to be 103cm−3, an order of magnitude less than reported by investigators who used experiments that, unlike ours, require assumptions about other ionic parameters to derive N+. The regions 83–88 km and 88–93 km are sequentially ionized by 2–8 A X radiation and the C VI line at 33.7 A that produce O2+ and N2+ ions. These ions are transformed into NO+ by processes involving charge transfer and/or ion‐atom interchange (N2+ + O2 → O2+ + N2 → NO+ + NO). From our results we compute the effective loss rate between 83 and 93 km to be approximately 2×10−8 cm3 sec−1, which quite likely represents the dissociative recombination rate for NO+. The 95–115 km region is ionized principally by extreme ultraviolet radiation leaving O2+ as one of the two dominant ions. Though never dominant, 40–75 A X radiation is an important ionizing source which indirectly produces some NO+ ions above 95 km through the process N2+ + O → NO+ + N. Our computed effective dissociative recombination rate between 95 and 115 km is about 1.8×10−7 cm3 sec−1. It is suggested that this value is higher than that computed for the region below 90 km because above 95 km the ionic content is richer in O2+.