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.

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+.

Data on atmospheric conductivity obtained experimentally in the altitude region between 35 and 80 km by use of rocket‐borne Gerdien condensers are presented. Analytic expressions based on ion equilibrium and ionization by cosmic rays only are derived for comparison. The experimental technique is described, and several factors that might influence the measurements are evaluated. There is good agreement between the measured and predicted values of negative conductivity at altitudes up to 50 km. Low conductivity values observed between 50 and 80 km are attributed to ionic diffusion to particulate matter, the reduction agreeing quantitatively with that calculated from present estimates of the radius and concentration of noctilucent cloud particles. It is suggested that meteoritic dust may be an important agent for electron destruction in the ionosphere.

Thermospheric temperatures deduced from satellite drag observations are compared with the intensity of extreme ultraviolet radiation measured on the first Orbiting Solar Observatory (OSO 1). The comparison leads to a conclusion that, for the two complete solar rotation periods during which the EUV data are available, the 27‐day periodic variation in upper atmospheric density was due to corresponding changes principally of ultraviolet rather than corpuscular radiation.