Ion composition and electron temperature data obtained from AE-C during a magnetically quiet period centered on the June 1976 solstice have been used in a statistical study of the southern winter F region poleward of-50 ø A at 300-km altitude. Prominent ionospheric features revealed by topographic maps of O +, NO +, and O•. + concentration and Te include the nightside main trough, an ionization 'hole' poleward of the nightside auroral zone, and ionization and Te enhancements in the dayside auroral zone-cusp region. The main trough, in which O + was the dominant ion, extended throughout the night between -60 ø and -70 ø A, the lowest trough densities, • 1 X 10 • cm -•, being detected near dusk. We attribute these low concentrations to the opposition, in the dusk sector, of plasma corotation and solar wind induced plasma convection velocities, leading to long plasma residence (and decay) times. That the distributions of NO + and O•. + in the trough region exhibited little correlation with O + suggests that drift-enhanced O+loss via reactions such as O + + N•. --, NO + + N played a minor role in the formation of the trough during this period. A band of enhanced electron temperature coincided with the trough throughout the night; this T, peak, which has been observed previously in the topside ionosphere, is attributed to heat conducted downward from the protonosphere. The ionization hole, a region poleward of the nightside auroral zone between -70 ø and -80 ø A, was characterized by depletions in all the measured ion densities and by a minimum in T,. The total ion concentration measured in this region exhibited extreme temporal variability, ranging from values as low as 2 X 10 •' to 6 X 10 • cm -•. We have c, oncluded that the hole forms as a result of slow antisunward plasma drift across the dark polar cap and attendant ion recombination; an average drift velocity of •0.1 km/s, corresponding to a convection electric field of less than 5 mV/m, could produce the deepest holes observed. The ion density variability in the hole is attributed to changes in the transpolar plasma convection configuration and the distribution of energetic particle fluxes. The dayside auroral zone-cusp region was characterized, in general, by enhanced levels of ionization and electron temperature associated with energetic particle precipitation. On some passes through this region, however, localized O + depletions and corresponding molecular ion increases were detected; we attribufe these features to the reactions O + + N•. --, NO + + N and O + + O•. --, O•. + + O, whose rates are enhanced by the high-speed plasma drifts observed in the cusp region.

The first in situ measurements of the details of the global composition and dynamics of the ionosphere of Venus have been obtained from the Bennett ion mass spectrometer on the Pioneer Venus orbiter during the period December 1978 through August 1979. These results include observations of three related plasma regimes, including (1) the bowshock‐ionosheath region, (2) the thermal ionosphere, and (3) a superthermal flowing ion layer interfacing with the ionosphere at the ionopause and extending outward to variable heights above the planet. During quiet periods an abundant ionosphere is observed both on the dayside and on the nightside, generally dominated by O+ above 200 km (except for a predawn region where H+ exceeds O+) and by O2+ down to typical periapsis heights of about 160 km. A sampling of some of the less disturbed data exhibits strong day to night variations in the distributions of the most prominent ions, including O+, O2+, CO2+, C+, N+, CO+(N2+), NO+, H+, He+, O18+, O++, and H2+. Important features of the day‐night variation measured at 200 km and at a fixed latitude of about 8°N include asymmetric nightside bulges in the distributions of the light ions H+ and He+, peaking near dawn at 110° and 90° solar zenith angle, respectively. These asymmetries are associated with dawn‐dusk asymmetries in the distributions of O+, O2+, and other molecular ions, which exist in higher concentrations near dusk, relative to dawn. Associated observations of the bowshock‐ionosheath and the ionosphere‐superthermal plasma layer regions indicate close coupling among these plasma regimes. The ionopause, identified as the boundary between the thermal ionosphere and the superthermal flow layer, is encountered near 250–400 km near the subsolar point and extends at times to heights greater than 1000 km in the flanks and nightside regions. Under disturbed nightside conditions, particularly noticeable in the dusk region, the ionosphere may exhibit randomly spaced concentration gradients of an order of magnitude associated with complex patterns of ion flow with velocities up to 10 km/s encountered within the main body of the ionosphere.

Measurements of O, He, and Ar from neutral gas mass spectrometers on four satellites (Ogo 6, San Marco 3, Aeros A, and AE‐C) and inferred O2 and H densities from an ion mass spectrometer on AE‐C have been combined with a neutral temperature and N2 density model, based on data from these four satellites and four incoherent scatter stations, to produce a global model of thermospheric composition in terms of inferred variations at 120 km. The data coverage is significantly improved over that of the similar Ogo 6 model in terms of altitude, seasonal, and solar parameters, and the usefulness is expanded by the inclusion of additional gas species. The overall data set covers the time period from mid‐1969 to mid‐1975. The seasonal He density variation is double that of the Ogo 6 model on average, with maximum variation in the northern hemisphere. Ar variations at 120 km tend to be in phase with temperature variations and inverse to the He, O, and H variations.

First results from the Ogo‐A positive ion spectrometer experiment are presented for the period September 23 through December 10, 1964. Thermal hydrogen and helium ion distributions extend from the lowest observations at 1500 km to an altitude of 30,000 km. The density obtained for H+ at 2000 km is of the order of 103 ions cm−3, and the He+ concentration is 1% of H+ over most of the altitude range. Whereas the concentration and distribution of H+ observed at the lower altitudes is in general agreement with theoretical models, the upper altitude profiles show significant departure from predictions based on diffusive equilibrium theory. Evidence is presented indicating that diffusion of ions is controlled by the geomagnetic field and that the ions are distributed in a beltlike region which exhibits a sharp gradient resulting in a ‘plateau’ at its outer boundary, which is characterized by a reduction in both the H+ and He+ concentrations by a factor of 10 or more. The ion belt is observed to expand and contract over an altitude range of 8000 to 30,000 km in an inverse relationship with the magnetic activity index Ap. There is significant correlation between these results and the knee whistler observations as well as with high‐altitude ionization gradients observed from other satellites. Although the data provide some indication of a direct coupling between the lower and upper ionosphere, more data will be required to describe this relationship adequately.

Midday ion composition measurements made by the Explorer 32 ion mass spectrometer in the midlatitude trough region (L = 5.0–10.5) are compared with measurements made within the plasmasphere (L = 1.8–2.2). In the trough the light ion (H+ and He+) concentrations are observed to decrease with altitude between 600 and 2500 km, a region in which O+ is the major ion. Within the plasmasphere, however, the concentrations of the light ions, when minor, increase with altitude. Model calculations indicate that the behavior of the plasmasphere profiles is consistent with a state of stationary diffusive equilibrium, whereas a dynamic plasma state is required to account for the trough distributions. Upward‐directed H+ fluxes with near sonic speeds above 2000 km are required to reproduce the measured trough variations. This ‘trough wind,’ consisting of a rapid upward flow of H+, can be interpreted as evidence for the convection of the supersonic polar wind on to closed trough field lines.