Behavior of the polar ionospheric F-layer as it is convected through the cleft, over the polar cap and through the night side F-layer trough zone is investigated. Passage through the cleft adds of the order of 2 x 10 s ions cm" 3 in the vicinity of the F 2 peak and redistributes the ionization above approximately kdO km altitude to conform with an increased electron temperature. The redistribution of ionization above kOO km altitude forms the "averaged" Plasma ring seen at 1000 km altitude.The F-layer is also raised of the order of 20 km in altitude by the convection electric field. The time required for passage across the polar cap (25°) is about the same as that required for the F-layer peak concentration to decay by e. The F-layer response to passage through the night side soft electron precipitation zone should be similar to but less than its response to passage through the cleft. The exception is that the layer will be lowered in altitude by the convection electric field.

Vector velocities of O+ ions in the Venus ionosphere are reported for the dawn and dusk terminator regions. The velocity vectors are generally directed antisunward and radially inward toward the planet with magnitudes ranging from approximately 1 to 8 km/s. The velocity generally increases with altitude. The velocity may increase to still larger values just below the ionopause, but experimental limitations prevent measuring such an increase. The Alfvén and ion Mach numbers are generally greater than 1. The estimated O+ flux across the terminator is equal within a factor of 2 to the total ion recombination rate on the nightside and is evidently a factor of 10 larger than that which could be exiting down the planet wake. The higher velocity flow near the ionopause is hypothesized to converge on the antisolar axis and, after thermalization, to descend into the ionosphere. The kinetic energy and O+ flux in this high‐velocity stream are sufficient to maintain, respectively, the high ion temperature and peak ion concentration, respectively, measured at solar zenith angles in excess of 150°. The nightside ionospheric variability is hypothesized to result from temporal and spatial variations in the ionospheric terminator and wake flow fields. Ion transport contributes substantially and possibly predominantly to the maintenance of the nightside ionosphere.

A model of the energy balance of the dayside ionosphere of Venus is presented. Calculations of the dayside electron and ion temperature profiles are carried out and compared with data from experiments on the Pioneer Venus orbiter. The coupled heat conduction equations for electrons and ions are solved for several values of the solar zenith angle. It is shown that thermal conductivities are inhibited by the presence of a horizontal magnetic field. A realistic model of the magnetic field that includes fluctuations is employed in deriving an appropriate expression for the thermal conductivity. The contributions of photoelectrons, ion chemistry, Joule heating, and solar wind heating to the energy balance of the ionosphere are considered.

Many authors have already noted that Mars' interaction with the solar wind may be like Venus', e.g., that of a supermagnetosonic plasma flowing past an effectively unmagnetized body having an ionosphere. However, at Mars, the incident solar wind dynamic pressure usually exceeds the peak ionospheric plasma pressure, while at Venus this condition prevails only when the incident dynamic pressure is extraordinarily high. With the aim of predicting what might be expected at Mars, this study examines the subset of Pioneer Venus Orbiter observations obtained during intervals of extremely high solar wind dynamic pressure. The characteristic features of this limit of the Venus‐solar wind interaction include a bow shock position that is not notably different from the norm, altitude profiles of the dayside upper ionosphere density without a sharp increase in gradient at the ionopause, and dayside electron temperatures that rapidly increase with altitude to consistently exceed the temperatures above 200 km which are present for lower solar wind pressures. Depleted nightside ionosphere densities, and a large‐scale horizontal magnetic field in both the dayside and nightside ionospheres, are among the previously identified responses to high dynamic pressure. Here, emphasis is placed on the dayside ionosphere because some data are available for the dayside Martian ionosphere from the Viking mission. The density and ion temperature trends are found to be similar to those seen in the Viking data. Overall, the Venus observations at high dynamic pressure provide a framework for reassessing the available Mars observations. In particular, this study shows that the observed absence of a distinct ionopause “cutoff” in the dayside plasma density gradient cannot be construed as evidence for an intrinsic magnetic field which stops the solar wind at higher altitudes. Similarly, the rules of thumb that the electron temperature is given by twice the ion temperature and that the ionospheric magnetic field pressure can only double the total ionospheric pressure, which have been applied to assess Mars' ability to standoff the solar wind, are not justified. Observations at Venus thus illustrate the expected modification of the common pressure balance picture of the solar wind interaction at an unmagnetized planet with a weak ionosphere. The available Mars observations appear to be consistent with this modified picture.

For three orbit paths of the Pioneer Venus orbiter the interaction between the solar wind and the Venusian ionosphere has been studied. Results of the retarding potential analyzer and the magnetometer are described for the boundary region between the solar wind and the planetary ionosphere. These are the first measurements that show that a transition region exists between the two plasmas of different origin. The observed magnetic field and current system producing it appear strong enough to stop the solar wind flow in front of the ionosphere and to separate the shocked solar wind from the ionosphere. The transition region between the ionosheath and the ionosphere is called the ‘mantle.’ The observed mantle electron energy spectra close to the ionopause show ionospheric character. With increasing height the number of electrons that have ionospheric energies decreases, and the number of electrons that have solar wind energies gradually increases toward the ionosheath boundary, where only solar wind energy spectra are observed. The mantle surrounds the frontside of the ionosphere and extends probably more than eight Venus radii downstream.