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 dynanic 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 construred 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. ¿ American Geophysical Union 1987