An analysis of Mars and Venus nightside electron density profiles obtained with radio occultation methods shows how the nightside ionospheres of both planets vary with solar zenith angle. From previous studies it is known that the dayside peak electron densities at Mars and Venus show a basic similarity in that they both exhibit Chapman layer-like behavior. In constrast, the peak altitudes at Mars behave like an ideal Chapman layer on the dayside (peak altitudes increase with solar zenith angle), whereas the altitude of the peak at Venus is fairly constants up to the terminator. The effect of major dust storms can also be seen in the peak altitudes at Mars. All Venus nightside electron density profiles show a distinct main peak for both solar minimum and maximum, whereas many profiles from the nightside of Mars do not show any peak at all. This suggests that the electron density in the Mars nightside ionosphere is frequently too low to be detected by radio occultation. On the Pioneer Venus orbiter, ''disappearing ionospheres'' were observed near solar maximum in the in situ data when the solar wind dynamic pressure was exceptionally high.

This condition occurs because the high solar wind dynamic pressure decreases the altitude of the ionopause near the terminator below ~250 km, thus reducing the ''normal'' nightward transport of dayside ionospheric plasma. While the ''disappearing ionosphere'' condition is usually considered important for the upper ionosphere only the peaks in the deep Venus nightside also show some dependence on the solar wind dynamic pressure. However, an additional persistent source (e.g., particle precipitation) makes a major contribution to these peaks. The observations suggest that this other source may in fact increase with solar wind pressure. This is different from the nightside ionosphere of Mars where even the peak cannot be detected on occasion. We do not know whether this more complete disappearance of the nightside ionosphere at Mars is, like the Venus ''disappearing ionosphere,'' a response to solar wind conditions because of the lack of solar wind data at Mars.

On the basis of the Venus observations, one might predict that if a positive correlation of nightside peak density with dynamic pressure was found, it could mean that transport from the dayside is the only significant source for the nightside ionosphere of Mars. The lack of a correlation would imply that the precipitation source at Mars is quite variable. ¿American Geophysical Union 1990