The dayside altitude profiles of the electron density obtained with the radio occultation experiments on Mariners 4, 6, 7, and 9 and the Viking 1 and 2 spacecraft are collectively reanalyzed to determine the global characteristics of the dayside ionosphere of Mars. These analyses concentrate on the comparison of the properties of both the electron density peaks and the topside profiles with the behavior expected for a Chapman layer and that observed at Venus with the Pioneer Venus orbiter radio occultation experiment. As at Venus, the peak densities at Mars behave much like Chapman layer peaks with only slight departure from a (cos&thgr;)1/2 dependence, where &thgr; is the solar zenith angle. In contrast, the peak heights depart from ideal Chapman layer behavior at Venus but not at Mars because the dayside neutral atmosphere at Venus depends on solar zenith angle. The global dust storm during the Mariner 9 main mission appears to have elevated the Martian ionosphere as a whole by ~20--30 km without otherwise notably altering its density profile.

These results gnerally corroborate the findings of earlier studies. An examination of the solar zenith angle dependence of density levels on the topsides of profiles obtained both at Mars and at Venus near solar minimum provides a new perspective on the solar zenith angle variation of the scale heights of the two ionospheres. The key contribution of this study is the improved picture of the solar zenith angle dependences of both the peaks and scale heights of the ionosphere of Mars and their comparison with Venus at solar minimum. In particular, this study illustrates that under the similar conditions where incident solar wind dynamic pressure exceeds peak ionospheric thermal pressure, the Martian dayside ionosphere peaks at higher altitudes in the flanks and has a greater scale height overall. Thus, even (magnetically) unshielded Martian and Venusian ionsopheres would present slightly different obstacles to the solar wind. ¿American Geophysical Union 1990