Abstract
Theoretical models of the Mars and Venus ionospheres are constructed and compared with the observed electron distributions in the upper atmospheres of these planets. It is found that a serious discrepancy exists between calculated and observed electron densities for all the Mariner flights if the presently accepted values of the solar extreme ultraviolet radiation are assumed to be the only ionization source. This result suggests the presence of a second source of ionization having an importance at least comparable to that of the ultra-violet. The proton flux that would be required to provide a source of ionization strong enough to bring the computed electron densities into agreement with observation is calculated, and a comparison of these flux values with those typical of the solar wind shows that the solar wind is a potentially important ionization source for the Mars and Venus upper atmospheres. A quantitative treatment of solar wind flow across the planetary bow shock and the subsequent ionization produced will be necessary before its actual importance can be established.
