A model for the variability of the Venus ionopause as a function of solar wind dynamic pressure and EUV flux during quiescent solar wind conditions is presented. The radio occultation measurements of the Venus ionopause from Mariner 5, 10, and Venera 9, 10 spacecraft, as well as recent in situ Pioneer Venus measurements are interpreted in terms of this model. An ionospheric model consistent with observations in the 400--1000 km region is predominantly 0+ with densities ~104 cm-3 and (Te+Tii) ~4500--6500 ¿K. For ionopause measurements below 400 km the ionosphere appears severely compressed and density and temperature profiles cannot be simply described, although a strong correlation with solar wind dynamic pressure is observed. Possible effects of IMF direction switching on the dynamics and structure of the ionosphere are also considered and compared with available ionopause data.