We argue, using new and previous experimental evidence that the height-integrated flux of O+ ions transported from the dayside Venus ionosphere into the nightside ionosphere is strongly reduced (shut off) at solar cycle minimum (SCmin) and that the electron impact source becomes the predominant nightside ionization source during this phase of the solar cycle. The height-integrated transterminator O+ flux, which is probably the dominant nightside ionization source at solar cycle maximum (SCmax), is shut off at SCmin by compression of the dayside ionopause to low altitude. We suggest that the ionopause is depressed because the dayside ionospheric kinetic pressure field is reduced by a factor of approximately 3 from its SCmax value and that the typical SCmin solar wind dynamic pressure, which is not less than the typical SCmax value, cannot be balanced by the ionospheric pressure field at altitudes above approximately 250 km. ¿ American Geophysical Union 1987