Solar cycle changes in the morphology of the Venus ionosphere

I present two median plasma density curves for the central nightside Venus ionosphere, one for solar cycle maximum (SCmax) conditions and one for solar cycle minimum (SCmin) conditions. The curves are constructed from Pioneer Venus retarding potential analyzer total ion density data. Although the two curves do not overlap in altitude because of orbit evolution, they constitute strong evidence that the nightside ionospheric density during SCmin is typically a factor of 10 smaller between 200 and 2000 km altitude than it is at SCmax. From these data, data presented previously (Knudsen et al., 1987), and Venera radio occultation data, I conclude that the Venus SCmin ionosphere is typically confined to an altitude below approximately 250 km at all solar zenith angles (SZA) with the possible exception of an SZA interval near 90°. The layer is composed primarily of molecular ions, O 2 + being the most abundant. At SCmax an extensive O + layer exists on top of the O 2 + layer with the typical ionopause altitude changing from 300 km in the subsolar region to 900 km at the terminator and to over 1800 km altitude in the antisolar region.