Solar zenith angle dependence of ionospheric ion and electron temperatures and density on Venus

Data from the first year of operation of the Pioneer Venus orbiter retarding potential analyzer are presented, showing the variation with solar zenith angle of median values of ion and electron temperatures and density. Ion density is found to decrease by about an order of magnitude from dayside to nightside. Median ion temperatures above 300-km altitude are essentially constant with solar zenith angle for solar zenith angles less than 150 degrees and have values near 2300 degrees K at the ionopause. Near the antisolar point, the ion temperature rises to 5000 degrees K. At altitudes below 300 km the median ion temperature is nearly constant with solar zenith angle throughout the dayside but increases with solar zenith angle on the night side. Ion temperatures on the dayside are nearly constant and are larger than the neutral temperature in the altitude range between 175 and 225 km. A strong heat source near 160 km is required to model the observations. Joule heating is suggested as the probable source. The median electron temperature at a given height is independent of both solar zenith angle and ionopause height. The electron temperatures suggest a constant heat flux into the electron gas at the ionopause. The flux may be provided by dissipation of energy by whistler mode plasma waves at the ionopause and/or conduction of heat from the ionosheath through the mantle.