Altitude profiles of median values of total ion density Ni and constituent ion densities [O+>, [H+>, and the molecular ion group density [O2++M29+> (=[O2+>+[NO+>+[N2+>+[CO+>) for the central nightside Venus ionosphere are constructed from measurements made by the Pioneer Venus (PV) orbiter retarding potential analyzer (ORPA) during the first five seasonal passages of the PV spacecraft through the nightside hemisphere. The profiles cover the altitude range 145-900 km altitude. The altitude profiles for Ni, [O+>, and [O2++M29+> are compared with previously predicted profiles derived from numerical models in which the ionization source was O+ transported from the dayside ionosphere and electron impact produced ionization, respectively.

The numerically derived profile for O+ transport reproduces the median profiles of Ni, [O+>, and [O2+, M29+> quite satisfactorily. The extent to which electron impact ionization is required remains uncertain. The temporal variability of a quantity Q from its median value Qm is characterized by a variance parameter such that two thirds of the measurements Q fall within the limits Qm/&sgr; ≤Q≤&sgr;Qm. The value of &sgr; for Ni, [O+> and [H+> above an altitude of 150 km is 2.5. Below 150 km the variance for Ni and [O2++M29+> is approximately 1.5, but the statistics from which it is derived are less reliable. ORPA-derived median plasma density Ni and [O2++M29+> agree closely with the average (and median) value of the peak nightside electron density Ne,max measured by the PV radio occultation experiment a short altitude [O2++M29+> are extrapolated a short altitude interval to that of Ne,max. The standard deviation of Ne,max also agrees well with the variances of Ni and [O2++M29+> calculated for altitudes below 150 km.

The ORPA median Ni profile at low altitudes is a factor of two smaller than the median Ne profile derived from published PV Langmuir probe (OLP) medians. At higher altitudes the Ni median profile is 80--85% that of Ne. The plasma densities measured by PV in situ instruments in the altitude range 200--250 km exceed those derived from Venera 9 and 10 radio occultation data by a factor of the order of 10. The Venera measurements were made at sunspot minimum and, if representative of the median density, imply an order of magnitude variation over a solar cycle in the median O+ flux transported from the dayside ionosphere.