PVO VENUS ELECT TEMP PROBE DERVD ELECT DENS LOW RES VER 1.0

Data from essentially every orbit in 1979 and 1980 included ionosphere transits. After the summer of 1980, however, periapsis could no longer be maintained at low altitudes, and it rose slowly.

After April 1981 periapsis was above the altitude of the dayside ionopause, so the spacecraft encountered the ionosphere only in the terminator regions and on the nightside where the ionosphere extends to much higher altitudes. Dayside measurements again became available early in 1992 when periapsis returned to low enough altitudes. The PVO Entry period of between July and October 1992 provided only nightside periapsis data.