Premium
Solar cycle variations of electron density and temperature in the Venusian nightside ionosphere
Author(s) -
Theis Robert F.,
Brace Larry H.
Publication year - 1993
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/93gl02485
Subject(s) - ionosphere , orbiter , venus , atmospheric sciences , solar maximum , solar minimum , solar cycle , physics , electron density , electron temperature , altitude (triangle) , atmosphere of venus , ion , environmental science , electron , solar wind , astronomy , astrobiology , plasma , geometry , quantum mechanics , mathematics
The return of periapsis to low altitudes during the Summer and Fall of 1992 provided a unique opportunity for the Pioneer Venus Orbiter (PVO) to make in situ measurements in the Venusian ionosphere at much lower levels of solar activity (F10.7 ≈120) than existed when periapsis was at low altitudes in 1979 and 1980 (F10.7≈220). We present the observations of electron density (N e ) and temperature (T e ) made by the Orbiter Electron Temperature Probe (OETP) during the Entry Period. Empirical models of the N e and T e height variations are presented and compared with similar models based on OETP measurements made at solar maximum. The median N e at the ionospheric peak (∼140 km) was essentially unchanged from its solar maximum value, but the ionosphere was increasingly depleted at higher altitudes, reaching a factor of 7 lower densities at 200 km. T e was lower by almost a factor of 2 at 140 km but was rather significantly enhanced at higher altitudes; exceeding its solar maximum values by a factor of 1.3 at 200 km and a factor of 2 at 500 km. In general these results support the earlier conclusions that the nightside upper ionosphere is depleted at lower levels of solar activity by a reduction of the nightward ion flow. The lack of N e variation near the peak (between solar maximum and entry) suggests that nightward ion transport does not play as large a role in the peak formation as does local ion production by energetic particles. The decrease in T e at low altitudes suggests that the low densities of the upper ionosphere at the time of PVO entry could no longer support the conduction of heat from the dayside ionosphere, thus allowing the lower nightside ionosphere to cool by collisions with ions and neutrals, and by heat conduction to the cooler regions below.