Median density altitude profiles of the major ions in the central nightside venus ionosphere
Abstract
Altitude profiles of median values of total ion density N_{i} and constituent ion densities [O^{+}], [H^{+}], and the molecular ion group density [O_{2}^{+}+M29^{+}] (=[O_{2}^{+}]+[NO^{+}]+[N_{2}^{+}]+[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 145900 km altitude. The altitude profiles for N_{i}, [O^{+}], and [O_{2}^{+}+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 N_{i}, [O^{+}], and [O_{2}^{+}, 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 Q_{m} is characterized by a variance parameter such that two thirds of the measurements Q fall within the limits Q_{m}/σ <=Q<=σQ_{m}. The value of σ for N_{i}, [O^{+}] and [H^{+}] above an altitude of 150 km is 2.5. Below 150 km the variance for N_{i} and [O_{2}^{+}+M29^{+}] is approximately 1.5, but the statistics from which it is derived are less reliable. ORPAderived median plasma density N_{i} and [O_{2}^{+}+M29^{+}] agree closely with the average (and median) value of the peak nightside electron density N_{e,max} measured by the PV radio occultation experiment a short altitude [O_{2}^{+}+M29^{+}] are extrapolated a short altitude interval to that of N_{e,max}. The standard deviation of N_{e,max} also agrees well with the variances of N_{i} and [O_{2}^{+}+M29^{+}] calculated for altitudes below 150 km.
The ORPA median N_{i} profile at low altitudes is a factor of two smaller than the median N_{e} profile derived from published PV Langmuir probe (OLP) medians. At higher altitudes the N_{i} median profile is 8085% that of N_{e}. The plasma densities measured by PV in situ instruments in the altitude range 200250 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.
 Publication:

Journal of Geophysical Research
 Pub Date:
 November 1986
 DOI:
 10.1029/JA091iA11p11936
 Bibcode:
 1986JGR....9111936K
 Keywords:

 Altitude;
 Electron Impact;
 Ionospheric Ion Density;
 Molecular Ions;
 Pioneer Venus Spacecraft;
 Venus Atmosphere;
 Electron Flux Density;
 Extreme Ultraviolet Radiation;
 Night;
 Plasma Density;
 Solar Radiation;
 Spacecraft Instruments