3. ION COMPOSITION AND ION DENSITIES

The orbiter ion mass spectrometer, OIMS (Taylor et al., 1980a), identified 12 ion species with O2 ⁺ as the major ion at altitudes below about 200 km, and O⁺ at higher altitudes (Taylor et al., 1980b). Figure 7 shows a plot of ion composition measurements from a single PVO passage through the subsolar ionosphere. The rich but complex variety of ions can be noted. Although CO2 is the major neutral constituent, CO2⁺ remains a minor ion at low altitudes because of charge exchange of CO2⁺ with O and of O⁺ with CO2 The photochemical and diffusive processes compete to produce an O⁺ maximum at about 200 km above which diffusion and bulk transport control the ion distribution. Model calculations have found a reason-able agreement with the experimental measurements (see Fox and Kliore 1997 and references therein). Figure 8 shows computed altitude profiles for the major ions for the low solar-activity and high solar activity models.

Fig.7:Ion composition measurements from a single PVO passage
through the subsolar ionosphere demonstrating that O2⁺ is the domination ion below 200 km,
while O⁺ dominates above it (from Taylor et al., 1980b).

Fig.8:Computed altitude profiles for the major ions (left) for the low solar activity model
and (right) for the hight solar activity model (from Fox and Kliore, 1997).

Large diurnal and solar activity variations have been seen in the ion composition. Figure 9 compares density profiles for important ions for solar maximum and the pre entry period. While O⁺ changed by about a factor of 10, O2⁺ showed a much smaller variation. H⁺ and He⁺ also showed decrease in their concentrations during the pre-entry period (Kar et al., 1994). These results indicate the importance of particle precipitation during solar medium (Kar et al., 1 994) and possibly during solar minimum too.

The ion densities have also been measured by the orbiter retarding potential analyzer, ORPA (Knudsen et al., 1980a) and the electron temperature probe, OETP (Krehbiel et al., 1980) which have shown a reasonable agreement with the OIMS measurements. Figure 10 shows median ion densities obtained from the ORPA data and sorted according to SZA, grouped into 30° intervals. A significant control of SZA is evident, but the densities change very little for SZA between 0 to 60° (central dayside) and 120 to 180° (central nightside). The OETP data gave similar results (Brace and Kliore, 1991). The EUV control on the densities, in terms of solar activity, can be noted from Figure 11 which shows electron density profiles obtained from radio occultation measurements during solar maximum and solar minimum.

Fig.9:Average altitude profiles of O+ and O2⁺ during the primary mission of PVO (solar max.)
and in 1992 during PVO entry with data from midnight to 04:30 (from Kar et al., 1994).

Fig.10:Midian total ion density profiles from ORPA fro 30 intervals of SZA:
There is little variation of the density between 0° to 60°(central dayside),and 120° to 180°( central nightside), but large changes occur for other values of SZA (from Miller et al., 1980)

Fig.11:Electron density profiles obtained by
PVO radio occulation at solar maximum(1980) and solar minimum (1986).
A large depletion in the dayside upper ionosphere at solar minimum
can be noted (from Knudsen et al., 1987)