Solar cycle variation of ion densities measured by SROSS C2 and FORMOSAT-1 over Indian low and equatorial latitudes

In situ measurements carried out by the SROSS C2 satellite during the ascending half of solar cycle 23 from 1995 to 2000 are used to study the effect of changes in solar activity on diurnal, seasonal, and latitudinal variations of O+, H+, He+, and O-2(+) ions at an average altitude of similar to 500 km over the Indian latitude and longitude sector. FORMOSAT-1 measurements at 600 km over Indian low and equatorial latitudes are also analyzed for diurnal and seasonal variations in O+, H+, and He+ concentrations from 1999 to 2003. O+ dominates these altitudes and exhibits the expected diurnal variations of minimum before local sunrise and maximum in noontime or afternoon hours. The magnitude and time of diurnal maximum vary with season and solar activity as well as latitude. H+ density, which is about 1 order of magnitude lower than that of O+ during the daytime in years of low solar activity, is further reduced with respect to O+ density as solar activity increases. The concentration of He+ is less than that of H+ during the solar minimum. However, during periods of moderate to high solar activity, the He+ concentration equals or is higher than that of H+. Day and nighttime O+ bear a good positive correlation with F-10.7 cm solar flux, while H+ declines with increases in solar flux. O-2(+) is a minor constituent at the altitude of similar to 500 km irrespective of solar activity and season of observation. It is observed that the O-2(+) concentration increases approximately twofold from solar minimum to solar maximum. Daytime He+ does not vary with solar flux, while the nighttime concentration of He+ increases marginally with increases in solar flux.