Diurnal, seasonal and latitudinal variation of ion concentrations as obtained from the SROSS C2 satellite over Indian low and equatorial latitudes during solar minimum Original Research Article

In situ measurements carried out by the Retarding Potential Analyzer on board the SROSS C2 satellite at equatorial and low latitudes during the low solar activity period of 1995 and 1996 are used to study the diurnal, seasonal and latitudinal variations of the concentrations of O+, H+, He+ and O2+ ions at the average altitude of ∼500 km in the Indian longitude sector. The concentration of the major ion O+ is minimum before local sunrise and reaches its peak near local noon or in the afternoon hours depending on season and latitude of observation. The minimum and maximum densities vary with season and latitude. The H+ concentration is found to be one order of magnitude less than that of O+ during the daytime at all latitudes. At night, hydrogen ion concentration is comparable to the concentration of atomic oxygen ion, which indicates a lowering of the O+ to H+ transition altitude in this period. The ratio of night to day H+ concentration is higher in summer than in winter and equinox. The measured concentration of He+ is one order of magnitude less than that of H+. Helium ion concentration is higher in the December solstice compared to that in the June solstice in the forenoon hours while in the afternoon the relative densities in the solstices reverse in magnitude. The He+ density is low at night and remains at a constant higher level during the daytime. The low nighttime He+ densities indicate a higher O+ to He+ transition altitude compared to O+ to H+ transition altitudes. O2+ concentration shows higher daytime values with peak in the midday or afternoon hours but it is a minor constituent at ∼500 km during solar minimum over the Indian longitude sector compared to H+ or even He+.