Equinoctial asymmetry in low latitude ionosphere as observed by SROSS-C2 satellite

Abstract The ionospheric plasma parameters (electron, ion temperatures and ion composition— T e , T i , O + and H + ) measured by SROSS-C2 satellite at an average altitude of ~500 km has been investigated to study the behaviour of the ionosphere in equinoxes during half a solar cycle (year 1995–2000, F 10.7 ~70–195). The region under study spans over 5–35°N geog. latitude and 65–95°E geog. longitude in the Indian sector. We found an equinoctial asymmetry in the diurnal behaviour of T e , T i , O + and H + varying with increase in solar activity. The strength of equinoctial asymmetry in T e and T i is strong during early morning and daytime and strength decreases with increase in solar activity whereas during night time no asymmetry/weaker is observed in low/high solar activity respectively. During the day time, a very strong equinoctial asymmetry in O + is observed during solar minimum which diminishes with increase in solar activity. The similar diurnal behaviour of H + as that of O + is observed during low solar activity but no clear equinoctial asymmetry is observed during solar maximum, as H + being highly dynamic. The transition height (O + /H + ) is the lowest in early morning during solar minimum, which increases during local day-time. The rate of increase in transition height is different in both the equinoxes (higher in vernal than autumn) with respect to dependence on the solar activity, during daytime. Hence equinoctial asymmetry is stronger during solar minimum period than maximum, with higher/lower transition height in vernal during daytime/nighttime respectively.