Equinoctial asymmetry in ionosphere over Indian region during 2006–2013 using COSMIC measurements

Abstract In this study, the ionospheric equinoctial asymmetry (EA, defined as different ionospheric behaviour in the two equinoxes) is investigated in the maximum electron density (NmF2) and the corresponding altitude (hmF2) over the Indian region (∼5–30° N geog. lat., 60–100° E geog. long.) during quiet geomagnetic conditions (Ap ≤ 20) and varying phase of solar activity (2006–2013, F10.7 ∼65–192 sfu) using COSMIC observations. Both NmF2 and hmF2 exhibited strong solar-activity dependence and which increased with the increase in solar activity throughout the day. We found EA in NmF2 is not just a daytime phenomenon but also occurs in morning and nighttime during high solar-activity. The EA in NmF2 over equatorial-low latitude is found solar-dependant in nighttime and morning (weaker/stronger in low-/high-solar activity) whereas stronger/weaker in noontime in low-/high-solar activity years (higher NmF2 in vernal in 2007–2009, 2013) respectively. Whereas for hmF2, EA is stronger in nighttime and weaker in morning for all years over both equatorial and low-latitudinal regions and weaker/moderate in low-/high-solar activity for noontime respectively. We found F10.7P and Dst index in the equinoxes playing a dominant role throughout the day and the chemical compositions (O, N 2 and neutral mass densities) also a responsible factor for the EA in NmF2 and hmF2 in the morning and nighttime respectively.