Interhemispheric asymmetry of the equatorial ionization anomaly in solstices observed by COSMIC during 2007-2012

The ionospheric radio occultation measurements from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites during 2007-2012 are utilized to analyze variations of the hemispheric asymmetry in the equatorial ionization anomaly (EIA) in solstice seasons. A hemispheric asymmetry index (A(h)) is introduced to quantitatively characterize the relative interhemispheric asymmetry or north-south differences of the EIA crests. The seasonal, solar activity, and longitudinal dependences of the hemispheric asymmetry are investigated during these low to moderately high solar activity levels. Our main findings are as follows: (1) The transition of stronger crest from the winter to summer hemispheres generally took place during noon to afternoon hours (similar to 11:00-14:00LT), whereas the change of the EIA hemispheric asymmetry pattern toward a stronger summer crest actually started 2-3h earlier. (2) Remarkable differences were observed in the EIA hemispheric asymmetry between June and December solstices. In June solstice, the transition from stronger winter to summer crests occurred later, and consequently, the duration with stronger winter crest lasted longer during the morning period. (3) In the afternoon hours, the magnitudes of relative interhemispheric asymmetry of EIA crests were obviously anticorrelated with the solar activity, while in the morning hours their solar activity dependence was weak. (4) Interhemispheric asymmetry of the crests and its transition time varied with longitudes. It is also found that the longitudinal and seasonal variations of the change rate of A(h) was generally consistent with the corresponding variations of hemispheric asymmetry in ion production rate. Our results indicate that the photochemical process may also play a role in the EIA hemispheric asymmetry, in addition to the important contribution from the transequatorial neutral wind and equatorial fountain effects.