The magnitude and inter-hemispheric asymmetry of equatorial ionization anomaly-based on CHAMP and GRACE observations

Based on nearly nine years (2001-2009) of observations from CHAMP and GRACE, a comprehensive study has been made on the morphology of the equatorial ionization anomaly (EIA), focusing on the EIA's magnitude, inter-hemispheric asymmetry by resolving their seasonal and local time variations at different altitudes and solar activity levels. The electron density and the magnetic latitudes of the EIA crests both peak around 1400 LT while the crest-to-trough ratio (CTR) of the EIA reaches its highest value post-sunset around 2000 LT, with a value almost twice the daytime level. The magnetic latitude of the EIA at CHAMP altitude (~400 km) can reach 13° around December solstice during both high and low solar activity years, while at GRACE altitude (~480 km) the crests are observed much closer to the dip equator during low solar activity years. During high solar activity years the averaged apex height of the EIA crests can reach 800 km. During solstice seasons a clear inter-hemispheric asymmetry of the EIA can be seen. At CHAMP altitude the electron density of the EIA crest is stronger in the winter hemisphere during morning to noontime hours. It reverses after the noontime and the transition time appears around 1400 LT and 1200 LT for high and low solar activity years, respectively. At higher altitude (GRACE), the electron density of the EIA crest is always stronger in the summer hemisphere over the whole daytime. Simulation results from the SAMI2 model also show the differences in EIA inter-hemisphere asymmetry at the two altitudes.