FORMOSAT-3/COSMIC observations of seasonal and longitudinalvariations of equatorial ionization anomaly and its interhemisphericasymmetry during the solar minimum period

摘要

Using an extremely valuable global data set from Formosa Satellite (FORMOSAT-3)/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)radio occultation experiment, a comprehensive study has been carried out on the seasonaland longitudinal variations of equatorial ionization anomaly (EIA) and the temporalvariation in the hemispheric asymmetry of EIA during the low solar activity period fromNovember 2006 to October 2007. The interesting result observed from this investigation isthe local-time-dependent variation in the hemispheric asymmetry of EIA. During thesolstices, it has been consistently observed that the EIA crest in the winter hemisphereappears stronger than that in the summer hemisphere during morning to noon hours. Incontrast to this, during noon to early afternoon hours, the ionization in the winter EIAcrest decreases rapidly, and the crest in the summer hemisphere becomes more intensifiedthan that in the winter hemisphere. Further, this transition of stronger EIA crest fromwinter hemisphere to summer hemisphere occurs around 1200-1300 LT during theDecember solstice months and is delayed by a couple of hours (seen around -4400 LT)during June solstice months. The causative neutral and electrodynamical mechanisms arediscussed in light of relative contributions from the field-aligned plasma transport dueto transequatorial interhemispheric neutral wind, the strength of the equatorial fountainprocess, and the ion drag effects during different local times and seasons. The results fromthe Sami2 is Another Model of the Ionosphere (SAMI2) model simulation also exhibitsimilar local-time-dependent variation in the hemispheric asymmetry of EIA, whichfurther supports our argument. Also, it was observed that the large magnetic declination ofthe field lines and the four-peaked longitudinal structure of EIA can significantly modulatethe interhemispheric asymmetry of EIA even during the equinoxes.