Seasonal and longitudinal (s/l) variations of ion density structure near 600 km altitude in the low-latitude ionosphere, observed by ROCSAT-1 for two similar high solar activity years of 2000 and 2002, are examined in five different local time (LT) sectors. The gross features in the s/l variations of density structure during the daytime are very similar to each other in these 2 years. A reproducibility of the daytime density structure can thus be assumed, including the shape of equatorial ionization anomaly (EIA), for similar solar flux input. The nighttime structure during the solstice shows a shape-preserving decay in the latitudinal variation and maintains similar density level for these 2 years. In contrast, the nighttime structure in the equinox indicates noticeable differences in the 2 years. A strong global transport effect that exists in the solstice can be identified as the cause of such difference in density structure between the solstice and the equinox. The model results from both the 2006 International Reference Ionosphere model and the thermosphere ionosphere electrodynamics general circulation model cannot reproduce the global, seasonal, and local time (LT) variations of the low-latitude ionospheric structure observed by ROCSAT. The observed longitudinal variation of the crest-to-trough ratio of the average background EIA structure in the 1800-1900 LT sector fails to correlate with the longitudinal variation of the postsunset equatorial irregularity occurrences in a season. There is also no correlation found between the seasonal variations of the crest-to-trough ratio and the irregularity occurrence at almost all longitudes, except at the longitudes of South America-Atlantic sector where some correlation seems to exist.
