Global, seasonal, and local time variations of ion density structure at the low-latitude ionosphere and their relationship to the postsunset equatorial irregularity occurrences

摘要

Seasonal and longitudinal (s/l) variations of ion density structure near 600 km altitudein the low-latitude ionosphere, observed by ROCSAT-1 for two similar high solar activityyears of 2000 and 2002, are examined in five different local time (LT) sectors. Thegross features in the s/1 variations of density structure during the daytime are very similarto each other in these 2 years. A reproducibility of the daytime density structure can thusbe assumed, including the shape of equatorial ionization anomaly (EIA), for similarsolar flux input. The nighttime structure during the solstice shows a shape-preservingdecay 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 the2 years. A strong global transport effect that exists in the solstice can be identified as thecause of such difference in density structure between the solstice and the equinox. Themodel results from both the 2006 International Reference Ionosphere model and thethermosphere ionosphere electrodynamics general circulation model cannot reproduce theglobal, seasonal, and local time (LT) variations of the low-latitude ionospheric structureobserved by ROCSAT. The observed longitudinal variation of the crest-to-trough ratioof the average background EIA structure in the 1800-1900 LT sector fails to correlatewith the longitudinal variation of the postsunset equatorial irregularity occurrences in aseason. 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 thelongitudes of South America-Atlantic sector where some correlation seems to exist.