Longitudinal effects on equatorial ionization anomaly observed by FORMOSAT-3/COSMIC and FORMOSAT-7/COSMIC-2 radio occultation data

The present study investigates the characteristics of the equatorial ionization anomaly (EIA) in terms of its structures and dynamics by using the ionospheric electron density profiles obtained from radio occultation (RO) measurements conducted by FORMOSAT-7/COSMIC-2 (F7/C2). The results are verified through the comparison with data from FORMOSAT-3/COSMIC (F3/C) RO profiles. The analysis focuses on four longitudinal regions that exhibit different magnetic declinations: (a) "DP" with the most positive-declination, spanning from -165°E to -105°E in geographic longitude, (b) "DN" with the most negative-declination, covering from -60°E to 0°E in geographic longitude, (c) "DH" with zero-and-negative-declination, ranging from 15°E to 75°E in geographic longitude, and (d) "DZ" with around zero-declination, covering from 90°E to 150°E in geographic longitude. In the DP, the EIA crest exhibits an earlier occurrence of the maximum in the local time, a more northern position, a stronger intensity, and an opposite trend in the DN. Moreover, except during the equinox, the altitude of the EIA crest maximum in the southern hemisphere is higher in the spring season, and lower during summer and winter, while the DN region exhibits an opposite trend. Additionally, while the trans-equatorial wind plays a crucial role in the different longitudinal sectors, the offset between the geomagnetic and geographic equators is another significant factor, particularly when the seasonal wind effect is not prominent.