Features of the F2 layer stratification at low-latitude ionosphere: Results from the COSMIC and GIRO

Global observations of electron density profile (EDP)from the COSMIC/FORMOSAT-3 satellites were used to investigate, for the first time, the additional stratification of the F2 layer over the equatorial ionosphere on a global scale, which is called F3 layer. The F3 layer in EDP was recognized through the altitude differential profile featured by two maxima existing from 220 km to the peak height of the electron density. Statistical results show that the highest occurrence of F3 layer appears at dip latitude 7~8°/-7~-8° for Northern/Southern Hemisphere and is more pronounced during summer months at 10:00-14:00 LT and 20:00-21:00 LT. The occurrence also has a clear longitude dependence during boreal summer, with relatively higher occurrence at -80~-100°, -20~20°, 80~120° and -160~-170° longitudes, that is possibly associated with the DE3 tide. Meanwhile, the characteristic of the sunset F3 layer is first detected in the COSMIC EDPs, and compared with the features derived from the GIRO stations at Jicamarca, Sao Luis, Fortaleza, Kwajalein, Sanya and Vanimo at IPS. Evidence shows that the local time distribution of the occurrence of the F3 layer can extend to the postsunset time (1800-2100 local time). The sunset F3 layer has a strong seasonal dependence occurring mainly during the summer time and with a preference at Jicamarca. Unlike the daytime F3 layer, the occurrence of the sunset F3 layer clearly increases with increasing solar activity. These features of the dependence on the season, solar activity, and longitude are clearly related to the geomagnetic control of the evening prereversal enhancement of the equatorial zonal electric field and meridional winds modulated by the geomagnetic configuration.