# Advances in Ionospheric Space Weather by Using FORMOSAT-7/COSMIC-2 GNSS Radio Occultations
## 3 Equatorial and Low-Latitude Ionosphere
### Equatorial Ionization Anomaly  
One of the fascinating features in the distribution of ionospheric electron density is the equatorial ionization anomaly, manifesting as two enhanced peaks (crests) of electron density surrounding a narrow belt of lower values of ionization (trough) roughly centered over the magnetic equator during the daytime [71; 72]. Being the locus of the largest electron density, the EIA crests often attract the attention of the ionospheric community, which offers a crucial vantage point to investigate the equatorial and low-latitude ionospheric electrodynamics dominated by the equatorial plasma fountain and neutral wind [73; 74]. These dynamic processes in the formation of EIA are susceptible to energy input from high latitudes [75; 76; 77; 78], waves, and tides that propagate from the lower atmosphere [79; 80; 81; 82; 83], making the variabilities in the EIA electron density an effective sensor to monitor coupling from other regions. Figure 2 depicts the horizontal and vertical distributions of the low-latitude electron density by combining 1-month F7/C2 measurements during a 1 h period centered at  
1200 UT. The figures, adopting a similar format used by [PERSON] et al. [38], highlight the potential of F7/C2 in providing the same information by combining only about a quarter of the time duration of observations that was required for the F3/C. The figure displays the monthly mean distributions of electron density in March, June, September, and December of 2021, revealing the seasonal characteristics. The largest electron densities occurred at the EIA latitudes over 250-300 km altitudes, spread across the longitudes of 60\({}^{\circ}\) W to 120\({}^{\circ}\) E. Notable seasonal differences existed in the low-latitude electron density, which was most pronounced in March and December with June yielding the lowest densities amongst the four months plotted. In the equinox months of March and September, the EIA peaks were more-or-less symmetric with respect to the magnetic equator. Though the overall distributions were mostly identical in the equinoxes, the electron density in March appeared slightly larger than that in September, revealing equinoctial asymmetry. Further, in September, there was an enhanced region of electron density over the west African and Asian longitudes of 60\({}^{\circ}\)-120\({}^{\circ}\) E in the Northern Hemisphere, causing a substantial deviation from the equinox distribution and resembling northern summer conditions. Figure 2: The 3D F7/C2 electron density structure at 12:00 UTC time in (**a**) March, (**b**) June, (**c**) September, and (**d**) December of 2021. The horizontal maps reveal well-developed EIA bands, being pronounced around 250–300 km altitudes over 60\({}^{\circ}\) W–120\({}^{\circ}\) E longitudes. The plasma caves appeared below 250 km in 0\({}^{\circ}\) E and 60\({}^{\circ}\) E longitude slices in the top panel. The median values of all the observations for each month at the selected hour are arranged in grids of 2.5\({}^{\circ}\)\(\times\) 2.5\({}^{\circ}\) longitude–latitude, and 5 km altitude. Note that equinox asymmetry in the electron density distribution has been reported with larger F-region electron density during March equinox compared to September equinox, which is attributed to asymmetries in neutral wind, temperature, and vertical plasma drifts [84; 85]. Further, [PERSON] et al. [86] examined the role of DE3 (diurnal eastward propagating wavenumber-3) tides in affecting the vertical ExB drifts and contributing to the asymmetry and suggested the asymmetry to be pronounced during low solar activities. [PERSON] [87] reported the north-south asymmetry in the electron density distribution in equinoxes over African and west Asian longitudes. [PERSON] et al. [43] examined the equinoctial north-south asymmetry over different longitudes and showed that the asymmetry was pronounced over the longitudes where the displacement of the geomagnetic equator from the geographic equator is larger.