# Advances in Ionospheric Space Weather by Using FORMOSAT-7/COSMIC-2 GNSS Radio Occultations
## 3 Equatorial and Low-Latitude Ionosphere
### Equatorial Ionization Anomaly
In addition to such equinox asymmetry, the figure also shows a large difference in electron density during solstices, with a much stronger electron density in December than in June. This is attributed to the December anomaly or the annual anomaly [88] with more electron density occurring in the December solstice than in the June solstice. The hemispheric asymmetry or winter anomaly of the EIA peaks in the solstice months is also evident in the figure. The vertical slices in the top panels illustrate the artificial plasma cave feature, first shown by [PERSON] et al. [39] by using F3/C observations. Interested readers may refer to [PERSON] et al. [60] and the references therein for a detailed discussion of the plasma cave feature and its significance. The illustrations given in Figure 2 and the features discussed show the importance of having more dense observations to unravel finer details of the ionospheric electron density distribution. A movie constructed by using such plots for all the 12 months in 2021 is provided in the Supplementary Materials (Movie S1), demonstrating the seasonal transition of the EIA development. The monthly electron density maps reveal further seasonal characteristics of electron density. For example, larger electron densities occurred in October, whereas earlier average plots combining 60 or 90 days of measurements would not be able to capture such seasonal transitions. Since the F7/C2 measurements are limited to the low-latitude regions, we further focused on the diurnal evolution of EIA crests in Figure 3a-d, which shows the development of EIA crests at 120\({}^{\circ}\) E longitude in the solstice months of June and December, as well as the equinoctial months of March and September. June and December months reveal the general EIA morphology discussed by [PERSON] et al. [38], with the electron density first appearing in the summer hemisphere followed by the characteristic winter anomaly showing enhanced EIA crests in the winter hemisphere in the morning sector, which gradually reverses in the afternoon period, yielding overall larger densities in the summer hemisphere. [PERSON] et al. [89] explained the combined theory of trans-equatorial neutral wind, subsolar point, and auroral equatorward wind in summer and winter solstices, producing such seasonal variations of the EIA crests. In the equinox months, there was a noticeable difference in the EIA development between March and September months. In March, the electron density started to enhance over the magnetic equator, and after 1000 LT, the northern EIA crest began to develop. The southern crest also built up by around 1200 LT, with both the crests enhancing symmetrically in the subsequent hours, exhibiting a typical equinox behavior. However, in September, the EIA development was somewhat similar to solstice conditions, with the Northern Hemisphere having much stronger density than in the Southern Hemisphere. Such strong hemispheric asymmetry is unusual in the September equinox period, which suggests a possible delay effect. The detailed results showed that the EIA development in October was similar to that in March (Movie S2 in the Supplementary Materials). Figure 3: _Cont._  
Figure 3: (**a**) Local time evolutions of the ionospheric electron density structure at the mid- and low-latitude regions at 120\({}^{\circ}\) E longitude sector from 0100 LT to 2300 LT in June 2021. The binning method was same as that for Figure 2. Gray curves represent the magnetic field lines. (**b**) Same as (**a**), but for December 2021. (**c**) Same as (**a**), but for March 2021. (**d**) Same as (**a**), but for September 2021. On the other hand, there was a clear seasonal asymmetry in the morning hours where the electron density in the winter hemisphere generally was greater than that in the summer hemisphere in June and December. The electron density in December was much larger than in June, which may partly be due to the December anomaly [88].