# Advances in Ionospheric Space Weather by Using FORMOSAT-7/COSMIC-2 GNSS Radio Occultations
## 2 Intense GNSS RO Soundings of the Ionosphere  
The scientific community, especially the atmospheric and ionospheric researchers, were widely anticipating the launch of F3/C because of the unique observations that, for the first time, offered 3D profiles of meteorological parameters as well as the ionospheric electron density spanning across the globe. Figure 1a provides a comparison of the distributions of the daily occultations from F3/C and those from F7/C2 until 1 March 2022. During the initial 1-year period after the launch, the F3/C averaged about 2500 occultations per day, which occasionally crossed 3000. Note that during this period, the satellites were mostly clustered together, and undergoing various orbital maneuvers to deploy each satellite to their final obits, approximately 60\({}^{\circ}\) apart. Since then, until the middle of 2010, the average occultations were about 1500, which afterward dropped to about 1000-1200 and remained mostly so until the end of 2015. The last 5 years of the F3/C saw the occultations falling to much smaller numbers, with a couple of satellites being forced to terminate operations due to technical problems. Figure 1: Comparison of F3/C and F7/C2 occultations and distribution of tangent locations. (**a**) All available occultations from F3/C mission from 2006–2020 (green) and the daily occultations from F7/C2 until 1 March 2022 (red). The number of occultations on a given day denotes the total number of retrieved electron density profiles (ionPrf). (**b**) The NmF\({}_{2}\) locations (green dots) from F3/C observations during January 2007 at 0600 UT. (**c**) Same as (**b**), but from F7/C2 during January 2022. The dashed curve denotes the magnetic equator. “nRO” stands for number of occultations, and “nRO (\(\pm 35^{\circ}\))” in panel (**b**) represents the number of occultations within \(\pm 35^{\circ}\) latitudes. In contrast, the number of occultations from the F7/C2 mission gradually increased after the launch, where the occasional intervals of reduced soundings corresponded to the phases when the satellites were being moved down from their initial parking orbits at about 700 km to their final mission altitudes at about 550 km altitude. The number of occultations gradually increased from the initial range of 1000-2000, reaching 3000-4000 daily occultations from early 2020, which have appeared to remain consistent at around 4000-4500 since the second quarter of 2021. Comparing the performances, except for the early months of the launch, F7/C2 achieves more than two times the number of occultations provided by F3/C. Clearly, the F7/C2, with its Tri-G capability, outperforms its predecessor in terms of the number of daily soundings, which is welcome news to the community. It is also worth comparing the spatial distributions of the occultations. In Figure 1b, the month of January 2007 was selected to examine the RO locations for F3/C, which falls within the period of its best performance (Figure 1a). For F7/C2, a corresponding 1-month period in 2022 is plotted in Figure 1c. The figure shows the geo-locations of the F-peak electron density (NmF\({}_{2}\)) during a 1 h period at 0600 UT. Though the spatial distribution of the occultation locations is comparatively sparse, the F3/C is advantageous when it comes to global coverage, scanning both high- and low-latitudes alike. Owing to the low-inclination orbit, the F7/C2 occultations are mostly within about \(\pm\)35\({}^{\circ}\) latitudes, which effectively enhances the spatial resolution. The figure demonstrates that owing to the denser sampling of the low-latitude regions, the F7/C2 could unravel finer details of the thermosphere-ionosphere dynamics when compared to F3/C. It can be further seen from Figure 1b that, though F3/C provided approximately 3000 occultations during the plotted period, only \(\sim\)1300 soundings out of that fell within \(\pm\)35\({}^{\circ}\) latitudes.