# Advances in Ionospheric Space Weather by Using FORMOSAT-7/COSMIC-2 GNSS Radio Occultations
## 3 Equatorial and Low-Latitude Ionosphere
### Wavenumber Four and Plasma Depletion Bay
It is expected that the denser RO observations by F7/C2 may play a major role in further exploring the different nature of the forcings and help to gain a better insight into the complex wave interactions in the upper atmosphere and how such different factors may contribute to the seasonal and inter-annual variations as well as hemispheric asymmetry of the wavenumber-4 modulation. In addition, the GIS electron density profiles constructed by using F7/C2 observations offer the unique opportunity to explore day-to-day variability of such longitudinal modulations [67] and a more detailed understanding of the electron density modulations forced from the variations below, such as the ionosphere EIA response to SSW events [68]. Apart from the wavenumber-4 longitudinal modulation of EIA crests that is widely investigated, a new type of electron density structure was reported by [PERSON] et al. [40], manifesting as a large-scale depletion of electron density over certain longitude regions. This new feature, named plasma depletion bay (PDB), was identified by examining the 3D global electron density maps during nighttime by combining about 60 days of F3/C observations at fixed local-time frames. The results showed a single and intense PDB originating from the Northern Hemisphere (north PDB) over 75-135\({}^{\circ}\) W during the months of October-March and three such PDB features originating from the Southern Hemisphere (south PDBs) over the North Atlantic (60\({}^{\circ}\) W to 30\({}^{\circ}\) E), Indian Ocean (45-110\({}^{\circ}\) E), and Southeast Asia (120-170\({}^{\circ}\) E) longitudes during April-September. Unlike the wavenumber-4  
Figure 5: Monthly plots of the F7/C2 electron density at 400 km, 1500 LT (**left two columns**) and 275 km, 2300 LT (**right two columns**) in 2020.  
modulations, the PDB structure pertained to nighttime plasma dynamics, being prominent at around 2300 LT, and their generation was attributed to the field-aligned plasma transport by the magnetic meridional wind from summer to winter hemisphere [40]. The global maps at 2300 LT in the right columns in Figure 5, constructed by using F7/C2 observations at 275 km altitude, illustrate the evolution of the north PDB and the south PDBs. When compared to the results using F3/C [40], the F7/C2 electron density offers more distinct monthly variations, with the PDB feature appearing pronounced in the solstice months and remnant wavenumber-4 modulation persisting in the equinox months. The PDB morphology, especially that of the north PDB, with the depleted plasma regions aligned normal to the magnetic equator, ascertains the peculiar role of the field-aligned component of the summer-to-winter neutral wind in removing the ionization in both the hemispheres alike. Additionally, note that the PDBs originate in the summer hemisphere over the longitudes where the separation between geomagnetic and geographic equators is largest. In other words, the PDBs occur near the longitudes where the magnetic equator is more offset to the summer hemisphere. Over such longitudes, the magnetic meridional wind is more effective in the inter-hemisphere plasma transport along the field lines [43]. Such features of the low-latitude ionosphere, especially over the EIA region, highlight how the GNSS RO measurements continue to play an important role in advancing the current understanding of the low-latitude plasma dynamics and atmosphere-ionosphere coupling. The comparison of the major findings by using the F3/C measurements provided here and re-producing similar results based on the F7/C2 data demonstrate how the increased observations help to unravel additional details of the low-latitude variations. The advantage of the current F7/C2 sampling is the ability to construct global electron density maps at every hour in each month, offering more detailed insights into diurnal and seasonal features in the electron density variations.