# Global GNSS-RO Electron Density in the Lower Ionosphere
## 3 Results
### E- to F-Region Transition  
The onion-peeling approach often produces negative or large oscillatory \(N_{e}\) values in the lower ionosphere [38]. These large errors make the onion-peeling retrieval unsuitable for studying the \(E\)- to \(F\)-region transition. The new \(N_{e}\) data, such as those with a higher RO top (e.g., Spire), can be used to describe the plasma transition from the \(E\)- to \(F\)-region with continuous height coverage and high spatiotemporal sampling. Figure 12 presents an example of monthly \(N_{e}\) statistics from such observations where high-RO-top (e.g., Spire) RO data are used. The retrieval covers an altitude range between 60 and 170 km and the data are averaged to 2-hourly bins in local time. More displays of Spire monthly climatology can be found in Figure S3 for the zonal mean and in Figure S4 for diurnal variations at the magnetic equator. One of the interesting features in the 2-hourly zonal mean (Figure 12) is the shift of the daytime \(N_{e}\) maximum from the SH \(E\)-region to the NH lower \(F\)-region. This shift implies a height-varying \(\chi\) dependence of the \(N_{e}\) morphology in the lower \(F\)-region. Again, the equatorial dip in \(N_{e}\) is obvious in the late afternoon and early evening hours (i.e., LST = 15 h, 17 h, and 19 h), continuing through all altitudes in the \(E\)- to lower \(F\)-region.  
Figure 12: \(E\)- to \(F\)-region transition in \(N_{e}\) for December 2021 from the Spire constellation. The Spire receivers acquire GNSS-RO routinely with a top up to \(\sim\)175 km, allowing the \(N_{e}\) retrieval to be extended to the \(F1\)-region.  
The polar features from auroral electron precipitation are more pronounced in the nighttime zonal mean in Figure 12. It is also known as the polar winter anomaly where ionospheric absorption of radio waves becomes extraordinarily strong [75]. Relatively speaking, they have a nearly uniform concentration in the lower \(F\)-region, but with a greater extension to the \(E\)-region compared to the low- to midlatitude \(N_{e}\). The penetration to the \(D\)/\(E\)-region seems to be deeper around the midnight hours in the NH. Compared to the \(D\)/\(E\)-region, the nighttime \(N_{e}\) distribution in the lower \(F\)-region is less symmetric about the equator. In the early evening hours, the lower \(F\)-region appears to have a higher \(N_{e}\) in the winter subtropics, but the distribution peak shifts to the summer subtropics during the predawn hours.  
The results presented in Figure 12 are based on the sampling from a subset of the Spire constellation data provided to the NOAA's CWDP program, which have \(\sim\)4000 daily RO profiles. Processed by the same COSMIC software to Level-1 and Level-2, these data have only become available on CDAAC for the research community since September 2021. As the constellation continues to expand, it is anticipated that more RO profiles will yield a further improved diurnal coverage on a monthly basis, perhaps to enable one-hour intervals in local time and a spatial resolution better than the \(8^{\circ}\times 4^{\circ}\) longitude-latitude grid used in this study. One of the advantages with dense spatial sampling is to allow a tomographic retrieval of \(N_{e}\), which can help to mitigate the horizontal inhomogeneity problem associated with the poor horizontal resolution of the GNSS-RO technique.