# Global GNSS-RO Electron Density in the Lower Ionosphere
## 4 Discussions  
### Comparisons with Ionosonde Data at Hermanus  
As a preliminary validation effort, the GNSS-RO \(E\)-region \(N_{e}\) profiles are compared against ionosonde \(N_{e}\). The Hermanus (HE13N), South Africa (34.4\({}^{\circ}\)S, 19.2\({}^{\circ}\)E) ionosonde site was selected for the comparison because of the low \(f_{\rm min}\) (minimum return frequency observed by the ionosondes) values over the period of interest: July 2008-August 2010. A low \(f_{\rm min}\) value in the ionogram measurement is vital to derive the \(E\)-region \(N_{e}\) profiles. To collocate the GNSS-RO observations, RO tangent points were required to be within 2\({}^{\circ}\) of the Hermanus site and within 30 min of an ionosonde measurement that has an \(f_{\rm min}\) below 1.2 MHz. The ionosonde data were obtained from the Digital Ionogram Database (DIDBASE, accessed 17 January 2022, [[https://giro.uml.edu/didbase/](https://giro.uml.edu/didbase/)]([https://giro.uml.edu/didbase/](https://giro.uml.edu/didbase/))), and all ionograms were hand-scaled using the SAO Explorer application for ionosondes ([[https://ulcar.uml.edu/SAO-X/SAO-X.html](https://ulcar.uml.edu/SAO-X/SAO-X.html)]([https://ulcar.uml.edu/SAO-X/SAO-X.html](https://ulcar.uml.edu/SAO-X/SAO-X.html)), accessed 1 March 2022) to eliminate uncertainties caused by automatic scaling and ensure the quality of ionograms used in the comparison. The ionogram with sporadic-\(E\) or an uncertain \(E\)-region profile was excluded in the comparison. Over this two-year period, a total of 60 ionograms were hand-scaled for the comparison against the RO profiles. Figure 13: Solar cycle variations of daytime **(top)** and nighttime **(bottom)**\(N_{e}\) at 70 km as a function of magnetic latitude from the combined MetOp-A/B/C and SAC-C (before 2008) monthly averages. The longterm variations at other altitudes (60 and 80 km) can be found in Figure S5. The time series of daily Ly-\(\alpha\) is the overplotted white line. The Ly-\(\alpha\) data are obtained from the GSFC’s Space Physics Data Facility (SPDF) OMNIweb service. Two examples of the ionosonde vs. RO \(N_{e}\) comparisons are displayed in Figure 14. The vertical line corresponds to the minimum return frequency measured by the ionosonde, \(f_{\rm min}\). The ionograms show \(E\)-region returns from \(f_{\rm min}\) up to \(f_{\rm o}E\), but no measurements are obtained for the \(E\)- to \(F\)-region valley. Therefore, this comparison is limited to the altitude range between the \(f_{\rm min}\) altitude and \(h_{m}E\), which varies with the time of day, location, and ionospheric conditions. As shown in Figure 14, the typical starting height for ionosonde measurements is 90 km, while GNSS-RO is capable of measuring densities at \(D\)-region altitudes (see Section 3). The \(N_{\rm m}E\) estimated from the RO and ionosonde techniques are in close agreement for these two examples, and the bottomside \(E\)-layer \(N_{\rm e}\) gradients with respect to altitude (d\(N_{\rm e}\)/dz) are also in agreement. While many of the profiles show agreement in d\(N_{\rm e}\)/dz and \(N_{\rm m}E\), the profiles are shifted in altitude with RO profiles being slightly higher, by 2 km on average, above the ionosonde profiles. This relatively small altitude difference may be due to the \"internal uncertainty of the starting height of the profile\" for ionosonde measurements [78]. The ionogram inversion assumptions and uncertainties were described in detail in several studies [79, 80, 81], revealing challenges in converting measured virtual heights to real heights for \(N_{e}\). On the other hand, errors from ionospheric bending and derivation of the Earth ellipsoid model may also induce an uncertainty in the GNSS-RO H\({}_{\rm SL}\), but it is expected to be \(\leq\)1 km. For this comparison, we show the results obtained for both unshifted (original) and shifted profiles. The shifted profiles are obtained by adjusting the ionosonde \(N_{e}\) altitude to minimize the altitude difference between profiles at \(f_{\rm min}\).