# Global GNSS-RO Electron Density in the Lower Ionosphere
## 3 Results
### Magnetic-Field Dependence  
In addition to the diurnal variation, the \(D\)/\(E\)-region \(N_{e}\) can be significantly modulated by the magnetic field. To illustrate the magnetic-field effect, we averaged the COSMIC-1 \(N_{e}\) data from all LSTs and produced a monthly map for each altitude as a function of geographical longitude and latitude. Because the \(N_{e}\) at low and midlatitudes is dominated by the daytime climatology, the all-time averages appear similar to the daytime maps except for high latitudes. Figure 10 shows the 70 km maps, but the \(N_{e}\) distributions at the \(E\)-region altitudes are similar. A complete monthly climatology of all-LST maps can be found in Figure S2. The features observed below are generally applicable to the COSMIC-1 data at 70-120 km. As shown in Figure 10, magnetic-field modulation of the \(D\)/\(E\)-region \(N_{e}\) is present in all months where the longitudinal variations follow the field line at most latitudes. There is an important distinction between the high-latitude and low- and midlatitude variations on what magnetic fields are in control. At high latitudes, as expected for the domination of auroral electron precipitation, the \(N_{e}\) distribution is correlated better with the magnetic field in the magnetosphere (e.g., at one Earth radius, \(R_{e}\)). As seen Figure 10 (left panels), the correlation appears worse if it is compared to the magnetic pitch angles from the field at 100 km, but the distribution of auroral electron precipitation correlates better with the pitch angle derived from the magnetic field in the magnetosphere. In addition to the direct contribution from electron precipitation, the winter polar vortex is capable of transporting the aurora-produced NO down to the mesosphere and further enhancing the nighttime \(D\)-region \(N_{e}\)[66]. The fact that the polar \(N_{e}\) enhancements are closely associated with the magnetic pitch angle implies that such vertical transport from the polar vortex only plays a secondary role in the polar winter anomaly. As shown in the RO observations, the auroral electron precipitation does not produce a very strong polar \(N_{e}\) distribution as simulated under different hypothesized scenarios for the \(D\)-region ion chemistry [67]. It is worth noting that the summer-midlatitude \(N_{e}\) distributions in Figure 10 (June and December) bear a striking resemblance to the global \(E_{s}\) morphology reported in the earlier studies with GNSS-RO [18; 19; 20]. As stressed by [PERSON] [31], the new \(D\)/\(E\)-region \(N_{e}\) data will allow a more comprehensive study of the \(E_{s}\) formation and its connection to the background \(E\)-region \(N_{e}\) concentration. The similar distribution between \(E_{s}\) and \(N_{e}\) suggests that a common process is modulating their longitudinal variations in the summertime midlatitudes. It remains challenging to fully understand what processes drive large longitudinal variations along the same field pitch angle (Figure 10). Planetary waves, such as the migrating and non-migrating tidal waves, can perturb the \(D\)/\(E\)-region \(N_{e}\) distribution by modulating the NO density and temperature in the upper mesosphere. In addition, some observations show that the lightning from tropospheric thunderstorms can induce a direct decrease in the \(D\)-region \(N_{e}\)[68] or sharpening in the \(N_{e}\) vertical gradient [69; 70]. The lightning-induced \(N_{e}\) reduction may explain the relatively lower \(N_{e}\) values (along the same pitch angle) in June over the American and Indian Monsoon regions where deep convection and lightning activity are likely higher than elsewhere. It remains challenging with the GNSS-RO \(N_{e}\) observations to separate the direct lightning effect from others, as the \(D\)/\(E\)-region \(N_{e}\) is strongly coupled with other processes (e.g., solar radiation, planetary waves). Figure 10: COSMIC-1 monthly \(N_{e}\) maps at 70 km from all local times for March, June, September, and December.