Global Ionospheric F Region Parameters From GNSS-POD Limb Measurements: Evaluations and Comparisons With Two Empirical Models - IRI-2020 and NeQuick-2

An optimal estimation (OE) technique has recently been developed for F region electron density (Ne) using Global Navigation Satellite System (GNSS) limb sounding on low Earth orbit (LEO) satellites (COSMIC-2, Spire, and FengYun-3). This method provides unprecedented spatiotemporal sampling for global monthly Ne climatology within 100–500 km in 2 hr intervals. The global dataset, collected during mid to moderately high solar activity, is compared with leading models: IRI-2020 and NeQuick-2. Diurnal variations in summer, winter, and equinoctial months are examined for the F2-layer peak, as well as the topside and bottomside of the F region. The observed and modeled NmF2 and hmF2 show good agreement during the daytime, but discrepancies appear with NeQuick-2 at night. The OE-retrieved dataset reveals distinct interhemispheric differences in topside scale height between the summer and winter hemispheres, which are not adequately captured by models. The estimated topside scale heights in IRI-2020 are $\sim$20–30 km higher than observations on regional scale, but this difference decreases to $\sim$12–20 km on global scale. In the bottomside, the agreement between observations and models varies significantly between daytime and nighttime conditions. During the daytime, the global bottomside thicknesses derived from OE-retrieved profiles agree within 10 km with the IRI-2020, but they are $\sim$10–15 km higher than NeQuick-2. The nighttime thicknesses differ substantially, with deviations reaching up to $\sim$30 km compared to IRI-2020 and $\sim$45 km compared to NeQuick-2. As models face challenges due to lack of reliable measurements, especially in the topside and bottomside, improvements in GNSS-LEO observing techniques can provide more accurate and comprehensive data to characterize the global ionosphere.