Global Validation and Characterization of the Ionospheric Bottomside Thickness (B 0 ) Using Fengyun-3 Radio Occultation Observations

The bottomside thickness parameter (B0) is a critical component for accurately representing electron density profiles in the International Reference Ionosphere (IRI) model. This study presents a comprehensive validation and global analysis of B0 derived from the Fengyun-3 (FY-3) GNSS radio occultation (RO) mission during the high solar activity period of 2022–2024. By integrating observations from the FY-3 constellation, this work effectively complements the data coverage of FORMOSAT-7/COSMIC-2 (F7/C2) by extending analysis to mid- and high-latitude regions. Validation against global digisonde measurements demonstrates that FY-3 derived B0 achieves high reliability, yielding correlation coefficients exceeding 0.86 and an RMSE of approximately 20 km in low and mid-latitudes. Comparisons with three IRI-2020 model options reveal that the ABT-2009 option offers the best overall agreement with observations, particularly in reproducing hemispheric asymmetries, whereas the Gul-1987 and Bil-2000 options exhibit notable deficiencies in capturing geomagnetic modulation and spatial variability. Global morphological analysis identifies a synchronized but inversely correlated relationship between B0 and peak electron density (NmF2) in the equatorial ionization anomaly region. Furthermore, distinct longitudinal structures are observed, characterized by wavenumber-4 patterns during equinoxes, and wavenumber-2 and -3 patterns during summer and winter solstices, respectively. In high latitudes, observations suggest a B0 enhancement near the South Magnetic Pole during the Southern Hemisphere winter, which likely reflects the influence of geomagnetic control. These findings confirm the utility of FY-3 RO data for characterizing the global ionosphere and offer valuable constraints for future refinements of the IRI model.