Global Characterization of Bottomside Ionospheric Parameters (B0 and B1) Using FORMOSAT-7/COSMIC-2 Radio Occultation Observations and Evaluation of IRI-2020 during Solar Cycle 25

Abstract

The bottomside thickness (B0) and shape (B1) parameters are crucial for representing ionospheric electron density profiles below the F2 layer in the IRI (International Reference Ionosphere) model. In the present study, the comprehensive analysis of diurnal, seasonal, longitudinal, and latitudinal variations of B0 and B1 is conducted using FORMOSAT-7/COSMIC-2 radio occultation electron density profiles over 2019–2024 and compared with corresponding IRI-2020 submodels, namely Bil-2000, Gul-1987, and ABT-2009, under different seasonal and solar activity conditions. The results show that B0 and B1 are strongly controlled by geomagnetic latitude and seasons, with maximum values near the equator and an apparent hemispheric asymmetry during the solstice. The consistent coupling among hmF2, NmF2, B0, and B1 is observed in geomagnetic latitude with local time variation, indicating that their variability is controlled by the equatorial dynamics. The magnitude of B0 and B1 primarily increases through solar activity without altering the spatial structure. The longitudinal analysis reveals pronounced modulation in B0, especially during the equinox and summer under high solar activity, whereas B1 exhibits weaker longitudinal dependence. On comparison with IRI-2020, it is observed that the model produces the spatial structure but with systematic differences in magnitude and variability from the COSMIC-2-derived B0 and B1 parameters. The results highlight the importance of these parameters for characterizing low and mid-latitude bottomside ionospheric structure and provide an opportunity for improvements of the empirical ionospheric model.