Study of the F‑region topside in situ ionospheric density using COSMIC‑2/ICON satellite observations and comparison with the IRI‑2020 model predictions

Abstract

This study presents a climatological evaluation of the International Reference Ionosphere (IRI‑2020) model predictions of low‑latitude topside F‑region O+ and H+ ion compositions, with particular emphasis on their fractional abundances. In situ O+ and H+ measurements from the Ion Velocity Meters (IVM) instruments from the Ionospheric Connection Explorer (ICON) (2020‑2022) and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC‑2) (2021‑2023) satellites missions were used as independent observational benchmarks. The IRI‑2020 model is sampled along the satellite trajectories at corresponding times, locations, and altitudes to ensure consistent climatological comparisons. Both satellite results revealed pronounced diurnal, seasonal, latitudinal, and longitudinal variability in topside ion fractions. IRI‑2020 successfully predicted the large‑scale climatological structure of the observed ion composition but exhibits reduced hemispheric asymmetries and smoother day‑night transitions relative to the measurements. ICON results showed systematic overestimation of O+ (approximately 21‑37%) and underestimation of H+ (approximately 12‑23%), particularly during equinoxes and around dawn and dusk. In contrast, COSMIC‑2‑based comparisons show smaller biases for both species. The differences in results, observed from both satellites, likely reflect different conditions of solar activity during the two missions. These results demonstrate that while IRI‑2020 provides a robust large‑scale global description of topside ion composition, some limitations persist, highlighting the need for improved representation of temporal and spatial resolution and better description of ion composition variation with geophysical parameters.