Impact of Solar Cycles on the Electron Density in the Ionosphere—An 18‐Year Combined Observation From FORMOSAT‐3/COSMIC and FORMOSAT‐7/COSMIC‐2

Abstract

The Earth's ionosphere is sensitive to variations in solar irradiance associated with the 11‐year and the 27‐day solar cycles (SCs). Previous studies have suggested that the peak altitude of the 27‐day SC response in ionospheric electron density is significantly higher than that of the 11‐year SC response, but the underlying mechanism remains unclear. In this study, we analyze an 18‐year data set combining observations from the FORMOSAT‐3/COSMIC and FORMOSAT‐7/COSMIC‐2 missions, spanning the descending phase of SC 23 through the maximum of SC 25. The vertically resolved observations between 150 and 500 km allow a detailed investigation of the vertical structure of solar‐induced electron density variability. We find that the peak altitude of the 11‐year SC response occurs near ∼350 km in the tropical region and ∼300–320 km at mid‐latitudes, whereas the peak altitude of the 27‐day response occurs at significantly higher altitudes, particularly during periods of strong solar activity. The relative alignment of the two responses varies systematically with latitude, season, and solar intensity. Comparisons with simulations from the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X) show that this apparent discrepancy largely disappears when pressure coordinates are used instead of altitude coordinates. These results indicate that the observed altitude differences primarily arise from the ionospheric thermal expansion associated with solar heating. The findings improve our understanding of ionospheric responses to solar variability and provide new insights into the interpretation of vertical electron density structures in the upper atmosphere.