UCLA

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. Plain Language Summary Solar activity varies on multiple time scales, including the 11‐year SC and the 27‐day SC. These variations strongly influence the Earth's ionosphere, a region of charged particles that affects radio communication and satellite navigation. In this study, we examine how these two SCs influence the vertical structure of ionospheric electron density. We used 18‐year combined observations from the FORMOSAT‐3/COSMIC and FORMOSAT‐7/COSMIC‐2 satellite missions. The observations reveal clear differences in how the 11‐year and 27‐day SCs affect electron density at different altitudes. In many cases, the peak response associated with the 27‐day cycle occurs at significantly higher altitudes than the peak response associated with the 11‐year cycle. The difference varies with season, latitude, and solar intensity. Global model simulations show that heating from solar radiation can cause the upper atmosphere to expand. This thermal expansion shifts ionospheric structures upward and is responsible for most of the altitude difference between the two SC signals. These results improve our understanding of how solar variability affects the ionosphere and help clarify the physical processes that control its vertical structure. Key Points COSMIC and COSMIC‐2 observations reveal distinct vertical structures of solar cycle (SC) signals in ionospheric electron density The 27‐day SC signal often peaks at higher altitudes than the 11‐year signal, depending on latitude, season, and solar activity Model results indicate that the altitude differences between the two SC signals largely arise from ionospheric thermal expansion