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Solar Activity Driven 27‐Day Signatures in Ionospheric Electron and Molecular Oxygen Densities

The complex interactions in the upper atmosphere, which control the height‐dependent ionospheric response to the 27‐day solar rotation period, are investigated with the superposed epoch analysis technique. 27‐day signatures describing solar activity are calculated from a solar proxy (F10.7) and wavelength‐dependent extreme ultraviolet (EUV) fluxes (Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Solar EUV Experiment), and the corresponding 27‐day signatures describing ionospheric conditions are calculated from electron density profiles (Pruhonice ionosonde station) and O2 density profiles (Global‐scale Observations of the Limb and Disk). The lag analysis of these extracted signatures is applied to characterize the delayed ionospheric response at heights from 100 to 300 km and the impact of major absorption processes in the lower (dominated by O2) and upper ionosphere (dominated by O) is discussed. The observed variations of the delay in these regions are in good agreement with model simulations in preceding studies. Additionally, the estimated significance and the correlation of the delays based on both ionospheric parameters are good. Thus, variations such as the strong shift in 27‐day signatures for the O2 density at low heights are also reliably identified (up to half a cycle). The analysis confirms the importance of ionospheric and thermospheric coupling to understand the variability of the delayed ionospheric response and introduces a method that could be applied to additional ionosonde stations in future studies. This would allow to describe the variability of the delayed ionospheric response spatially, vertically and temporally and therefore may contribute further to the understanding of processes and improve ionospheric modeling.