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Solar Activity Driven 27‐Day Signatures in Ionospheric Electron and Molecular Oxygen Densities
(2022)
Abstract
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.
Abstract
Based on the analysis of electron density Ne profiles (Grahamstown ionosonde), a case study of the height‐dependent ionospheric response to two 27‐day solar rotation periods in 2019 is performed. A well‐defined sinusoidal response is observed for the period from 27 April 2019 to 24 May 2019 and reproduced with a Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model simulation. The occurring differences between model and observations as well as the driving physical and chemical processes are discussed based on the height‐dependent variations of Ne and major species. Further simulations with an artificial noise free sinusoidal solar flux input show that the Ne delay is defined by contributions due to accumulation of O+ at the Ne peak (positive delay) and continuous loss of O2+ ${\mathrm{O}}_{2}^{+}$ in the lower ionosphere (negative delay). The neutral parts' 27‐day signatures show stronger phase shifts. The time‐dependent and height‐dependent impact of the processes responsible for the delayed ionospheric response can therefore be described by a joint analysis of the neutral and ionized parts. The return to the initial ionospheric state (and thus the loss of the accumulated O+) is driven by an increase of downward transport in the second half of the 27‐day solar rotation period. For this reason, the neutral vertical winds (upwards and downwards) and their different height‐dependent 27‐day signatures are discussed. Finally, the importance of a wavelength‐dependent analysis, statistical methods (superposed epoch analysis), and coupling with the middle atmosphere is discussed to outline steps for future analysis.