Solar variability and delayed ionospheric response: Insights from complex 27-day solar EUV signatures

The solar extreme ultraviolet (EUV) radiation drives the major ionization processes in the upper atmosphere. Its variability causes a related response in ionospheric observables. Especially of interest is the delayed response of electron density (Ne), integrated total electron content (TEC), and the density of major neutral and ionized species to the 27-d solar rotation period. But this solar signature is often influenced by underlying trends on shorter and longer time-scales. Therefore, this study examines the ionospheric response to a solar 27-d signature superposed with a long-term increase in solar EUV, showing that complex composition changes in the upper atmosphere influence the expected response significantly. Using high-resolution simulations of the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM), we compare two different 27-d solar rotation periods from the year 2014 with enhanced solar activity. This allows us to compare an almost ideal solar activity input with one that is superposed with an increase in solar activity. The main results show that the accumulation of ionized species O⁺ and O₂⁺ in the lower ionosphere, especially up to the maximum density of ionized oxygen (O⁺) at about 230 km, is significantly affected by the long-term increase in solar activity.  Nevertheless, the 27-d solar rotation period dominates the ionization in both, ideal and complex model run for altitudes above 230 km. Thus, our results are in good agreement with preceding studies and extend the study of the delayed ionospheric response to more complex cases.