Comparative case study of delayed ionospheric response to a superposed 27-day solar rotation signal

Abstract

Major ionization processes in the upper atmosphere are driven by the solar extreme ultraviolet (EUV) radiation causing corresponding responses in ionospheric observables. The response to the 27-day solar rotation period is of particular interest, as this variation occurs with a delay, which depends on the level of solar and geomagnetic activity as well as atmospheric processes. The 27-day signature is also frequently superimposed with long-term variations, which further impact the length of the delay. For a better understanding of these interactions, the present study investigates the delayed response of ionospheric total electron content (TEC) and the concentrations of major neutral and ionized species. Using high-resolution simulations from the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) and the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model, two distinct 27-day solar rotation periods from the high solar activity year of 2014 are analyzed. This comparison allows us to examine the effects of an ideal 27-day solar activity cycle alongside one characterized by increasing solar activity, while also comparing model results with observed IGS data. Our detailed analysis, based on TIE-GCM simulations, presents the ionospheric response for various ionized and neutral species across different altitudes and latitudes. Notably, we find that the accumulation of ionized species, such as $O^{+}$ and $O_2^{+}$, in the lower ionosphere, particularly at approximately 230 km, where ionized oxygen density peaks—is significantly influenced by the long-term increase in solar activity. However, the 27-day solar rotation period predominantly governs ionization processes at altitudes above 230 km in both, ideal and complex model runs. Thus, our results are in good agreement with previous studies and extend the understanding of the delayed ionospheric response to more complex cases.