Delay in Response of Global Electron Content and Electron Concentration at Various Altitudes to 27-Day Variations in Solar Activity

We investigate the influence of 27-day variations in solar activity related to the rotation of the Sun around its axis on the thermosphere–ionosphere system at different latitudes and heights, using the results of the calculations of the model of the Earth’s upper atmosphere. Based on the results of the model calculations, related processes in the thermosphere–ionosphere system were analyzed in the period from June 20 to July 21, 2014. There is a clear reaction to the daytime electron concentration Ne in the ionosphere for 27-day variations of the solar radiation flux (index F10.7). Using comparative and correlational analyses, we revealed the delay in the variations of the daytime electron concentration values calculated in the Ne model at different heights, including at the maximum of the F2-layer of the ionosphere (NmF2) and the total electron content and global electron content regarding changes F10.7. It is shown that changes in the O/N2 ratio are the main possible reasons for the delay. The revealed two-day lag in the global electron content is consistent with the results obtained earlier from the observational data. The height structure of the delay Ne relative to F10.7 is discussed. The results of the calculations over the ionospheric stations of the Northern Hemisphere showed that the maximum delay of variations Ne relative to F10.7 is obtained in high and low latitudes, and less at the subauroral and middle latitudes. It is shown that the lag of variations in the total electron content relative to F10.7 is always less than in the case of NmF2.