This study explores how solar sunspot number (Sn) compares with other solar indicators like solar radio fluxes in predicting changes in Earth's ionosphere, particularly its critical frequency, over more than 60 years. The work finds that Sn, despite recent fluctuations in other proxies, remains the most stable predictor across all time periods. When adjusting for potential data saturation, Sn outperforms other proxies, providing a more accurate forecast of long-term ionospheric trends.

This study analyzes changes in the ionospheric response to solar flux over five complete solar cycles (1957 to 2023). We use Juliusruh hourly data of the peak electron density of the F2 layer, NmF2, and three solar extreme ultraviolet (EUV) radiation proxies. The response is better represented by a cubic regression, and F30 shows the highest correlation for describing NmF2 dependence over time. These results reveal a decrease in NmF2 influenced by the intensity of the solar activity index.

Total electron content (TEC) affects the GPS (Global Positioning System) signal propagation and applications of GPS signals like positioning. Here, we study long-term trends in TEC. TEC trends are regionally different and predominantly negative; all statistically significant trends are negative. TEC trends reveal a clear wavenumber 2 longitudinal structure in low/equatorial latitudes, with strong negative trends in belts 0–60° E and 180–240° E and weak trends in belts 90–150° E and 270–330° E.

Our research investigates how different proxies of solar activity influence long-term trends in the Earth's ionosphere. By analyzing data from two mid-latitude stations up to 2022, we found that the choice of solar activity measures significantly affects trends in ionospheric electron density, while trends in ionospheric height remain more stable. Selecting the correct solar activity measure is crucial for accurate density trend predictions and improving space weather forecasting models. ​

We investigate the ionospheric response to the temporal and spatial dynamics of the solar activity using total electron content (TEC) maps and multiple solar proxies. The maximum correlation at a 16–32-d timescale is observed between the He-II, Mg-II, and F30 with respect to global mean TEC, with an effective time delay of about 1 d. The most suitable proxy to represent the solar activity at the timescales of 16–32 d and 32–64 d is He-II.