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.

The amplitude of subionospheric very low-frequency (VLF) radio signals does not show a symmetrical behavior over the year, which would be expected from its dependency on the solar position. The VLF amplitude rather shows a distinctive sharp decrease around October, which is hence called the "October effect". This study is the first to systematically investigate this October effect, which shows a clear latitudinal dependency.

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 examines solar wind interactions with magnetic structures from the Sun. Observations at different longitudes near the ecliptic plane show the progression of interactions between a co-rotating interaction region and a magnetic-cloud-like structure. These interactions cause significant changes, including compression and wave formation, enhancing our understanding of the complex nature of solar wind impacting Earth's magnetic environment.

The study uses data from soundings of the SABER satellite to examine the mesospheric inversion layer (MIL) and the related gravity wave energy and instability. In the mesosphere and lower-thermosphere (MLT) region, upper inversions occur with less than 40 % frequency, while the lower inversions are below 20 %. High gravity wave energy (~100 J/kg) in the upper MLT (85–90 km) is linked to instability, driving inversion phenomena. Conversely, the trend is less pronounced in the lower-MLT region.

This study, for the first time, convincingly attests to the two-step geospace storm being the best possible solar–terrestrial event of opportunity for realizing a technique for estimating the interplanetary magnetic field (IMF) B_z component threshold for the formation of the substorm current wedge. The data from INTERMAGNET magnetometer network on the two-step geospace 23–24 April 2023 storm yield the IMF B_z threshold for the formation of the substorm current wedge of –(22–30) nT.

The dispersion equation and the reflection coefficient are elegant tools for investigating acoustic–gravity waves (AGWs) in stratified atmospheres. AGWs are used to study the the ionospheric D region–lower thermosphere interaction. This is important because these waves, and especially gravity waves, can produce ionospheric disturbances and an increase in lower thermospheric temperature. The results of this article can be applied to the stratified atmospheres of the planets and the Sun.

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. ​