Long-term changes in the dependence of NmF2 on EUV solar flux at Juliusruh

Abstract

Research on the ionosphere and thermosphere has been conducted to investigate indications of the impact of climate change and other changes. Various studies have found long-term changes in different parameters of the ionosphere and thermosphere, which depend on factors such as magnetic latitude, seasons, time of the day, etc. However, analyzing these changes poses several challenges. The most significant challenge is determining an appropriate model for the ionosphere parameters. Linear and polynomial fits to data sets spanning several decades are the most commonly used approaches. Another challenge is selecting the most appropriate solar flux index. This work contributes to the debate on the best choice of fitting models and solar flux index, while also evaluating the stability of the NmF2 dependence on solar flux throughout different solar cycles. Hourly values of the peak electron density of the ionospheric F2-layer (NmF2) from the ionosonde at Juliusruh station (54.6°N, 13.4°E) are analyzed. The analysis period is between 1957 and 2023. Geomagnetic perturbations are removed based on a kp with a threshold of 4. Linear regression and 3rd order polynomial fit models dependent on different solar flux indices (MgII, F30, and F10.7) are generated for each solar cycle and for winter (January), summer (July), and two equinox months (March and October) separately. The coefficient R^2 is used to measure the quality of the NmF2 model dependent on the solar flux proxies. It shows a well-pronounced local time dependence in January with R^2 value being maximum around noon hours. During the other months, R^2 is rather constant throughout the day. R^2 is highest for F30 and MgII in general and especially during winter noon conditions. Comparing the slopes of the model curves for the different solar cycles, a clear decrease in the slope with each solar cycle becomes visible. We conclude that the best solar flux proxies for describing the NmF2 dependence at all LT hours are MgII and F30. In addition, the response of NmF2 to solar flux shows a clear long-term change as the model slope decreases with time for each solar cycle.