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

Kurzfassung

Understanding ionospheric dependence on solar activity is crucial for comprehension of the upper atmosphere. The response of the ionosphere to solar extreme ultraviolet (EUV) flux has previously been considered stable. Subsequent studies have revealed long-term changes that are not yet fully understood. This work evaluates the stability of the ionospheric F2 layer (NmF2) dependence on solar EUV indices throughout different solar cycles (SCs). Hourly values of the peak electron density of NmF2 from Juliusruh station (54.6° N, 13.4° E) are analyzed between 1957 and 2023. Geomagnetic perturbations are removed. Third-degree polynomial-fit models dependent on different solar EUV proxies (MgII, F30, and F10.7) are generated separately for each solar cycle, each season, and each local time (LT) hour. The saturation effect is visible in our data and starts at lower F30 values in the ascending phase than in the descending phase. A highly pronounced local time dependence in January with the R2 (goodness of the description for each fit) value being maximum around the noon hours has been observed. The correlation is highest for F30 and MgII, especially under winter noon conditions, supporting the findings of recent studies that they are the best solar flux proxies for describing the NmF2 dependence at all LT hours. Most importantly, the response of NmF2 to solar flux shows a clear long-term change as the slopes of the model curves decrease with time for each solar cycle. Between SC20 and SC24, the observed decrease is consistently higher than 2.9 % per decade, reaching 4.4 % per decade at 90 sfu between 1964 and 2019.