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

Understanding the ionospheric dependence on solar activity is crucial for the comprehension of the upper atmosphere. The response of the ionosphere to solar EUV flux has been previously considered stable. Subsequent studies have revealed long-term changes that are not yet fully understood. This work evaluates the stability of the NmF2 dependence on solar EUV indices throughout different solar cycles. Hourly values of the peak electron density of the ionospheric F2-layer (NmF2) from Juliusruh station (54.6° N, 13.4° E) are analysed between 1957 and 2023. Geomagnetic perturbations are removed. Third-degree polynomial fit models are dependent on different solar EUV proxies (MgII, F30, and F10.7) and generated separately for each solar cycle and season separately. The saturation effect is visible in our data and starts at lower F30 values in the ascending phase than in the descending phase. A well-pronounced local time dependence in January with the R2 value being maximum around noon hours has been observed. The correlation is highest for F30 and MgII especially during winter noon conditions, supporting 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 slope of the model curves decreases with time for each solar cycle. Between SC20 and SC24, the observed decrease is consistently higher than 16 %, reaching 24 % at 90 sfu which means a decal reduction of 3–4.4 % between 1954 and 2019.