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

Abstract

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. The NmF2 response to solar EUV proxies (F10.7 or R) was found to be linear in early studies (Bremer, 1992). However, later studies (e.g., Balan et al., 1994; Liu et al., 2003) discovered that the linear increase of NmF2 with solar EUV proxies at low and moderate solar activity levels breaks down at higher activity levels, indicating a “saturation effect” and a nonlinear dependence. Recent publications (e.g., Danilov and Berbeneva, 2023) show that the dependence of foF2 to solar flux is better represented with a third-degree polynomial regression and that F30 and MgII are the most reliable proxies for long-term analysis (Laštovicka, 2021).