Long-term behavior of NmF2 over the German ionosonde station Juliusruh in comparison with solar radio flux indices F10.7 (a, b) and F30 (c, d). Panels a and c: Yearly means of NmF2 monthly medians and solar indices F10.7 (a) and F30 (b). Panels b and d: Sliding solar cycle averages (previous 11 years) of NmF2 estimates (NmF2*) and linear models of NmF2* (until 1984) run by sliding solar cycle averages of F10.7 (F10.7*) and F30 (F30*). The differences between observations and linear NmF2* models, ΔNmF2, are plotted in the lower part. scaled at the right side. The dashed lines indicate the RMS ± error (RMSE) range of the linear models taken as reference.