Long-term behavior of NmF2* over the ionosonde stations Slough/ Chilton from 1952 to 2021 (a, b) and Boulder from 1958 to 2022 (c, d). Panels a and c: yearly means of NmF2 and solar index F30. 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 F30 (F30*). The differences between observations and corresponding linear NmF2* models, ΔNmF2 are plotted in the lower part, scaled at the right side. Dashed lines indicate the RMS ± error range.