Mean differences ΔNmF2 between estimates of NmF2* at ionosonde stations in the Northern (a and c) and Southern (b and d) hemispheres and corresponding models LM(NmF2*) as a function of F30*. Panels a and b zoom in on the early phase of measurements until 1982. Panels c and d show differences between the linear NmF2* models and observations over the full period 1963–2022, including linear regression lines starting in 1980.