Time series of the time derivative of three components of the magnetic field for (a) Niemegk (NGK) in Germany, together with the near-by observatories SED and POT, and (b) Hermanus (HER) in South Africa. Comparison between the first differences of observatory annual means (open symbols) and the predictions for several models: quadratic in space and time (d_S= 10⁴ μT and d_T= 10⁶ nT yr⁻¹, black curve), quadratic in time and maximum entropy in space (d_S= 30 μT and d_T= 10⁶ nT yr⁻¹, blue curve), and maximum entropy in space and time (d_S= 30 μT and d_T= 300 nT yr⁻¹, red curve). At Niemegk the black and blue curves are nearly invisible as it coincides almost exactly with the red curve. At Hermanus the black curve only is invisible, superimposed with the blue curve. Note on (a) that we have chosen to show three distinct model predictions for the observatories POT, SED and NGK (these result in discontinuities at the transition from POT to SED and from SED to NGK).