Fig. 1
From: Physics-based secular variation candidate models for the IGRF
Forecast error measuring the difference between the true field at epoch t and a linear extrapolation between \(t_0\) and t, with \(t_0 = t / 5\) (integer division). Each Gauss coefficient has a constant rate of change, which is set either to the value of its time derivative at \(t=t_0\) (blue curve) or to the average rate of change between \(t_0\) and \(t_0 + 5\) years (orange curve). Fields are expanded up to spherical harmonic degree and order 8