Fig. 2
From: Physics-based secular variation candidate models for the IGRF
a Intensity secular variation error (see text for its definition) for past generation IGRF SV models (in blue) and those SV candidates submitted for IGRF-11 and IGRF-12 (in grey), computed using fields expanded up to spherical harmonic degree and order 8. b Energy of the second derivative of the magnetic field, the secular acceleration, at Earth’s surface, up to spherical harmonic degree and order 8, according to the MCM model by Ropp et al. (2020) and the CHAOS model by Finlay et al. (2020), version 7.5