Simple additional constraints on regional models of the geomagnetic secular variation field

Standard ways of building regional models of the geomagnetic field fail when one wishes to model secular variation (SV). The reason for this is that the SV is mainly a large-scale feature, while regional modelling is most appropriate for characterizing small-scale features. The new idea presented in this note consists in reducing this effect by requiring that the spatial derivatives of the SV produced by the regional model fit the values given by global (and smoother) models such as the international geomagnetic reference field.