Models of the magnetic field at the core‐mantle boundary for 1715, 1777, and 1842

Models of the magnetic field at the core‐mantle boundary for 1715.0, 1777.5, and 1842.5 are presented, derived from original observations rather than from charts or grid point values. The models, produced using the method of stochastic inversion, have surprisingly good resolution of the core field, resolving small‐scale features which have only recently been identified in modern data. The models show that secular variation is low over much of the core‐mantle boundary, with clearly identifiable static, slowly varying features in the core field. Westward drift is confined to a relatively small region, in which the secular variation is more vigorous. It is concluded that the effects of magnetic diffusion should not be ignored in studies of the secular variation.