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Direct measurements of the geomagnetic field are available through ground observatories and navigators' data for the last 400 years (Jonkers et al., 2003) and satellite measurements for the last decades. For earlier periods, the data are obtained through the archeomagnetic study of archeological baked materials and volcanic rocks that acquire, during their last cooling, a thermoremanent magnetization (TRM) whose direction is parallel, and intensity is proportional to the intensity of the local geomagnetic field.Global models of the Earth's magnetic field have been constructed from direct, historical, and archeomagnetic data by inversion with spherical harmonic analysis in space (e.g.,