Drifting and standing fields in the geomagnetic field for the past 400Â years

Based on the field model by Jackson et al. (2000), we reanalyzed the data of historical times and decomposed the geomagnetic field into drifting and standing fields. It has been confirmed that the results are consistent with the earlier analyses. When the field is expressed in a spherical harmonic series, the drifting field is characterized by two remarkable features. One is that the drifting field is represented mostly by sectorial harmonics, and the other is that the drift velocity does not differ significantly between the harmonics. The standing field that remains after subtraction of the drifting field from the main geomagnetic field is stronger than the drifting field. The predominance of sectorial terms in the drifting field means that the vertical component, for instance, of the drifting field is symmetric about the equator. On the other hand, the standing field is antisymmetric. Due to this difference, the drifting field largely controls the geomagnetic secular variation in the low latitude region, whereas the standing field dominates in the middle and higher latitudes.