Abstract
The geomagnetic field is comprised of drifting and standing fields. The drifting field has two remarkable features. One is predominance of sectorial harmonics when the field is expressed in a spherical harmonic series, and the other is uniform drift rate irrespective of harmonics. We consider that the drifting field is a product of interaction of the core flow with the axial dipole field near the surface of the core. The key to the predominance of sectorial harmonics is in the boundary condition on the electric current at the core–mantle boundary. If we take the mantle to be an electrical insulator, the electric current normal to the boundary must vanish. This strongly constrains the surface flow. The toroidal flow becomes the flow with the sectorial harmonics predominant. Then, the sectorial toroidal flow, interacting with the axial dipole field, induces the poloidal field in which the sectorial harmonics are predominant. This is the observed type of drifting field. The uniform drift rate, the second nature of the drifting field, seems to suggest that the surface part of the core is rotating westwards as a whole. Subsequently, the sectorial type toroidal flow embedded in the westward-rotating surface layer is considered as the cause of the drifting field.
Highlights
Geomagnetic secular variation and westward drift It is well known that some specific features of the geomagnetic field are drifting westwards, while many other features are standing at the same place, changing their intensity
These observations seem to suggest that the westward drift in the low latitudes is a special feature characteristic in the equatorial region
Infinite conductivity in Region II Predominance of sectorial harmonics in the toroidal flow The flow structure in the surface layer is greatly affected by the boundary condition on the electric current at the core–mantle boundary
Summary
Geomagnetic secular variation and westward drift It is well known that some specific features of the geomagnetic field are drifting westwards, while many other features are standing at the same place, changing their intensity. For example, the intense focus of the vertical component anomaly in the equatorial region near the western coast of African continent observed at the Earth’s surface. It has been moving westwards steadily for the past several hundred years (e.g., Bullard et al 1950; Yukutake and Tachinaka 1968a). The drifting foci are distributed in a narrow equatorial region (Jackson 2003; Finlay and Jackson 2003). These observations seem to suggest that the westward drift in the low latitudes is a special feature characteristic in the equatorial region
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