The westward drift of the geomagnetic field caused by length‐of‐day variation, and the topography of the core‐mantle boundary

We propose that the westward drift of the geomagnetic field is caused by length‐of‐day (1.o.d.) variation through topographic coupling between the mantle and the core. Fluctuations of the rotation velocity of the mantle induce radial flow in the core through topographic coupling at the core‐mantle boundary (CMB). The induced flow bends the toroidal field to generate a poloidal field which appears to drift westwards. An analytical model is constructed to describe this situation on the basis of a quasi‐geostrophic approximation. We compare our model with observations of the geomagnetic field over the last several hundred years. When the sectorial harmonic components of the Gauss coefficients, g and h, are plotted on a diagram with g as the abscissa and h as the ordinate, the observed trajectories follow a clockwise ellipse. The clockwise motion on the g‐h plot signifies that the pattern of the magnetic field moves westwards. This behaviour is described well by our model if 1.o.d. variation is assumed to consist of one Fourier component. We find that the period of the Fourier component of 1.o.d. variation is about 700 yr. The relationship between the ellipticity of the trajectories and the eigenfrequencies of the slow magnetohydrodynamic oscillations enables us to infer the dispersion relation to the oscillations, which signifies that the strength of the toroidal field of the outer core is 50–100 Gauss. We derive the topography of the CMB from the directions of the axes of the ellipses. The CMB topography is found to correlate well with the lower mantle structure.