Geomagnetic decadal variations caused by length-of-day variation

Sectorial components of the geomagnetic decadal variations correlate well with the length-of-day (LOD) variation. Furthermore, when the Gauss coefficients g_m^m are plotted against h_m^m (g-h plot), the trajectories of the geomagnetic variation lie almost on a straight line. The linearity indicates the existence of a spatial preferential direction, which can be interpreted as the pattern of the core-mantle boundary (CMB) topography. We therefore propose that the sectorial components of the geomagnetic decade variations are caused by LOD variation through bumps at the CMB. We examine the geomagnetic variations using the topographic interaction model developed by Yoshida and Hamano (Geophys. J. Int., 114: 696-710, 1993) for modeling the westward drift of the geomagnetic field. Accurate observations of the decadal LOD variation over the past century enable us to assess the advantages and disadvantages of the model. The patterns of the time variations of the sectorial geomagnetic fields are found to be explained well by our model, if the basic toroidal field is assumed to be symmetric about the equator. However, the inferred variation of the time delays with the wavenumber is difficult to explain. The model allows us to infer the sectorial components of the CMB topography from the preferential direction of the g-h plot. The resulting pattern correlates well with the seismic heterogeneity of the lower mantle, although the inferred amplitude of the CMB undulation is unrealistically large.