15. Geomagnetic Secular Variations are caused by Length-of-day Fluctuations

We propose that one of the main causes of the secular variation of the geomagnetic field is length-of-day (LOD) variation, namely, the variation of the rotation velocity of the mantle. We developed an analytical model, in which fluctuations of the rotation velocity of the mantle induces flow in the outer core through topographic coupling at the core-mantle boundary (CMB). The flow in turn bends the toroidal field to produce a poloidal field. Secular variation of the sectorial components of the geomagnetic field, from decadal to millennial time scale, is explained very well by our model. By comparing our theory with observations, we inferred important properties of the outer core and the mantle: the strength and the direction of the toroidal field, the dispersion relation of the slow magnetohydrodynamic oscillation, and the topography of the CMB.