On electromagnetic core-mantle coupling

The induced magnetic field in the Earth's mantle is computed for an electrical conductivity of the mantle of form σ = σa(a/r)α where a is the core radius. When the inducing core motion is a uniform westward drift of −10−10s−1 relative to the mantle, the retarding couple reaches −1019 Nm. This result can be obtained with values σα and α such that the time scale of screening, μσa2/α2, is still below 10 years. The retarding couple, and in particular the contribution from the higher field harmonics, increases with increasing concentration of the conducting layer towards the core-mantle boundary. Deviations from the average torque balance of order 10% of the retarding couple can account for the “decade fluctuations” in the Earth's rate of rotation.