抄録
The time-scale of the variation in the length of the day is comparable to, or shorter than, the electromagnetic decay time, τη, of the mantle. This suggests that an electromagnetic theory for the origin of these fluctuations should not only depend on the electromechanical coupling time, τI, introduced by ROCHESTER (1960), but also on τη. It is shown that, when τI is small compared with τη, the fluctuation time for perturbations in the angular velocity of the mantle is of order τc=τI2/3τη1/3, and not τη. The possibility that τI is not large compared with τη for the case of the Earth is considered. Also, by means of a simple shellular model of the core, the rôle of Alfvén waves in modifying the coupling of core to mantle is discussed. The relationship of this theory with that of BRAGINSKII (1970) is briefly noted. A new account is given of mantle induction which, although only valid at small magnetic Reynolds numbers (based on the conductivity of the mantle), is sufficiently general to permit the coupling of the mantle to an arbitrary slow motion on the core surface to be treated.
