ELECTROMAGNETIC CORE-MANTLE COUPLING - I - EXPLAINING DECADAL CHANGESIN THE LENGTH OF DAY

Measured changes in the Earth's length of day on a decadal timescale a re usually attributed to the exchange of angular momentum between the solid mantle and fluid core. One of several possible mechanisms for th is exchange is electromagnetic coupling between the core and a weakly conducting mantle. This mechanism is included in recent numerical mode ls of the geodynamo. The 'advective torque', associated with the mantl e toroidal field produced by flux rearrangement at the core-mantle bou ndary (CMB), is likely to be an important part of the torque for match ing variations in length of day. This can be calculated from a model o f the fluid flow at the top of the outer core; however? results have g enerally shown little correspondence between the observed and calculat ed torques. There is a formal non-uniqueness in the determination of t he flow from measurements of magnetic secular variation, and unfortuna tely the part of the flow contributing to the torque is precisely that which is not constrained by the data. Thus, the forward modelling app roach is unlikely to be useful. Instead, we solve an inverse problem: assuming that mantle conductivity is concentrated in a thin layer at t he CMB (perhaps D ''), we seek flows that both explain the observed se cular variation and generate the observed changes in length of day. We obtain flows that satisfy both constraints and are also almost steady and almost geostrophic, and therefore assert that electromagnetic cou pling is capable of explaining the observed changes in length of day.