Consistency Between Geomagnetic Secular Variation Data And A Stably Stratified Layer At The Core-Mantle Boundary

Recent calculations (Pozzo et al., 2012) of outer core thermal and electrical conductivity from first principles have raised their values by a factor of three. This has significant implications for geodynamo operation, in particular, forcing the development of a stably stratified layer at the core-mantle boundary (CMB). The lack of convection in this layer means there should be no large-scale upwelling and downwelling of core fluid at the CMB. In principle, this can be tested against geomagnetic secular variation (GSV) data, by examining whether a purely toroidal core flow (consistent with no upwelling and downwelling) can fit them. When this was first attempted (Whaler, 1980), there were insufficient data to provide a rigorous test; moreover, the main field models implicit in the process were not consistent with the frozen-flux hypothesis on which core flow inversion is predicated. Subsequent studies have shown that approximately 90% of the energy in the CMB flow is in the toroidal component, and that there is very little difference in the data fit that is achieved by a purely toroidal flow compared to a less restrictive one allowing upwelling and downwelling. Therefore, adherence to the underlying assumptions and careful examination of the error budget is necessary to address the question. A new inversion method that solves for the time-dependent main field consistent with frozen-flux alongside the CMB flow (Lesur et al., 2010; Wardinski and Lesur, 2012) allows us to repeat the earlier test, but implemented properly. We apply it to a data set including large quantities of recent high quality satellite data that provides the most detailed constraints on the core field and its evolution, and advective flows that can explain the GSV.