The Low Geomagnetic Secular Variation in the Pacific and the Inhomogeneous Conducting Lower Mantle

Reconstructions of the temporal changes in Earth's magnetic field for the past 400 years show that the secular variation has been lower in the Pacific region, a feature further confirmed by recent satellite measurements. Core flow reconstructions suggest that one of the main reason for this is connected to a planetary scale geostrophic gyre which is offset from the rotation axis towards the Atlantic hemisphere. Because of the general downflow in the Pacific region, fewer magnetic flux anomalies are advected to the surface of the core resulting in a reduced secular variation intensity. Topographic and thermal core-mantle coupling with an inhomogeneous lower mantle, as well as hemispherical differences in inner core growth, have been proposed as possible explanations for this geographic pattern of flow and magnetic field. Here, we investigate whether electromagnetic coupling between core flows and an inhomogeneous conductivity structure at the base of the mantle can explain the lower magnetic secular variation in the Pacific as well as the offset gyre. Regions with larger conductivity tend to "screen" a greater portion of the secular variation - indeed this was one of the earliest suggestion to explain the Pacific low secular variation. However the more important effect is that a larger conductivity exerts a larger electromagnetic drag on convective structures, thus leaving little eddy action to act on the magnetic field. Using a quasi-geostrophic model of the dynamics in the core, we show how effective electromagnetic coupling is at attenuating convective columns. We also show how the planetary gyre tends to be deflected away from regions of higher conductivity in an effort to reduce the drag acting on it. Our results suggest lateral differences in the material properties of the lower mantle, and therefore offering clues and constraints on the evolution and dynamics of the mantle.