Lateral temperature variations at the core‐mantle boundary deduced from the magnetic field

Recent studies of the secular variation of the Earth's magnetic field over periods of a few centuries have suggested that the pattern of fluid motion near the surface of Earth's outer core may be strongly influenced by lateral temperature variations in the lowermost mantle. In this paper we introduce a self‐consistent method for finding the temperature variations near the core surface by assuming that the dynamical balance there is geostrophic and that lateral density variations there are thermal in origin. The calculation is accomplished by using the radial and horizontal components of the magnetic field at the core‐mantle boundary to calculate the fluid flow and its radial derivative near the core surface, which are related to the pattern of lateral temperature variations by the thermal wind equation. As expected, these lateral temperature variations are very small. We find some agreement between this pattern and the pattern of topography of the core‐mantle boundary, but are unable to answer conclusively to what extent core surface motions are controlled by the mantle, rather than being determined by processes in the core.