Effects of thermally heterogeneous structure in the lowermost mantle on the geomagnetic field strength

We have conducted a study of numerical dynamos in a rapidly rotating spherical shell with prescribed non-uniform heat-flux patterns at the outer boundary to examine effects of thermal structure at the core-mantle boundary (CMB) on the geodynamo, especially on the magnetic field strength. Large heat-flux heterogeneity with equatorial symmetry enhances strength of the dipolar magnetic field for a sufficiently small Ekman number, whereas that with equatorial anti-symmetry does not change the strength but affects the tilt of the magnetic dipole axis. The thermal wind induced by heat-flux heterogeneity at the CMB has strong influence on the flow structure and on the magnetic field intensity. These results suggest that thermally heterogeneous Structure of the lowermost mantle might give rise to an anomalously strong geomagnetic field such as that during the Cretaceous Normal Superchron. (c) 2008 Elsevier B.V. All rights reserved.