Fluid Motion of the Outer Core in Response to a Temperature Heterogeneity at the Core-Mantle Boundary and Its Dynamo Action

Abstract

We perform a simple linear analysis of the response of the outer core fluid to a sectorial temperature heterogeneity at the core-mantle boundary (CMB). The locations of upwellings and downwellings are shown to be controlled by the Elsasser number. When the Elsasser number is less than the order of unity, upwellings occur to the east of hot regions, and downwellings occur to the west of hot regions. When the Elsasser number is more than the order of unity, upwellings occur beneath hot regions, and downwellings occur beneath cold regions. Our theory provides an explanation for the correlation between the observed magnetic feature and the lateral heterogeneity of the seismic velocity in the lower mantle, which would correspond to the temperature heterogeneity at the CMB. From the correlation, we infer that the Elsasser number is a little less than unity in the Earth's outer core. Moreover, we investigate whether the resulting flow can sustain dynamo action or not. A temperature heterogeneity of the order of 10-6 K over a length scale of 103 km would be sufficient for the generation of dynamo action.