The Magnetostrophic Rise of A Buoyant Parcel In the Earth's Core

Abstract

The dynamics of a buoyant parcel (or blob) of fluid released from the mushy zone on the inner core boundary (ICB) is considered. Estimates of the density defect and of the rise velocity are obtained from consideration of mass conservation and magnetostrophic force balance. When Lorentz and Coriolis forces are of comparable orders of magnitude, the disturbance remains localized in a neighbourhood of the blob even in the absence of viscous effects, and an inviscid analysis is possible. The instantaneous velocity and magnetic field associated with a given localized buoyancy distribution are then determined, and on the assumption that the velocity is approximately uniform throughout the blob, its trajectory from ICB to core-mantle boundary (CMB) is deduced. Both westward drift and poleward migration of erupting field loops are indicated. The effects of turbulent diffusion on the blob are considered. The prospects for constructing a dynamo theory based on this ‘blob scenario’ are discussed.