Force balances and convective state in the earth's core

Kuang and Bloxham [Kuang, W., Bloxham, J., 1997a. An Earth-like numerical dynamo model. Nature 389, 371-374] found that numerical strong-field dynamo solutions in the earth's core depend strongly on viscous couplings introduced in simulation. In this paper, we discuss in detail how the force balances, the convective patterns and the magnetic field inside the core vary with the viscous couplings on the boundaries. We find that dynamo action region depends on the viscous couplings on the boundaries. When strong viscous couplings are retained in simulation, dynamo action is confined in the region near the inner core boundary (ICB) and within the tangent cylinder (the co-axial cylindrical surface across the core tangent to the inner core at the equator). Dynamo action occurs in the bulk of the fluid core when viscous couplings are neglected. The convective flow inside the tangent cylinder is very sensitive to the viscous couplings on the ICE, while the flow patterns outside the tangent cylinder do not vary qualitatively with the viscous couplings. We also find that when the viscous couplings are eliminated, the axial Lorentz torques acting on the co-axial cylindrical surfaces balance almost the fluid inertia, resulting in strong zonal flow (differential rotation) outside the tangent cylinder. However, they are strongly damped when the viscous couplings are retained. The thermal wind and the magnetic wind inside the tangent cylinder vary significantly with the viscous couplings on the ICE. With strong viscous couplings on the ICE, the magnetic wind is small compared with the thermal wind. Both winds are unidirectional, resulting in a zonal flow increasing with the depth and a super-rotating inner core. Without viscous coupling, both winds are comparable, Their directions vary with depth and with time, resulting in a zonal flow varying non-monotonically with the depth and an oscillating inner core. (C) 1999 Elsevier Science B.V. All rights reserved.