Fluid Motion in the Earth's Core Derived from the Geomagnetic Field and Its Implications for the Geodynamo

Abstract

An attempt is made to derive fluid motion in the Earth's outer core from geomagnetic field data. We implicitly incorporate the energy source for the geodynamo in the prescribed radial dependence of poloidal velocity field. Then the Navier-Stokes equation for the toroidal constituent and the induction equation for the toroidal and the poloidal magnetic fields are solved so as to fit the magnetic field at the core-mantle boundary estimated through downward continuation on the assumption that the mantle is an insulator. The basic standpoint is that the large-scale magnetic field is maintained by induction processes associated with large-scale fluid motion within the core. We also impose the condition that time variations of the velocity and the magnetic fields are very slow. The generation balance in the induction equation, for the derived velocity and magnetic fields, suggests that the geodynamo is likely to be of a2w-type.

Time dependency of fluid flow near the top of the core

“…The author prescribed the radial dependence of the poloidal velocity field and minimized the temporal variations of the velocity and the magnetic fields. Thus at any particular epoch, the time derivative of the magnetic field at the CMB calculated using the velocity field derived by Matsushima (1993) was much smaller than given by the secular variation model for that epoch. Since secular variation of the poloidal field at the CMB contains important information about the geodynamo, it is necessary to consider it in attempts to estimate the temporal evolution of fluid flow in the outer core, which would shed light on the existence of lateral temperature variations at the base of the mantle.…”

“…It was also assumed in this model that the slowly varying axisymmetric part of the flow is dominated by the toroidal flow and U P ≈ U R −1 m . With the assumption of large scale, slowly varying velocity and magnetic fields, Matsushima (1993) had found the toroidal velocity to be dominant at the CMB. In Braginsky's (1965) hydromagnetic dynamo the magnetic field was also represented in a manner similar to the velocity field:…”

“…It was pointed out by Matsushima and Honkura (1992) that the ω-effect may not be as strong as was assumed in the earlier papers, so that it is necessary to consider the other interaction terms also in the induction equation. Matsushima (1993) solved the induction equations for the toroidal and poloidal magnetic fields together with the Navier-Stokes equation for the toroidal velocity field to estimate fluid motion in the Earth's outer core from a geomagnetic field model. The author prescribed the radial dependence of the poloidal velocity field and minimized the temporal variations of the velocity and the magnetic fields.…”

A Green's function for the excitation of torsional oscillations in the Earth's core

“…Changes in the magnetic field can be related to fluid flow at the surface of the core when restrictions are imposed to resolve the nonuniqueness [ Bloxham and Jackson , 1991]. Flow in the interior of the core has also been inferred from field variations [e.g., Matsushima , 1993; Olson and Aurnou , 1999], although the interpretations are usually subject to large uncertainties. These difficulties are compounded if part of the surface motion is driven by temperature anomalies at the base of the mantle [ Bloxham and Gubbins , 1987; Zhang and Gubbins , 1992; Christensen and Olson , 2003; Lister , 2004].…”