On the Role of Rotation of the Internal Core Relative to the Mantle

Summary

The outer fluid core of the Earth can be considered as a fluid between two hard spheres (the internal core and the rock mantle) rotating with different but close angular velocities. In the incompressible, nonconducting almost inviscid limit a singular cylindrical surface having the radius of the internal sphere appears (the Proudman solution). A shear layer forming around this surface in the non-ideal fluid may have important implications for the geodynamo.

INTRODUCTION

The aim of this short paper is to attract attention to one feature in the Earth's fluid core. The feature is an internal shear layer induced by a relative rotation of the inner core. Large gradients of the velocity around this layer may be important for the geodynamo. Note, in particular, that in the geodynamo model-Z without an account of the inner core rotation one of the basic sources (the α-effect) is assumed to be concentrated near the core-mantle boundary (Braginsky 1993).

The inner core of the Earth can be considered as a hard iron ball of radius approximately 0.2R, where R is the Earth's radius. The rest of the planet is occupied by the outer liquid core and the rock mantle in the form of spherical shells of almost equal width, 0.4R. The other iron-rock planets (Mercury, Mars), except probably Venus, also have inner cores (Stevenson 1983). As the source of compositional convection (Loper & Roberts 1983) the inner core is apparently a necessary part of the planetary dynamo.