Elliptical instability of the Earth's fluid core

The elliptical instability of a rotating fluid contained in a thick spherical shell has been excited in our laboratory by a tide-like perturbation of the flexible inner boundary. For an inviscid fluid, the growth rate of the instability is approximately proportional to the perturbation amplitude and the rotation rate. Development of the instability appears to be limited by the spherical outer surface and the relatively small perturbation applied over the inner surface. If the corresponding instability were excited in the Earth's fluid core by tidal forces, in the absence of dissipation the e-folding time for growth would be on the order of several thousand years. Although this time scale is similar to current estimates for the time needed for the geomagnetic field to undergo a reversal, the instability would grow at a rate equal to the difference between the ideal growth rate and the overall decay rate. The rates of viscous and electromagnetic damping are determined by material properties of the core fluid that are not well known. If elliptical instability plays a central role in geomagnetic reversals, upper limits on the viscosity and conductivity of the fluid core might be inferred. (C) 1997 Elsevier Science B.V.