Abstract:

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.