Inertial waves driven by differential rotation in a planetary geometry

Abstract

Dynamics occurring in the Earth's outer core involve convection, dynamo action, geomagnetic reversals, and the effects of rapid rotation, among other processes. Inertial waves are known to arise in rotating fluids, and their presence in the core has been previously argued using seismological data (Aldridge and Lumb 1987). They may also be involved in flows affecting the geodynamo. We report experimental observations of inertial wave modes in an Earth-like geometry: laboratory spherical Couette flow with an aspect ratio 0.33, using liquid sodium as the working fluid. Inertial modes are detected via magnetic induction and show good agreement with theoretical predictions in frequency, wavenumber, and magnetic induction structure. Our findings imply that linear wave behavior can dominate the dynamics even in turbulent flows with large Reynolds number Re, where nonlinear behaviors might be expected (here Re ∼ 107). We present evidence that strong differential rotation excites the modes via over-reflection. Earth's inner core may also super-rotate and thereby excite inertial modes in the same way. Zonal flows in the core, likely to have higher speeds than the super-rotation, may be a stronger source for exciting inertial modes in the Earth.