Experiments on convection in Earth’s core tangent cylinder

Results of thermal convection experiments in a rotating fluid with a geometry similar to the Earth’s core tangent cylinder are presented. We find four different states. In order of increasing Rayleigh number, these are: (1) subcritical (no convection), (2) helical plumes around the tangent cylinder, (3) helical plumes throughout the tangent cylinder, and (4) fully three-dimensional convection. The convection generates a retrograde (westward) azimuthal thermal wind flow below the outer spherical boundary, with maximum velocity on the tangent cylinder. The velocity of the thermal wind scales with convective buoyancy flux B=αgq/ρCp and Coriolis parameter f=2Ω as U≃2(B/f)1/2. Tangent cylinder convection can explain the origin of polar vortices beneath the core–mantle boundary inferred from geomagnetic secular variation, if a large fraction of the buoyancy produced by inner core solidification remains within the tangent cylinder, and also if the inner core growth rate is high. According to our experiments, the tangent cylinder can act as a reservoir for products of inner core growth, although the effect is probably far too small to detect from surface observations.