Experiments on Convection in Earth's Core Tangent Cylinder

Results of thermal convection experiments in a rotating uid with a geometry similar to the Earth's core tangent cylinder are presented. We nd four di erent regimes of convection. In order of increasing heat ow, these are: (1) subcritical (no convection), (2) helical plumes (hetons) around the tangent cylinder, (3) hetons throughout the tangent cylinder, and (4) fully three-dimensional convection. The convection generates a retrograde (westward) azimuthal thermal wind ow below the outer spherical boundary, with maximum velocity on the tangent cylinder. The velocity of the thermal wind scales with buoyancy ux B and Coriolis parameter f = 2 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 solidi cation is available to drive tangent cylinder convection, and also if the inner core growth rate is high. According to our experiments, the tangent cylinder acts as a reservoir for products of inner core growth, and may have a slightly di erent composition than the rest of the outer core.