EXPERIMENTS ON PRECESSING FLOWS IN THE EARTHS LIQUID-CORE

Experiments simulating flow in the Earth's liquid core induced by luni -solar precession of the solid mantle indicate, to a first approximati on, that the core behaves like a rigidized fluid sphere spinning slowe r than the mantle and with its spin axis lagging the mantle spin axis in precession. Secondary flow patterns are always present. At low prec ession rates the fluid sphere is subdivided into a set of cylinders co axial with the fluid spin axis, the cylinders rotating alternately at slightly faster and slower rates relative to the net retrograde motion of the fluid as a whole. Slow non-axisymmetric columnar wave patterns develop between the differentially rotating cylinders. Axial flows be tween the spheroidal cavity boundary and the interior are observed. Fl uid motion becomes turbulent only at precession rates large enough to cause the fluid spin axis to align nearly with the precession axis. Th ere is no evidence that the Earth's liquid spin axis direction departs more than a fraction of a degree from geographic north. Our observati ons suggest precession induces a complex variety of laminar flows, inc luding slowly varying and/or periodic patterns, in the Earth's liquid core.