Core precession: flow structures and energy

Abstract

Experiments using a precessing liquid‐filled oblate spheroid with ellipticity (a − b)/a =1/400 extend and clarify earlier research. They yield flow data useful for estimating flows in the Earth’s liquid core. Observed flows illustrate and confirm a nearly rigid liquid sphere with retrograde drift and lagging a cavity (mantle) axis in precession. The similarities of the observed lag angle with that computed for a rigid sphere, and earlier energy dissipation research both support the use of a rigid sphere analytical model to predict energy dissipation and first‐order flow within the core–mantle boundary (CMB). Second‐order boundary layer and interior cylindrical flow structures also are photographed and measured. Interior flows are never turbulent or unstable at near‐Earth parameters, although complex and transient flow patterns are observed within the boundary layer. Other mechanisms proposed to explain net heat loss from the Earth and maintenance of the geodynamo typically require acceptance of some critical but unproven premise. Precession and CMB configuration are known with certainty and precision. Analytical difficulties have been the obstacle. Experiments illustrate the consequences of precession and ellipticity, provide criteria for validating analytical and numerical models, and may yield direct knowledge of the Earth’s deep interior with careful scaling.

On effect of precession-induced flows in the liquid core for early Earth's history

“…That is why experimental results obtained by Vanyo et al (1995) and Vanyo and Dunn (2000) differ significantly from that of Malkus (1968). Specifically, Vanyo et al have been able to match the Earth's parameters more closely than in previous experiments.…”

“…Specifically, in the first investigations of precessioninduced flow in oblate spheroid by Malkus (1968) the ellipticity of containers took values 1/10 and 1/25. The last experiments by Vanyo et al (1995) and Vanyo and Dunn (2000) have included spheroids with ellipticities 1/100 and 1/400. Minimal Poincare number achieved in experiments by , and Ekman number was 3.6×10 −6 and 2×10 −7 , respectively.…”

“…The following flow features have been identified in Vanyo et al (1995) and Vanyo and Dunn (2000) experiments: 1) A uniform rotation component with an angular rotation vector deviating (at a very small angle) from the container's rotation axis. The fluid spin axis precesses at the same rate as the container spin axis, but with a lag.…”

“…Using evidence from experiments (Vanyo et al, 1995;Vanyo and Dunn, 2000) we consider the background flow structure with differentially rotating cylinders (shear structure), and ignore for simplicity a deviation of fluid angular rotation from the container's rotation axis.…”

“…It is a commonly accepted hypothesis that buoyancy powers stellar and planetary (for example, the Earth) dynamos while precession has a weak influence on the Earth's outer core flows and geodynamo. From recent experiments (Vanyo et al, 1995;Vanyo and Dunn, 2000) we know, however, that flows in precessing containers (liquid-filled spheroids with the Earth's like ellipticity) can become unstable and even turbulent. The flow data illustrate the real consequences of precession and ellipticity, which together with hypothetical thermal and compositional convection may give insight to direct knowledge of the Earth's deep interior and analytical solution of the geodynamo problem.…”