HR: 1330h
AN: G22A-0299    [PDF]
TI: Effect of an Overlying Stable Layer on Planetary Dynamos
AU: * Schubert, G
EM: schubert@ucla.edu
AF: Department of Earth and Space Sciences, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 595 Charles E. Young Drive East, Los Angeles, CA 90095-1567 United States
AU: Chan, K H
EM: khchan@ust.hk
AF: Department of Mathematics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, 0000 China
AU: Liao, X
EM: xhliao@center.shao.ac.cn
AF: Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai Astronomical Observatory, Shanghai, 200030 China
AU: Zhang, K
EM: KZhang@maths.ex.ac.uk
AF: School of Mathematical Sciences, University of Exeter, North Park Road, Exeter, EX4 4QE United Kingdom
AB: A fully three-dimensional, nonlinear, time-dependent, multi-layered spherical kinematic dynamo model is used to study the effect on the observable external magnetic field of flow in a stable layer above a spherical turbulent dynamo region. For a rapidly rotating planet with sufficiently large magnetic and ordinary Reynolds numbers it is reasonable to assume that turbulence in the magnetic field generation region is associated with an $\alpha\/$ effect having a symmetry reflecting the rapid rotation. In this case, we would expect the observed planetary magnetic field to be predominantly a dipole aligned with the rotation axis. Except for Saturn, observed planetary magnetic fields are more complicated. We show that the structure of the observed field is essentially determined by the flow in the overlying stable layer. It is also shown that a strong-field planetary dynamo can be readily produced by the circulation in a stable layer above the turbulent convective region. Such stable layers might exist at the top of the Earth's core due to chemical or thermal causes, in the cores of other terrestrial planets for similar reasons, and in Saturn due to the differentiation of helium from hydrogen. An electrically conducting and differentially rotating layer could exist above the metallic hydrogen region in Jupiter especially if the observed near surface zonal winds extend to great depth. Lateral temperature gradients resulting in thermal winds drive the flow in the stable layers. It is the amplitude and structure of the flow in the stable layer that mainly determines the nature of the observable magnetic field. Saturn's axisymmetric, rotation-aligned dipole field could indicate either the presence of axisymmetric flow in an overlying stable shell (Stevenson, 1982) or the absence of such a stable layer. The structure of the Earth's magnetic field could simply reflect the structure of stable-layer thermal winds driven by lateral thermal heterogeneity in the lower mantle. Planetary dynamos might have more in common with the solar dynamo than is generally thought. It is the differential rotation in the solar tachocline below the turbulent convective dynamo region that largely determines the characteristics of the solar magnetic field. A unified model could therefore explain all solar system dynamos. Stevenson, D.J., Reducing the non-axisymmetry of a planetary dynamo and an application to Saturn, J. Geophys. Astrophys. Fluid Dyn., 21, 112--127, 1982.
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 1507 Core processes (8115)
DE: 1510 Dynamo theories
SC: Geodesy [G]
MN: 2003 Fall Meeting