Magnetohydrodynamics of the Earth's core. 1: Steady, rotating magnetoconvection. 2: Magnetic Rossby waves

Details Magnetohydrodynamics of the Earth's core. 1: Steady, rotating magnetoconvection. 2: Magnetic Rossby waves Magnetohydrodynamics of the Earth's core. 1: Steady, rotating magnetoconvection. 2: Magnetic Rossby waves

May 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........30b&link_type=abstract

Ph.D. Thesis Massachusetts Inst. of Tech., Cambridge.

Physics

Geophysics

Earth Core, Earth Mantle, Geomagnetism, Geophysical Fluids, Geophysics, Magnetic Variations, Magnetohydrodynamics, Planetary Waves, Rotating Fluids, Boundaries, Buoyancy, Coriolis Effect, Coupling, Destabilization, Forced Convection, Free Convection, Lorentz Force, Magnetic Dipoles, Models, Rayleigh Number, Secular Variations

Scientific paper

We review the evidence for core-mantle thermal coupling, which includes spatial correlation between the static magnetic field features, lateral thermal anomalies in the lower mantle, core-mantle boundary topography, and possibly virtual geomagnetic pole (VGP) path during magnetic dipole reversals. Maps of the main magnetic field at the Earth's surface suggest that anomalous electrical currents may be present in the equatorial zone, with a large azimuthal wavenumber m = 1 contribution. We invert the surface magnetic field for the magnitude of ideal magnetic dipoles distributed in various ways throughout the outer core. We find a better, smoother fit with equatorial dipoles than with polar dipoles. The equatorial anomalies in the surface main field correlate with regions of high surface magnetic secular variation, further suggesting persistent motions in the core. Although the convective equations are highly non-linear due to the high Rayleigh number of the Earth's core, so that time-dependent motions are certainly present, we are interested here in the possibility of steady motions that result from either free convection or forced convection and that might cause static features in the magnetic field. We provide destabilization in our model by uniformly fluxing buoyancy across the bottom boundary with a concominant buoyancy sink at the top boundary. The Rayleigh number Ra is a measure of the destabilization. We first apply our method to a non-rotating, electrically insulating spherical fluid shell in order to demonstrate the method's viability. We then look for solutions in a rapidly rotating spherical fluid shell (high Taylor number Ta). We next return to the rotating spherical shell, impose a poloidal magnetic field, and look for the magnetic analog of the polar models. When the Lorentz force is comparable to the Coriolis force (Elsasser number El = O(1)), the modes fill the shell and most efficiently transport buoyancy. Finally, we study axisymmetric flows forced by a high buoyancy flux across the upper boundary near the equator and a low flux near the poles. A second topic that we study in this thesis involves a possible consequence of compositional convection in the Earth's core: the formation of a stably stratified layer at the top of the outer core.

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