Physics – Geophysics
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt........10a&link_type=abstract
Thesis (PhD). THE JOHNS HOPKINS UNIVERSITY, Source DAI-B 60/11, p. 5403, May 2000, 203 pages.
Physics
Geophysics
Scientific paper
Three aspects of deep Earth geophysics are investigated in this dissertation: (1)heterogeneous, electromagnetic coupling between the core and mantle, (2)rotational dynamics of the inner core, and (3)experimental rotating magnetoconvection in liquid gallium. Analytical calculations are performed to determine whether spatially heterogeneous electromagnetic torques between the core and mantle can explain virtual geomagnetic pole (VGP) paths found in some paleomagnetic studies of polarity reversals. These calculations show that during a reversal, the VGP is attracted to low conductivity regions in the lower mantle and repelled from high conductivity regions. Further calculations are made in which the core is free to rotate with respect to the mantle in response to the heterogeneous electromagnetic torques. In these calculations, resistive torques are induced by the differential motion between the mantle and the core and prove to be far greater than the heterogeneous torques. Thus, it is concluded that heterogeneous core-mantle torques have little effect on the position of the VGP during polarity reversals. The rotation of the solid inner core is studied with analytical and numerical models. These models show that it is possible to explain the seismically-inferred prograde rotation of the inner core [Song and Richards, 1996] by electromagnetically coupling the inner core to the thermally-driven, outer core fluid that is within the tangent cylinder, the imaginary cylinder tangent to the inner core equator. In contrast, the anomalous rotation of the inner core is not explicable in terms of a cylindrical model of flow, inferred from the geomagnetic westward drift. The interaction of electromagnetic, gravitational and mechanical inner core torques is also numerically modelled. The highest estimates of the gravitational torque produce a locked inner core that remains fixed in azimuth at the bottom of the gravitational well. For the lower bound estimate of the gravitational torque, electromagnetic torques can dominate and the inner core travels in and out of successive gravitational wells. The effect of mechanical torques is shown to be small in all cases. In a study of rotating magnetoconvection, experiments are performed in a plane layer of liquid gallium that is heated from below and cooled from above. The rotation axis and the imposed magnetic field are both aligned parallel to laboratory gravity (in the vertical direction). These experiments model the convection dynamics in the metallic outer core fluid located within the tangent cylinder. In magnetoconvection experiments, the onset of convection is inhibited by vertical magnetic fields. In rotating convection experiments, convection is strongly suppressed at high rotation rates and the onset of convection is oscillatory. The simultaneous effect of magnetic field and rotation always suppress thermal convection in the range of parameter space studied. Oscillatory convection is detected in the rotating convection experiments. In all of the convection experiments, time-dependent temperature signals are recorded very close to the onset of convection and, therefore, a regime of stationary convection is not detected. Thus convection in liquid gallium is inferred to be chaotic or even turbulent just beyond the onset of convection.
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