Thermal coupling between the mantle, outer core and inner core: an experimental model

Details Thermal coupling between the mantle, outer core and inner core: an experimental model Thermal coupling between the mantle, outer core and inner core: an experimental model

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmdi24a..02s&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #DI24A-02

Physics

1560 Time Variations: Secular And Longer, 7207 Core (1212, 1213, 8124), 8115 Core Processes (1213, 1507), 8147 Planetary Interiors (5430, 5724, 6024)

Scientific paper

Thermal coupling between the mantle and the outer core has been proposed based upon the correlation between the patterns of stationary geomagnetic field and the seismic heterogeneity of the lower mantle (Bloxham and Gubbins, 1987). We have studied how such thermal heterogeneity can affect the outer core flow, using laboratory experiments in a rapidly rotating hemispherical shells (Sumita and Olson, 1999, 2002). Here I review the results obtained from these experiments, and offer their implications to the Earth's core. We use a hemispherical shell with an outer diameter of 30 cm and spin it at 207 rpm to achieve an Ekman number of 4.7 × 10-6. By circulating a cooling water through the inner sphere, we impose a radial temperature gradient, and achieve a Rayleigh number of up to 44 times the critical value. For most Rayleigh numbers (Ra/Rac > 8), thermal convection consists of meandering plumes that originate from inner and outer boundaries and are advected westward by the mean zonal flow (Sumita and Olson, 2000). When we impose a thermal anomaly at the outer boundary using a strip heater we find that the warm fluid generated by the heater flows eastward. When Q\ast = (Applied total heat flow)/(Total heat flow at ICB) > 0.7, we find that a stationary front forms at the east of the heater which separates the warm eastward flow and cold westward flow. The stationary front take the form of a spiral and extends from the outer boundary towards the inner boundary, along which a jet flows towards the inner boundary. Simple estimate shows that the condition Q\ast > 1 can be satisified in the Earth's core. Since centrifugal force is used to simulate the radially dependent gravity, a heater in the experiment corresponds to a cold anomaly at the CMB. Seismic tomography suggest that such an anomaly exists beneath east Asia. Our experiments suggest that there is a cold eastward flow in the Pacific and a warm westward flow elsewhere which is consistent with the core flow model obtained from geomagnetic secular variation (Bloxham and Jackson, 1991). Our experiments also suggest that inner core growth rate is fast at the western hemisphere, which coincides with the region of large P-wave anisotropy of the inner core (Tanaka and Hamaguchi, 1997). According to the model of Yoshida et al. (1996), elastic strain energy of deformed crystals, which is the cause for preferred orientation, scales as \propto (growth rate)2, and thus explains the region of large anisotropy in the western hemisphere.
Sumita and Olson, 1999, Science, 286, 1547-1549. ibid, 2002, J.Geophys. Res., 107, 10.1029/2001JB000548.

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