Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmgp12a1092g&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #GP12A-1092
Other
1507 Core Processes (8115), 1510 Dynamo Theories, 1522 Paleomagnetic Secular Variation, 1555 Time Variations: Diurnal To Secular
Scientific paper
The Rayleigh number (Ra) for the Earth's core is a dimensionless measure of the heat flux crossing the core-mantle boundary (CMB). Recent calculations on the properties of liquid iron mixtures at core pressures and temperatures have provided reliable data for thermodynamic estimates of core heat flow, all of which suggests that heat budget is rather tight: Either the core is cooling so fast that the inner core is young and formed only 1~Gyr ago, or the core contains substantial radiogenic heating from K40. These heat flow estimates translate into Ra≈ 1000Rac, the critical value for convection assuming a magnetic field of typical core strength and turbulent values of viscosity and thermal diffusivity [Gubbins, Phys. Earth Planet. Int., 128 3--12, 2001]. Magnetic fields facilitate convection in a rotating system; for core parameters they lower Rac by a factor of about 1000. The estimate of Ra is therefore close to the marginal value, the minimum heat flux needed for convection in the core if the magnetic field were absent. Another estimate was made by Jones [Phil. Trans. R. Soc. Lond., 358 871--872, 2000] by considering the heat transported by convection in the core. This gives a similar value of 500Rac; it is independent because the basic information used is a typical estimate of core flow speed rather than CMB heat flux. The third estimate comes from the observation of low secular variation in the Pacific. Seismic tomography shows low velocity, probably hot material, at the base of the mantle in the Pacific region. Using this as a boundary condition for simple convection calculations shows that the usual Busse rolls are completely suppressed in the Pacific when lateral variations are 30%\ of the mean heat flux and total heat flux is just above critical. While much work needs to be done on extending these simple calculations, they imply that the heat flux from the Pacific region is subadiabatic. Converting the variations of seismic velocity within D'' to heat flux, and assuming this to 30%\ of the mean, again gives a Rayleigh number of about 1000Rac. All 3 independent estimates are low in that they are close to the lowest possible value required to sustain core convection without a magnetic field. This has important implications for geodynamo models, which should be easier in the low Ra regime.
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