Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p44a..08g&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P44A-08
Other
[1507] Geomagnetism And Paleomagnetism / Core Processes, [1510] Geomagnetism And Paleomagnetism / Dynamo: Theories And Simulations
Scientific paper
Core convection is driven by a combination of thermal and chemical buoyancy. Both sources of buoyancy are likely to be weaker at the top of the core than at the bottom, the former because the adiabatic gradient is steeper at the top and conducts more heat away, the latter because it is unlikely that light elements pass through the CMB to any large degree, making zero flux the appropriate boundary condition there. This raises the possibility of stable stratification at the top of the core. Furthermore, light elements may still be dissolving into the core from the mantle, forming a thin stratified upper layer. Reverse flux patches on the core-mantle boundary arise from expulsion of toroidal magnetic field: no other mechanism seems big enough to explain their formation in the Southern Hemisphere during the last 200 years. This requires upwelling within the top 70 km or so of the core, giving evidence against a stably stratified layer. Furthermore, absence of flux inside the tangent cylinder requires upwelling in a region where convection is hard to drive. On the other hand, thermal core-mantle boundary interaction is inhibited by strong convection in the uppermost core: thermal anomalies in the mantle can only influence deep core convection by thermal conduction through the upper layers of the liquid core. These effects are demonstrated using numerical geodynamo simulations with varying stratification at the top. Extrapolation to Earth-like parameters suggests that we would be very unlikely to observe any thermal core-mantle interaction with convection occurring all the way to the core-mantle boundary. The conflict between these observations, of reverse flux formation, low polar flux, and thermal core-mantle interaction, can be resolved with a thin stratified layer 50-100 km thick at the top of the core.
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