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Pacific Secular Variation: A Result of Hot Lower Mantle

Wednesday, March 20, 2002 - 10:30am - 11:30am
Keller 3-180
David Gubbins (University of Leeds)
The Earth's magnetic field is generated by convection in the liquid core, which in turn is driven by internal sources of buoyancy strongly influenced by rotation, magnetic forces, and the boundary conditions. The core-mantle boundary is an isothermal surface, but convection in the mantle causes variations in the heat flux across the boundary, variations that may exceed the average heat flux out of the core. These variations can influence core convection and place an imprint of lower mantle heat flux onto the geomagnetic field itself. Observational evidence for this comes from the modern field, which is concentrated where lower mantle has high heat flux, beneath the Pacific rim, the paleomagnetic time average for the last 5 million years, and persistent patterns in the transition field during polarity reversals, which appear to have poles which track around the Pacific rim.

Geomagnetic secular variation has been low in historical times. Paleomagnetic results from Hawaii show that this anomaly has persisted for 5 thousand years and probably longer, and is therefore likely to be a permanent feature also associated with heat flux anomalies on the core-mantle boundary. Core convection calculations show that a heat flux boundary condition derived from lower mantle seismic velocities causes convection to be suppressed beneath the Pacific but leaves convection rolls drifting around the Atlantic hemisphere. This is very similar to the appearance of secular variation over the last 400 years, where westward drifting features form near the Pacific rim, drift west, and disappear when they reach the west coast of the Americas. Low Pacific secular variation may therefore be one more result of the lower mantle's influence on the dynamo.