EVIDENCE FOR A GEODYNAMO, DRIVEN BY THERMAL ENERGY AT THE CORE MANTLE BOUNDARY AND BY PRECESSION DEEPER IN THE OUTER CORE

VGP paths during reversals are preferentially located in roughly north south circum-Pacific arcs, above where the lower mantle is coolest. This indicates that the movement of the corresponding magnetic flux concentrations in the outermost core responds during the reversal to thermal constraints. Hence thermal energy is involved in driving the geodynamo. The preponderance of the occurrence of reversals within the last 5 Ma, when the obliquity is lower than the average, and their preferential onset during the decreasing half of the obliquity cycle also provide evidence for a role of precession in driving the dynamo. The geodynamo therefore appears to be driven by a combination of thermal and precession energy. An essential part of its operation is coupling of the two mechanisms through the exchange of flux from the deeper outer core precession controlled region into the outermost flow pattern. This appears to be primarily in a region beneath the Eastern Indian Ocean towards Indonesia.