Local Reversal Durations and Inverse Magnetic Field Patches in Geodynamo Simulations

Paleomagnetists commonly infer reversal properties based on records from one site only. In an attempt to gain more global information about geomagnetic field reversals Bradford Clement (Nature, 2004) analyses the latitude dependence of reversal durations in several paleomagentic records. He finds that the field transitions takes signifincantly longer close to the poles than near the equator. Such a piece of information is important for testing numerical geodynamo simulations. Many simulated reversals show a comparable latitudinal dependence. These models offer the opportunity to identify the cause of such an effect. I have tried to link the reversal duration to the transitional magnetic field at the core-mantle boundary. Unfortunately, the connection is complex. Reversals tend to last longer then average where the first inverse patches appear at the core-mantel boundary. They are shorter than average where secondary inverse patches tilt over an already weak core-mantel boundary field. Any equatorial and large scale azimuthal transitional field asymmetries can be identified in a global map of reversal duration. It has been speculated that the intensifying inverse patches at the Earth's core-mantel boundary may constitute the start of a field reversal. So far, they are too weak to lead to significant changes in field inclination. This, however, may change if the inverse patches close to the equator move to higher latitudes. Such a behavior is frequently found in dynamo simulations and indeed can mark the start of a field reversal. However, more often the inverse patches disappear again causing only slight oszillations in inclination.