The Fluid Mechanics of Astrophysics and Geophysics

Paleomagnetic data on geomagnetic reversals are divided into two general categories: times of occurrence, and records of directional and/or intensity changes for transitions at individual locations. Despite consider-able efforts expended in acquiring paleomagnetic reversal records, a detailed picture of the reversal process is still lacking, along with any means of clearly identifying when the magnetic field has entered a transitional state destined to lead to a reversal. Accurate dating remains critical to making inferences about timing and structure of reversals and excursions. Controversy remains about the significance of such features as the preferred longitudinal paths that virtual geomagnetic poles at some sites seem to follow during excursions and reversals. Reversal rates are estimated under the assumption that reversal occurrence times can be described as a Poisson process. Correlations are sought between reversal rates and other properties of the paleomagnetic secular variation, and more general models for reversals and secular variations are being developed to provide predictions of the power spectrum of geomagnetic intensity variations for comparison with those derived from long paleomagnetic records. These analyses may ultimately allow the identification of any characteristic time scales associated with the geomagnetic reversal process, and should prove useful in evaluating the behavior observed from numerical simulations of the geodynamo. 1