Abstract

Paleomagnetic studies reveal that Earth’s magnetic field is sustained by a dynamo in its liquid outer core and is intimately linked to the history and thermal evolution of our planet. The field has reversed polarity many times: these apparently random reversals occur when its overall strength decays, and there are departures from the usual structure, which at Earth’s surface is to first order that of an axially aligned magnetic dipole. We know surprisingly little about the origin of these radical events, which are the most extreme elements of a continuum of magnetic field changes that also includes geomagnetic excursions, rapid changes in field strength, and other less spectacular features collectively known as paleosecular variation. Stochastic models based on paleofield observations are developed to describe field changes on all timescales and provide a bridge to enhance physical and mathematical understanding for dynamo modelers engaged in theoretical work and in direct numerical simulations. There is scope for significant future advances in this arena.