The cause of superchrons

Superchrons – long periods in the geomagnetic record when the Earth's magnetic field did not reverse its polarity – are a challenge to observers and theorists. Jack Jacobs outlines the problems and some possible solutions.
Reversals of polarity are a feature of the geomagnetic record for all the time it has been documented. Although not regular, reversals are sufficiently frequent for their absence to be noticeable. When the Earth's magnetic field retains the same polarity for over 20 million years, a superchron is established. Superchrons demand the attention of geophysicists concerned with the generation of the Earth's field: either they must result from an intrinsic feature of the geodynamo, or they reflect the influence of some external force. Here I discuss internal and external mechanisms for the formation of superchrons, including the role of the inner core, true polar wander, Earth's orbital variations and tides.