The two states of paleomagnetic field intensities for the past 320 million years

Virtual dipole moments (VDM) from the past 320 million years show bimodal distributions when averaged over time intervals of several tens of millions of years. The two peaks in the bimodal distributions occur at about (4–5)×1022 A m2 and at about twice that value, these two values vary little with time. However, the relative contributions of the two modes do vary significantly with time. It is possible that one of these modes is associated with an as yet unidentified rock magnetic artifact. Alternatively, the observation admits the intriguing interpretation that as well as having two polarity states the geodynamo has two (relatively constant) intensity states. Our perception of the paleofield intensity, and its variation, in a given time interval then depends on how much time the dynamo has spent in each of the intensity states and on how well our observations have sampled this.