Temporal aspects of the last reversal of Earth's magnetic field

Spatial palaeomagnetic field data associated with the Matuyama-Brunhes reve rsal reveal a global pattern of virtual geomagnetic poles (VGPs) so distinc t that inferences about the temporal nature, and, possibly, the energetics of the dynamo process, can be drawn. Specifically, what is observed are two pairs of low- and mid-latitude VGP concentrations mirrored about the Equat or. Yet, while the path of the virtual pole associated with any given site must pass through the Equator at least once during the polarity transition, remarkably few data record the actual crossing. We propose that early in t he reversal process a transitional state was reached within the dynamo and held for a considerable time; only much later was it possible for field cha nges to take place whereby VGPs crossed the Equator into the Northern Hemis phere. Thus, a barrier to reversal may exist, related, perhaps, to the stat e of magnetic Aux emerging from the solid inner core. Regardless, these pal aeomagnetic data suggest that when this apparent barrier was successfully s urmounted, field directions at sites about the globe rotated quickly in suc h a manner that the associated VGPs transited the Equator at minimum rates of speed approaching 1 deg yr(-1). The contention that the Matuyama-Brunhes reversal was dominated by quasi-stationary transitional field states of lo w harmonic order is supported by the data; however, the stage of rapid VGP movement needs to be associated with a predominantly driving dynamo process .