A model for transition fields during geomagnetic reversals

Abstract

Palaeomagnetic studies of the reversal of the geomagnetic field have produced a number of palaeomagnetic records of intermediate or transition fields during reversals. These transition fields may provide an important clue as to the nature of the geodynamo. During the last reversal, the geomagnetic field was no longer dipolar¹, but the field appeared to be dominated by low-order harmonics^2–4. Such transition-field geometries predict that all palaeomagnetic records will include high inclinations of either + or − 90°, which give the virtual geomagnetic pole paths passing through the site or its antipode. Subsequently, when transition records were found not showing these characteristics, the idea of the dominance of low-order zonals was questioned. Here we show that if the low-order zonals are combined with a drifting non-dipole field, similar to that seen at present, most of the features of reversal transitions can be simulated. We therefore suggest that low-order zonals may indeed dominate transition fields, but they are accompanied by some form of drifting non-dipole field.