A Model for transitional field geometries involving low‐order zonals and drifting nondipole harmonics

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Abstract
It is now clear that the transitional fields during reversals are not dominated by dipole fields nor are they purely zonal harmonics. Zonal fields generate paleomagnetic reversal records in which inclination reverses by a gradual change through a vertical inclination and the declination instantaneously switches from 0 to 180° or vice versa. In terms of VGP paths these directions define either a “near sided” path, in which the pole passes through the site, or a “far sided” path, in which the pole passes through the sites antipode. Although some records approximating this character have been found, other types of records reveal that the transitional field cannot be purely zonal. An earlier zonal model (Williams and Fuller, 1981) has been modified to include a drifting nondipole field and is successful in simulating many features of the paleomagnetic reversal records. A prediction of this model is that paleomagnetic reversal records at particular sites depend upon the phase relation between the decay and reversal of the low order harmonics and the passage of drifting anomalies of the nondipole field. Therefore, records obtained from different latitudes and longitudes will have quite different appearances. Only by looking at multiple records of a single reversal will the nature of the field become apparent. The model is nonunique, but is, at least, consistent with the data for the Matuayama/Brunhes transition. We suggest that it supports the idea that zonal harmonics dominate transition fields, but they are accompanied by secular variation which may involve drifting nondipole fields.