Fluctuation of the nondipole magnetic field and its implication for the process of geomagnetic polarity reversal in the Cox model

Abstract

In the Cox model on the mechanism of polarity reversal of the Earth's magnetic field, the nondipole magnetic field is assumed to behave differently from the dipole magnetic field; the former fluctuates at random with a time scale of the order of 1000 years, whereas the latter varies systematically with a longer time scale. Our analyses of Gauss coefficient data for various epochs show that the characteristic time of nondipole fluctuation is about 500 years, while paleomagnetic studies for lake sediments indicate 1000–2000 years. These estimates are comparable to the time scale for nondipole field fluctuation presumed in the Cox model. This result supports one aspect of the Cox model. It is also important to examine whether the Cox model actually presents a physically plausible mechanism of polarity reversal, in particular from the viewpoint of nonlinear dynamo action. In this respect, we first show that the Cox model is represented by a disk dynamo which is subject to magnetic perturbations exerted externally. Then we solve disk dynamo equations numerically and examine time evolution of the magnetic field of the disk dynamo with special attention to polarity reversals. We finally conclude that the mechanism of polarity reversal presented in the Cox model is basically operative. It is surprising, however, that excursionlike variations also appear as often as polarity reversals in this specific dynamo model. Detailed examination shows that the probability of magnetic field growth from the transitional state either in the previous direction (excursion) or in the opposite direction (reversal) is 1/2. We also conclude that the Cox model is capable of accounting for the exponential distribution of polarity intervals which has been found from paleomagnetic data.