The Occasional Reversal of the Geomagnetic Field

It is presently believed that the geomagnetic field is generated by the combined effects of the cyclonic convective motions and the nonuniform rotation of the liquid core of Earth. Fossil magnetism shows that the fie'd remains steady for periods of the order of 1O6~1O7 years, and then, at apparently random times, reverses abruptly, thereafter remaining steady for another 1O~-1O~ years, etc. We do not believe that the reversal of the field is caused by a temporary change in the sign of the cyclonic or nonuniform rotations, since the signs of both are determined by the Coriolis forces of the rotating Earth. We demonstrate that a statistical fluctuation in the distribution of the fifteen to twenty cyclonic con~ vective cells in the core produces an abrupt reversal of the geomagnetic field. The simplest fluctuation lead- ing to reversal is a general absence of cyclones below about 300 latitude for a time comparable to the life (`-%`1O~ years) of the individual cell. Assuming that the convective cells are statistically independent of each other, we estimate that such a fluctuation might occur once in 1O~ years. The simplicity of the mechanism recommends it as the explanation for the reversal of the field at random intervals