Alternative to the geomagnetic self-reversing dynamo

Abstract

A PRIMARY task of geomagnetism is to account for the magnetic field of the Earth and for the occasional reversals of that field. Present research in geomagnetic dynamo theory aims to develop a physically realistic, mathematically solvable, self-reversing dynamo model¹—one that contains the seeds of its own reversal. Here I point out that there is an alternative method of analysing the geomagnetic field which can account for magnetic reversals but which does not require a self-reversing dynamo. The analysis is based on the assumption that the geomagnetic field arises from two separate sources. Each source has a mathematical representation in terms of dipoles, quadrupoles, and so on. The magnetic field which is observed at or above the surface of the Earth is the sum of the fields arising from the two sources: in particular, the observed dipole component of the Earth's field is the vector sum of the dipole components of each source If the dipole components of each source are oppositely directed, one pointing essentially towards the north rotational pole and the other essentially towards the south rotational pole, then The prevailing magnetic polarity reflects the polarity of the dominant source component (Fig. 1). A magnetic reversal, which corresponds to a change in the sign of M₀, represents a shift in the relative sizes of M₁ and M₂. If both M₁ and M₂ are very large compared with M₀, then only small fluctuations in M₁, M₂, or both will result in a reversal.