An Alternative Picture of the Geomagnetic Field

Abstract

Spherical harmonic models of the Earth's magnetic field are a convenient way of describing the characteristics of the field. It has been common practice to use the Lowes/Mauersberger function to describe the power of the field in each degree of harmonic. There are several characteristics of this power spectrum which are well known from recent analyses of the geomagnetic field. Firstly, there is a general decrease of power as the degree of harmonic is increased. This general pattern is that the power varies as a function of a constant raised to the power n where n is the degree of the harmonic. However, there are some exceptions to this rule of thumb. Firstly, the dipole term (n = 1) is higher than expected. Secondly, the degree two term is lower than expected. Thirdly, the degree eight term is lower than expected. Although these facts might be considered to be just a chance happening, characteristic of today's field but not of other fields, it is shown that these characteristics are true for the last 200 years, during which time both the degree two and the degree eight individual harmonic terms change drastically. It is therefore suggested that these patterns represent a more permanent feature of the field, which may be caused by the sources approximating current loops located close to the core-mantle boundary (CMB). Another interesting fact about the spherical harmonic analyses of the field over the past 200 years is that the low order harmonics tend to be significantly stronger than the high order harmonics. This probably reflects sources which are preferentially located at higher latitudes, or are stronger at higher latitudes than at lower latitudes. The rates of change of the field are also studied, and it is shown that these rates of change are similar to what might be expected if the foci of the field at the CMB moved without changing their strength greatly. These simple observations based on analysis of the Gauss coefficients give information which may be useful for dynamo theories of Earth's magnetic field.