Recent trends in the Earth's magnetic field

Abstract

This work is an abridgment of a more extensive analysis of the earth's magnetic field based on observed data from the time of Gauss to the most recent epochs. An attempt was made to collect all Gauss-Schmidt coefficients of previous investigators, and extensive computations were made using selections of these data. Root-mean-square representations of the geomagnetic field elements are employed to depict time trends and structural dissymmetries at the earth's surface and core-mantle boundary. It is established that the magnetic dipole field is being driven destructively to smaller values by fluid motions which transform its magnetic energy into that of the near neighboring modes rather than expend it more directly as Joule heat. Concurrent with the continual decrease in field energy in the atmosphere, at least since 1900, sixth-degree analyses indicate that the mean magnetic energy density in the outermost layers of the core fluid has been increasing at a small rate. An associated phenomenon is the continued increase in hemispheric dissymmetry, the southern hemisphere preponderating. Defining an asymmetry ratio, we find by extrapolation that the geomagnetic field was sensibly symmetric in A. D. 1670. The average rate of westward drift of the geomagnetic poles during the past 130 years is 0.042°/year in azimuth angle, corresponding to a polar rotation period of 8.6 thousand years. The polar angle shows no progressive motions.