An Analysis of the Earth's Magnetic Field from 1835 to 1965

Résumé du livre

This work is an analysis and description 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 geomagnetic field elements are employed to depict time trends and structural dissymetries at the earth's surface and core-mantle boundary. Our analysis establishes that the magnetic dipole field is being driven destructively to smaller values by fluid motions which transform its magnetic energy into near neighboring modes rather than expend it more directly as joule heat. Concurrent with the continual decrease in field energy in the atmosphere, since 1900 at least, sixth-degree analyses indicate that the mean magnetic-energy density in the outermost layers of the core fluid has been increasing. An associated phenomenon is the continued increase in hemispheric dissymetry, the Southern Hemisphere preponderating. Defining an asymmetry ratio, our analysis infers by extrapolation that the geomagnetic field was sensibly symmetric in the year 1670 A.D. The average rate of westward drift of the geomagnetic pole during the past 130 years is 0.042 degree/year in azimuth angle, corresponding to a polar rotation period of 8.6 thousand years. The motion appears, however, to have slowed somewhat since about 1900. The polar angle shows no progressive motions.