Spherical harmonic analyses of the palaeomagnetic field

Analytical models of the palaeomagnetic field have been constructed for a number of geological periods (Quaternary Neogene, Jurassic, Triassic, Permian and Permo-Carboniferous) by spherical harmonic analysis using the present-day world map as a basis and (for the earlier periods) using also a palaeogeographic reconstruction. The use of the palaeogeographic chart for the earlier periods simplifies the models, and its use appears to be valid. The low accuracy, small number and uneven distribution of palaeomagnetic data severely limit the conclusions which can be drawn from the analyses. Nevertheless the results for all periods indicate that throughout the past 300 million years the geomagnetic field has maintained its global structure, and has remained similar to the field of a dipole slightly shifted from the Earth's centre. It appears that there have not been any persistent systematic anomalies or variations in the Earth's magnetic field throughout that time, but rather that the field has been oscillating around a mean level not greatly different from that of the present epoch.