THE GEOMAGNETIC HELD OVER THE PAST 5 MILLION YEARS

The modern geomagnetic held is usually expressed as a spherical harmon ic expansion. Although the palaeomagnetic record is very incomplete in both space and time, sufficient data are available from a span of age s to generate time-averaged spherical harmonic field models with many degrees of freedom. Here three data sets are considered: directional m easurements from lavas, inclination measurements from ocean sediments, and intensity measurements from lavas. Individual data are analysed, as well as site-averages, using the same methods that have been develo ped for the modern held, to give models for the past 5 Myr. The normal -polarity held model has an axial-dipole intensity similar to that of the modern-day held, whilst the equatorial-dipole component is very mu ch smaller. The held is not axisymmetric, but shows flux concentration s at the core's surface under Canada and Siberia similar to those obse rved in the held over historical timescales. Tests on synthetic data s how that it is unlikely that these similarities result from the overpr inting of the palaeomagnetic held due to inadequate cleaning of the sa mples. The reverse-polarity held model does not show such obvious feat ures, but this may be due to the sparsity of the data. The patterns ob served in the normal-polarity held, with persistent features in the no rthern hemisphere and a smooth southern hemisphere, could be explained if the present pattern of secular variation is typical of the past se veral million years. This would reveal itself as large variations over time in the direction of the magnetic vector in regions of high secul ar variation, with relatively little change over quieter regions. Howe ver, we have been unable to find any evidence for a geographical patte rn of secular variation in the data.