An analysis of the geomagnetic field over the past 2000 years

Using the archeomagnetic database of Daly, L. and Le Goff, M. [An updated and homogeneous world secular variation database. Smoothing of the archeomagnetic results, Phys, Earth Planet. Inter, 93 (1996) pp. 159-190.] complemented by archeomagnetic data from Peru, by sediment data from Argentina and New Zealand and by lava data from Hawaii and Sicily, we compute a time-varying spherical harmonic model of the geomagnetic field between 0 and 1700 A.D. The computation has been validated with the help of artificial data produced by a stochastic process having the statistical properties of the historical field. With the sites available, a model up to degree 2 plus the degree 3 order 3 Gauss coefficients are derived. We recover the well-known fact that on average, the dipole field has been decreasing over the past 2000 years. A statistical analysis of our model also confirms an earlier suggestion (based on 300 years of historical data), that the typical time scales involved in the degree 2 and 3 components are of the order of 150 years, suggesting that these components are unrelated to their past values after a period of about 450 years. These estimates have been used to assess the statistical meaning of average values derived from the model: The average nondipole field does not differ significantly from zero. However, the observed average g(2)(0), of the order of 3% the observed average g(1)(0), is more compatible with the average value required by the palaeomagnetic data than with the null assumption. Such a g(2)(0) is therefore a likely lasting feature of the magnetic field. The average energy for the degree 1, 2 and 3 Gauss coefficients available from our model have been used to plot an average spectrum for the archeomagnetic field. At the Core-Mantle Boundary, this spectrum displays the familiar V shape observed in modern,geomagnetic spectrum and corresponding to the fact that there is less energy in the degree 2 than in the degree 3, Plotting the bounds within which the spectrum is expected to fluctuate suggests that this low energy degree 2 could be a second characteristic feature of the present regime of the geodynamo. We therefore conclude that despite the marginal statistical significance of our claims, there is a suggestion in the archeomagnetic data for some g(2)(0) component in the average field and for some significant structure in the spatial spectrum. (C) 1998 Published by Elsevier Science B.V. All rights reserved.