An analysis of the geomagnetic field over the past 2000 years

Abstract

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.