How complex is the time-averaged geomagnetic field over the last 5 million years? Geophys

A basic tenet of palaeomagnetism is that the Earth’s magnetic field behaves on average like that of a central axial dipole (g0 1). Nevertheless, the question of possible persistent second-order features is still open. Recently Johnson & Constable (1995, 1996) performed a regularized non-linear inversion and found evidence for persistent non-zonal features. Formal uncertainties would indicate that there are significant (non-zero) terms at least up to degree and order 4. Using a recent compilation of two diVerent data sets from lavas (0 to 5 Ma) and the Johnson & Constable codes, we test the robustness of this result. The data set has been divided into three subsets: the Brunhes polarity data (B), all normal polarity data (N) and all reverse data (R). In each subset of data, a prominent g0 2, of the order of 5 per cent of g0 1, is clearly present, as previously established by several authors. In some subsets, smaller terms appear: g2 2 and g