Andrew J. Biggin
SUMMARY Robust, unbiased statistics describing long-term geomagnetic behaviour are sorely needed to elucidate the dynamics and evolution of Earth’s magnetic field and the geodynamo which produces it. While palaeomagnetic data are available across much of Earth’s history, their utility is hampered by highly inhomogeneous distributions in space and time and by associated uncertainties. To address this, a set of parameters based on robust statistics and describing the average strengths of the axial dipolar and non-axial dipolar components of the field, the time variability of the total field and certain ratios of these are proposed. A framework for estimating these parameters is developed whereby global data sets of palaeomagnetic directions and intensities are compared to outputs from an ensemble of numerical geodynamo simulations. A bespoke Monte–Carlo proxy-based approach allows measurement uncertainties and spatial inhomogeneity in the data to be accounted for and the framework further allows for independent validation tests to be performed. Estimates of these parameters obtained for three intervals: 0.1–1, 1–4 and 4–15 Myr ago, provide benchmarks against which field models and geodynamo simulations may be compared. Furthermore, the values obtained suggest that: (1) 0.9 Myr is an insufficient duration for fully defining the time-averaged field; (2) average axial and non-axial dipole field strengths in the interval 0.1–1 Myr were ∼50 per cent higher than in the two preceding intervals; (3) the time-variances of the total field in the three intervals were not distinguishable. In addition to demonstrating the utility of the new framework, these findings can potentially address a longstanding question in geomagnetism: why is a polarity reversal ‘overdue’?