Earth's Dynamic Magnetic Field: Centennial to Million Year Time Scales

Among his many contribution to geophysics Edward Bullard studied the geomagnetic secular variation on human time scales and was the first to attempt direct numerical simulation of the geodynamo with the Bullard-Gellman dynamo in 1954. This lecture will focus on the considerable progress since then in characterizing the geomagnetic field on timescales ranging from centuries to millions of years using paleomagnetic data and how such results should impact our current consideration of "standard geodynamo models" that exhibit properties considered representative of the geomagnetic field. The past few decades have seen the development of time-varying global spherical harmonic geomagnetic field models at varying levels of spatial and temporal resolution, that allow study of features such as Unusually Rapid Geomagnetic Events (e.g., the Levantine Spike during the Iron Age), and of the structure and evolution of the instabilities that have produced several geomagnetic excursions in the interval 0-100 ka. Such features have been characterized by the paleosecular variation index, PI, and its variability in both space and time. Large values of PI reflect a combination of low dipole moment and large deviation from the directions expected of a geocentric axial dipole, and can reflect the regional onset and variable duration of excursions with location. A feature of geomagnetic excursions over 0-100 ka is a somewhat rapid drop in dipole moment and relatively slow recovery following the excursion that may expose the possibility of directional instability over extended time intervals of the order of 10 kyr. This contrasts with the asymmetric behavior seen in growth and decay of lower resolution axial dipole moment variations at periods of ~30 ky and longer, and also identified during polarity reversals over the past 2 Ma, which exhibit a longer general decay in field strength followed by a rapid recovery once the opposite polarity is established. Such differences might call into question a perception that excursions and reversals are produced by essentially the same processes in Earth's core, although differing in the polarity outcome, and overall duration. One property of paleofield variations that has recently been characterized across frequencies ranging from 0.1 to 106 / Myr is the power spectrum of variation in the axial dipole moment, which is flat in the low frequency range, f, corresponding to geomagnetic reversal frequency, and falls off as f-2, f-4, f-6, in successively higher bands providing a potential means to distinguish frequency dependent behavior of distinct physical processes. This broad structure can be well characterized by a stochastic model of an autoregressive process of order 3, suitable for comparison with the products of numerical simulations. Small bumps in the spectrum provide tantalizing suggestions of additional power being supplied to the axial dipole moment spectrum by more complex processes. Prospects for future improvements in linking core dynamics in numerical simulations to paleofield results will outlined.