The Time-Averaged Field and Paleosecular Variation

Abstract

Paleomagnetic measurements of direction and intensity recorded in crustal rocks enable investigations of Earth's dynamo field, in particular over timescales for which direct measurements (surface, aeromagnetic, and satellite) are unavailable. These data allow characterization of spatial and temporal variations in the geometry and intensity of the field. For the past 5 Myr, paleomagnetic data of adequate spatial and temporal sfampling and sufficiently high quality exist to permit global modeling of the paleofield; on longer timescales, deciphering the magnetic field history is complicated by geographic effects. The focus of this chapter is the behavior of the magnetic field during stable polarity periods over the past 5 Myr. We describe data sets available for such studies and review investigations of the time-averaged field (TAF) and temporal variations about that mean or paleosecular variation (PSV). Central to paleomagnetism and tectonics is the hypothesis that the field can be approximated to first order by that due to a geocentric axial dipole (GAD). Long-lived departures from GAD and the spatial characteristics of PSV are of interest as they may shed light on the influence of the outer core's boundaries on magnetic field generation. The presence of the inner core influences the flow geometry in the outer core leading to possible observable differences in field behavior at Earth's surface inside and outside the tangent cylinder. Both the inner core and the core–mantle boundary (CMB) can influence magnetic field generation through electromagnetic coupling, and the CMB may also influence the field through thermal coupling. Descriptions of PSV have been greatly facilitated by the introduction of models in which statistical distributions for the Gaussian coefficients in a spherical harmonic expansion are prescribed. This model formulation allows the prediction of any paleomagnetic observable at Earth's surface and so allows investigations of geographic variations in summary statistics or local investigations of the distributions of a particular observable. Current models match the latitudinal variations in summary statistics, for example, dispersion of virtual geomagnetic poles, and the variance in intensity data globally. However, they do not predict local distributions of field directions. Among investigations of the 0–5 Ma TAF, the agreed-upon signal is that due to an axial quadrupole term, whose size is 2–5% of the axial dipole term. Longitudinal structure in the TAF has been suggested, and such models show features similar to those seen in the time average of historical models, notably regions of increased radial field at high latitudes. We discuss the issues that are the source of differences among current PSV and TAF models, along with the successes and limitations of models to date. We conclude with some thoughts on the current status of data sets and modeling tools and future directions.