Quantitative estimates of average geomagnetic axial dipole dominance in deep geological time

A defining characteristic of the recent geomagnetic field is its dominant axial dipole which provides its navigational utility and dictates the shape of the magnetosphere. Going back through time, much less is known about the degree of axial dipole dominance. Here we use a substantial and diverse set of 3D numerical dynamo simulations and recent observation-based field models to derive a power law relationship between the angular dispersion of virtual geomagnetic poles at the equator and the median axial dipole dominance measured at Earth's surface. Applying this relation to published estimates of equatorial angular dispersion implies that geomagnetic axial dipole dominance averaged over 10⁷-10⁹ years has remained moderately high and stable through large parts of geological time. This provides an observational constraint to future studies of the geodynamo and palaeomagnetosphere. It also provides some reassurance as to the reliability of palaeogeographical reconstructions provided by palaeomagnetism.

Paleomagnetic Evidence for Inverse Correspondence between the Relative Contribution of the Axial Dipole Field and CMB Heat Flux for the Past 270 Myr

We provide an evaluation of the paleolatitudinal dependence of the paleosecular variation throughout the Paleozoic-Mesozoic transition - linked to the high geomagnetic reversal frequency interval Illawarra Hyperzone of Mixed Polarity (IHMP; ~266.7-228.7 Myr). Our findings were compared with those for intervals of distinctive geomagnetic reversal frequencies within the Phanerozoic. Our results for the IHMP were conducted through estimates of angular dispersion (S_B) of virtual geomagnetic pole (VGP) data groups, taken from a high quality paleomagnetic database. Model G was fitted to these data, providing its shape parameters a and b (respectively related to the antisymmetric and symmetric harmonic terms for the time-average geomagnetic field). Results for the IHMP exhibited compatible patterns with two well-known intervals of higher reversal frequency - Jurassic and the last 5 Myr. A comparison of b/a ratio results - considered as an efficient indicator for the relative contribution of the axial dipole field - for the last 270 Myr, indicated an inverse correspondence with the relative core-mantle boundary (CMB) heat flux, according to recent discussions, clarifying the physical meaning of the Model G shape parameters a and b.

A Statistical Model of the Fluctuations in the Geomagnetic Field from Paleosecular Variation to Reversal

The statistical characteristics of the local magnetic field of Earth during paleosecular variation, excursions, and reversals are described on the basis of a database that gathers the cleaned mean direction and average remanent intensity of 2741 lava flows that have erupted over the last 20 million years. A model consisting of a normally distributed axial dipole component plus an independent isotropic set of vectors with a Maxwellian distribution that simulates secular variation fits the range of geomagnetic fluctuations, in terms of both direction and intensity. This result suggests that the magnitude of secular variation vectors is independent of the magnitude of Earth's axial dipole moment and that the amplitude of secular variation is unchanged during reversals.

Paleomagnetism and the Nature of the Geodynamo

Records of direct observations of the earth's magnetic field cover less than a ten-millionth ofthe known lifetime of the field. Thus our knowledge of several geomagnetic phenomena, critical to our understanding of the geodynamo, must come firom the paleomagnetic record. A combination of substantial advances during the past decade or so both in dynamo theory (previously the domain of the mathematician) and in paleomagnetism (previously the domain of the geologist) has led to provocative models of the earth's magnetic field and a better understanding of the geodynamo.