Observations and Models of the Long-Term Evolution of Earth’s Magnetic Field

Abstract

The geomagnetic signal contains an enormous temporal range-from geomagnetic jerks on time scales of less than a year to the evolution of Earth's dipole moment over billions of years. This review compares observations and numerical models of the long-term range of that signal, for periods much larger than the typical overturn time of Earth's core. On time scales of 10 5 -10 9 years, the geomagnetic field reveals the control of mantle thermodynamic conditions on core dynamics. We first briefly describe the general formalism of numerical dynamo simulations and available paleomagnetic data sets that provide insight into paleofield behavior. Models for the morphology of the time-averaged geomagnetic field over the last 5 million years are presented, with emphasis on the possible departures from the geocentric axial dipole hypothesis and interpretations in terms of core dynamics. We discuss the power spectrum of the dipole moment, as it is a well-constrained aspect of the geomagnetic field on the million year time scale. We then summarize paleosecular variation and intensity over the past 200 million years, with emphasis on the possible dynamical causes for the occurrence of superchrons. Finally, we highlight the geological evolution of the geodynamo in light of the oldest paleomagnetic records available. A summary is given in the form of a tentative classification of well-constrained observations and robust numerical modeling results.

Testing the geocentric axial dipole hypothesis using regional paleomagnetic intensity records from 0 to 300 ka

“…We used a compilation of 86 sediment RPI records (Tauxe and Yamazaki, 2007), and absolute paleointensity data from the 2014.01 version of the PINT database (Biggin and Paterson, 2014) and the Geomagia50.v2 database (Donadini et al, 2009). The RPI compilation was previously used to construct the PADM2M model , which spans 2 million years.…”

Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation

“…Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime 6 and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations.…”

Instability of thermoremanence and the problem of estimating the ancient geomagnetic field strength from non-single-domain recorders

“…Paleointensity data are essential for constraining the conditions in the core (1)(2)(3)(4), for studying the role that the geomagnetic field plays in controlling Earth's atmosphere (5)(6)(7), and as a geochronological tool (8,9). Despite the necessity for a large amount of reliable paleointensity data, there are still significant ambiguities in the available paleointensity information (10)(11)(12); these call for a reevaluation of the existing database (13).…”