Geomagnetic Secular Variation and Its Applications to the Core

TLDR: In this paper, the authors review the observational constraints on the morphology and evolution of the magnetic field of the Earth over the last few centuries; these changes are referred to as the secular variation.

Abstract: We review the observational constraints on the morphology and evolution of the magnetic field of the Earth over the last few centuries; these changes are referred to as the secular variation. Starting with a description of the available sources of original observations of the field, we then discuss the mathematical models of the field's evolution that can be derived from them. We discuss the prominent features of the field's evolution, both at the Earth's surface and at the surface of the liquid core. The final section concerns itself with a discussion of the interpretation of the field's evolution, in terms of physical core processes. These divide themselves into processes associated with movement of core fluid, which is capable of advecting the field, and processes associated with the finite resistivity of the core, commonly termed diffusive processes. We lay the foundations for some of the more theoretical subjects covered in Volume 8.