查看文章

Timescales of processes in the Earth tend in general to be very short (eg, the rupture associated with an earthquake) or very long (millions of years or longer)(eg, the movement of the tectonic plates). The geomagnetic field provides the primary example of intermediate timescales. The internal magnetic field, with origin in the Earth’s fluid core, varies on timescales from yearly or less (the so-called geomagnetic jerks) through decadal variation, secular change over centuries, to more significant variations (excursions and reversals) on timescales of millenia. These changes are named the secular variation. This chapter is concerned with variations on timescales of years to centuries, and what can be deduced about the interior of the Earth and processes in the core from such changes. This subject has a long history: the first attempt to infer motion in the Earth’s interior from geomagnetic secular variation was by Halley (1692). Modern analysis interprets the secular variation as being primarily due to advection of magnetic field at the top of the core, and so uses magnetic observations to constrain flow there. In Section 4.2, we begin by outlining how neglect of diffusion in the core allows the simplification of the magnetic induction equation to a form that can be used to solve for flow. This solution is highly