Introduction

Over four centuries ago it was realized that the time-averaged direction of a compass needle is not affected by a force emanating from the sky, but by a magnetic field that is intrinsic to the Earth. The basic structure of the geomagnetic field and its slow variation with time was characterized long before magnetic fields were detected on other celestial bodies. By the middle of the twentieth century, the study of remanent magnetization of natural rocks had firmly established that the principal dipole component of the Earth's magnetic field had reversed its direction many times in the past.

Our understanding of the origin of the field by a dynamo process in the Earth's core has developed at a much slower pace, basically in parallel with that of astrophysical dynamos in general. Aside from understanding the intricate details of how a magnetic field is generated by a dynamo, we must ascertain that some fundamental requirements are fulfilled inside our planet. Geophysical observations have shown that one condition, namely the existence of an electrically conducting fluid region, is met inside the Earth, which has an outer core consisting of a liquid iron alloy. It is likely, but not completely certain, that all big planets have conducting fluid cores (see Fig. 7.5). However, some planets may not conform with another basic condition for a dynamo, namely sufficiently fast motion in the fluid layer.