Measurement and description of the geomagnetic field and its spatial and temporal variations comprise one of the oldest geophysical disciplines. However, our ability to describe the field far exceeds our understanding of its origin. All plausible theories involve generation of the geomagnetic field within the fluid outer core of the Earth by some form of magnetohydrodynamic dynamo. Attempts to solve the full mathematical complexities of magnetohydrodynamics have driven some budding geomagnetists into useful but nonscientific lines of work. In fact, complete dynamical models have not been accomplished, although the plausibility of the magnetohydrodynamic origin of the geomagnetic field is well established Quantitative treatment of magnetohydrodynamics is (mercifully) beyond the scope of this book, but we can provide a qualitative explanation.
The first step is to gain some appreciation for what is meant by selfexciting dynamo. A simple electromechanical disk-dynamo model such as that shown in Figure 1.11 contains the essential elements of a self-exciting dynamo. The model is constructed of a copper disk rotating on an electrically conducting axle. An initial magnetic induction field, present in an upward direction perpendicular to the copper disk. Electrons in the copper disk experience a Lorenz force,
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