Recent geodynamo simulations and observations of the geomagnetic field

numerical integrations of fully three-dimensional, fully nonlinear dynamos. Their papers were precursors of a stream of such models that have focused particularly on the geodynamo. They provide us, in unprecedented de-tail, with spectacular realizations of interesting geomag-netic field behaviors, such as secular variation and even polarity reversals. The proliferation of models has, how-ever, created some confusion and apparently conflicting results. This can be partly attributed to the different ways in which different groups have modeled the core, normalized their equations, defined their dimensionless parameters, chosen their boundary conditions, and se-lected their energy sources. This has made it difficult to compare the results of different simulations directly. In this paper, we first try, as far as possible, to overcom