ABSTRACT

Magnetohydrodynamic experiments with liquid metals and natural dynamos share a common property, a very low magnetic Prandtl number (Pm = ν/η, see Chapter2). Typical values of the magnetic Prandtl number for liquid metals in the core of terrestrial planets are indeed very small (< 10−4), despite the extreme pressure and temperature conditions (Chapters 4 & 5 and Poirier, 1988). In gaseous planets, the hydrogen gas presents a metallic phase at large pressure, with again a low value of Pm (Chapter 5 and Guillot, 1999). Further, both stellar convection zones (Chapter 6) and diluted gas in magnetospheres are in the low Pm regime. As liquid metals share this low Pm property, we believe that most of natural dynamos may be modelled and studied in a laboratory. A number of groups in the world have therefore focused their liquid experiments on fundamental aspects of the magnetohydrodynamics of natural objects.