No abstract

Data from the Galileo orbiter suggest that two of Jupiter's moons, lo and Ganymede, have intrinsic magnetic fields. The magnetic field of Jupiter alters the nature of the magnetohydrodynamic processes generating these intrinsic fields. Such an imposed field allows appreciable internal fields to be generated in cases where convection cannot otherwise maintain a dynamo. The dipole moment of the internal field can then become aligned with the background field, as is observed for lo and Ganymede, lo might not have a self-sustained intrinsic field in the absence of the ambient jovian field; Ganymede is almost certainly operating as a dynamo in its own right.

Abstract.Some fundamental difficulties in the construction of an Earth-like dynamo model and the role of hyperviscosity in attempting to overcome these difficulties are discussed. An Earth-like convection model influenced by an imposed toroidal magnetic field is investigated both with and without the effect of hyperviscosity. It is shown that hyperviscosity affects the dynamics of convection significantly; the effect is similar in some respects to that of a strong magnetic field. The effective Ekman number, a key parameter for an Earthlike dynamo model, is substantially increased when hyperviscosity is employed.