Abstract :
Io, Europa, Ganymede and Callisto all interact with a flowing magnetized plasma, that of the nearly corotating jovian magnetosphere. The plasma conditions at each differ as there is a strong variation in the properties of the magnetosphere with radial distance. The properties of the moons vary as well. Thus each of the moons has a unique interaction. In order to understand how to interpret these signatures we examine first the limiting field induced in a permeable moon to determine what size fields demand intrinsic sources. We also examine the implications for remanent field sources imposed by the Runcorn theorem. Electrical conductivity in the ionosphere and interior to the moons can exclude the time varying portion of the external magnetic field. This exclusion can be used to probe for subsurface oceans beneath the crust. In addition to these effects, Europa and Io have sufficient atmospheres that they add mass to the incoming flow. The evidence for this mass loading and its effects on the jovian magnetosphere are reviewed as are its effects on the more classic signatures described above. When all these effects are considered we conclude that Io may have an intrinsic field with a moment as large as 1013 Tm3 although it is possible that this signature results from permeable material within Io. Europa has a conducting interior, most probably due to a saltwater ocean near its surface. Ganymede has an intrinsic field that is dynamo driven and Callisto has a conducting layer like that of Europe. Earlier observations by Voyager can readily be interpreted in terms of this mass loading scenario. The slowed cold flow behind Io provides a suitable source via the interchange instability for the cold torus well inside the orbit of Io.
