The Current Systems of the Jovian Magnetosphere and Ionosphere and Predictions for Saturn

Magnetized plasmas in motion inevitably generate currents and the magnetized plasmas that form the magnetospheres of the outer planets are no exception. Although a focus on the current systems tends to distract from the underlying dynamics, many elements of magnetospheric structure can be organized by discussing them in terms of the large scale currents present in the system. This paper starts with a digression on the pitfalls of a current-based description of a planetary magnetosphere but then proceeds to characterize the magnetospheres of Jupiter, Earth, and to some extent Saturn by the currents that flow within them. Emphasis is placed on the field-aligned currents that couple the equatorial magnetospheres to the ionospheres and the conditions that call for the development of field-aligned electric fields.