Why Jupiter? Is a book devoted solely to the magnetosphere of Jupiter too narrow, too specialized? With the present emphasis on solar-terrestrial relationships, why should we be studying other magnetospheres, and why Jupiter's? The primary reason is that Jupiter's magnetosphere is so unlike the Earth's in its fundamental workings. We study the Jovian magnetosphere because it is different. The difference challenges our understanding of magnetospheric physics. It leads us to a broader and more basic insight regarding both magnetospheric physics and the behavior of matter on a cosmic scale.

Jupiter is not an ordinary planet, nor does it have an ordinary magnetosphere. Although Jupiter's magnetosphere does most of the things Earth's does, it does them differently. For example, the Earth's magnetosphere extracts essentially all of its energy and some significant fraction of its plasma from the solar wind. In contrast, Jupiter's magnetosphere is powered by the slowing of Jupiter's spin, and nearly all of the magnetospheric plasma comes from internal sources – the satellite Io and the Jovian ionosphere. Jupiter also exhibits weak but genuine pulsar behavior. If we did not have the Earth's magnetosphere as a model, most theoretical work on the Jovian magnetosphere would probably be directed toward pulsar-type models.

The brief encounters of the two Pioneer and the two Voyager spacecraft with Jupiter have opened new frontiers of research in magnetospheric physics. Jupiter offers more than just another magnetosphere; it functions in a different mode and allows us to stretch our conceptions and develop better theories of the Earth's magnetosphere.