Theoretical ideas concerning Jovian magnetospheric phenomena are at least as diverse as the phenomena themselves, and there presently exists no single comprehensive model that encompasses all known phenomena within a unified theoretical framework. We identify here a number of important theoretical concepts, some subset of which (together with perhaps others yet unidentified) will ultimately provide the elements of such a comprehensive model. A number of ideas have been advanced to account for the copious plasma source associated with Io, but none of these has yet accounted satisfactorily for both the magnitude and the morphology of the inferred source. Nevertheless, given the observed fact that Io supplies the bulk of the magnetospheric plasma mass, and the corollary that the net plasma transport is predominantly outward, it follows that the rotational energy of Jupiter is an important if not dominant source of energy for magnetospheric phenomena. This rotational energy is expended in a variety of phenomena, including the electrodynamic Io-Jupiter interaction and associated radio and auroral emissions, the acceleration of charged particles to MeV energies, and the generation of a wide variety of spin-periodic phenomena as observed both remotely and in situ. The spin periodicities observed within the magnetosphere can be explained for the most part as resulting from the diurnal wobble of the magnetospheric current sheet caused by the offset between Jupiter's magnetic dipole axis and its spin axis. However, remotely observed spin periodicities (the “pulsar” phenomena) apparently require the existence of an intrinsic longitudinal asymmetry in the Jovian magnetosphere that corotates with Jupiter.

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.

Jovian coordinate systems are not complicated or cabalistic, but they are different. The following is a description of these systems, as relevant to this book. I will also try to explain why things are as they are. There is logic behind the present system, even if some of the results seem curious or unfortunate.

Jovian longitude conventions

Latitude and longitude coordinates are usually established relative to some solid surface. Because Jupiter does not have a solid surface (at least none that is visible through the clouds), arbitrary, but convenient, coordinate grids have been prescribed. A spin equator is rather easily made out from observations of cloud motion, so the direction of the planetary spin axis is determined with relatively good accuracy. However, the determination of longitude is an entirely different matter.

Longitudes on a planet are fixed relative to an arbitrary, but well defined, prime- or zero-longitude meridian. For example, the Earth's prime meridian is the one that passes through the central cross-hair of the transit telescope at the Greenwich Royal Observatory. Its location is unique, and it stays put. The selection of this meridian as the prime or zero-longitude meridian was initially arbitrary, but the selection, once made, fixes the longitude grid with precision. The problem immediately faced in establishing a Jupiter longitude system is that the mean rotation period of the clouds is a function of latitude. The equatorial region rotates faster than the temperate and polar regions, as is common in all planetary upper atmospheres.