Jupiter's magnetosphere.

Jupiter has a huge magnetic field. The region of space dominated by a planet's magnetic field is called "magnetosphere." Its magnetosphere extends more than 650 million km (it goes beyond the orbit of Saturn!). Jupiter's magnetosphere isn't spherical, it extends "just" a few million kilometres in the direction of the Sun. The planet's environment, its magnetic field, and a swarm of energetic particles and gases dominate the magnetosphere. The low energy ions, protons, and electrons are called "plasma." The plasma forms a sheet through which concentrated electric currents flow. The Pioneers and Voyagers observed a giant doughnut-shaped collection of charged particles surrounding Jupiter at about the distance of the orbit of Io. This is known as "the Io plasma torus." It results from material escaping from Io's atmosphere or surface and then being caught up in Jupiter's magnetic field. At Jupiter, the plasma within the magnetosphere tends to rotate along with its rotating magnetic field.

Unfortunately for future space travelers and to the frustation of the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter's magnetic field. This "radiation" is similar to, but much more intense than, that found within Earth's Van Allen belts.

The Galileo atmospheric probe discovered a new intense radiation belt between Jupiter's rings and the uppermost atmospheric layers. This new belt is approximately 10 times as strong as Earth's Van Allen radiation belts. Unexpectedly, this new belt was also found to contain high-energy helium ions of unknown origin.