Short answers, pulled from the story.
The magnetosphere of Jupiter is the largest planetary magnetosphere in the Solar System and the largest known continuous structure apart from the heliosphere. It extends up to seven million kilometers toward the Sun and sometimes stretches almost to the orbit of Saturn in the opposite direction. The Sun and its visible corona would fit inside it with room to spare.
The first evidence for Jupiter's magnetic field came in 1955 with the detection of decametric radio emission from the planet. Synchrotron radiation confirming a radiation belt was identified in 1959. The definitive direct measurement of the field was made in December 1973 when the Pioneer 10 spacecraft flew past Jupiter.
Io loads Jupiter's magnetosphere with as much as 1,000 kilograms of new plasma every second through its volcanic eruptions, which blast sulfur dioxide into space. The resulting Io plasma torus forces Jupiter's magnetic field into a flattened disk-like structure called the magnetodisk and roughly doubles the size of the magnetosphere compared to what it would be without Io.
Radiation levels vary dramatically across the Galilean moons. Io receives about 3,600 rem per day, Europa about 540 rem per day, and Ganymede about 8 rem per day. Callisto, the most distant of the four, receives only about 0.01 rem per day, making it the only Galilean moon where human surface exploration is considered feasible.
Jupiter's aurorae are driven primarily by the planet's own rotation rather than by the solar wind. Electrons accelerated by electric potential drops between the magnetodisk plasma and the ionosphere continuously energize the main auroral ovals, independent of solar activity. Earth's aurorae, by contrast, depend on solar wind input and fade when solar activity is low.
Europa, Ganymede, and Callisto each generate induced magnetic moments in response to changes in Jupiter's ambient magnetic field. Scientists attribute this induction to subsurface layers of electrically conductive salty liquid water. Evidence for these underground oceans gathered in the 1990s was considered one of the most significant planetary discoveries of that decade.