MAGNETIC FIELDRotation and currents within the metallic hydrogen interior of Jupiter generate a magnetic field, much as the
molten iron core of the Earth does. Jupiter's field is 4,000 times stronger than the Earth's. It is roughly dipolar, like a bar magnet, with its axis offset by 10,000 km (6,200 mi) from the center of the planet and tipped 11¡ from Jupiter's rotation axis. As the planet rotates, the magnetic field wobbles up and down with the electrically charged
particles trapped within it. This results in a radio emission whose periodicity reveals the bulk rotation period of the planet. The plasma, or gas of charged particles, is locked to the magnetic field so that it rotates with it as well. This
magnetosphere extends at least 20 Jovian radii away from the planet, forming an extremely large, intense radiation region in which some particles are accelerated to speeds of tens of thousands of kilometers per second. The Galileo probe discovered a radiation belt between the uppermost atmosphere and the distance of Jupiter's rings that is ten times as strong as the Earth's Van Allen radiation belts.The satellite Io and, to a lesser degree, the other satellites sweep up the energetic electrons, which move roughly perpendicular to their orbits. Electric currents probably generated in Io may be responsible for long-puzzling radio bursts received on Earth from Jupiter. The energetic particles striking Io probably help remove atoms and ions of sodium, sulfur, and other elements from the satellite and put them into the doughnut-shaped cloud of these materials surrounding Jupiter along Io's orbit. Some of this material may also be ejected by volcanism on Io combined with the electromagnetic interaction with the magnetosphere. High-energy particles from the Io plasma torus spiral in along magnetic-field lines to Jupiter's atmosphere, where they stimulate auroral-light emissions. Galileo images of auroras encircling the north pole fit the expected footprints of the charged-particle flow in the magnetosphere.