Jupiter's Magnetic Field

The first clue to Jupiter's extraordinary magnetic field came with the detection of strong radio frequency emissions from the planet. This implied accelerating charged particles and a charged particle region around the planet similar to the Earth's van Allen belts. But the emissions were several thousand times stronger than those from the van Allen belts. When spacecraft reached the vicinity of Jupiter, they found that its magnetosphere was about 30 million km across, larger than the entire Sun. The interaction of the solar wind with this magnetosphere elongates it on the outward side to a distance beyond Saturn's orbit, about 4 A.U. past Jupiter.

From analysis of the radio frequency emissions, it is implied that the magnetic pole strength of Jupiter is nearly 20,000 times stronger than the Earth's pole strength. That does not, however, mean that the surface magnetic fields are that much stronger than those of Earth. According to E. G. Smith of Jet Propulsion Laboratories, the surface magnetic field strengths are about 14G at the north pole, 11G at the south pole and 4G at the equator.

While 20,000 times the intrinsic strength of Earth's magnetic dynamo is incredible, it is approximately a dipole field. Given that Jupiter's radius is almost 12 times that of Earth, and that dipole field strength drops off with the cube of the radius perpendicular to the dipole axis, one would project a surface field on the order of 7 Gauss at the equator from just the ratios of the equatorial diameters.

Io's Interaction with Jupiter

Io, the closest of the Galilean moons of Jupiter, is about the size of Earth's moon but in relation to the planet's diameter, it is much closer to Jupiter. It's orbit at just 5.9 times the radius of Jupiter puts it in a region of intense magnetic field. It has a significant and unique effect on the magnetosphere of the planet.

Although all four of the large Galilean moons have some effect on the magnetosphere, Io's effect is much larger because of its intense volcanism. It is seen as a major source of the heavy ions in the inner charged particle region of the magnetosphere. The particles ejected by Io's volcanoes have interacted with the magnetic field of the planet to produce a ring of charged particles around the planet, following Io's orbit. It is referred to as the Io plasma torus. This torus is detectable from Earth, but it's origin was unknown until Voyager discovered the intense volcanic activity of Io. The Galileo spacecraft made detailed studies of the torus. The energetic charged particles in the torous make it an extremely high radiation area, lethal to either manned or unmanned space vehicles. Spectroscopic evidence of sulfur in the torus makes the volcanism of Io the likely source of the charged particles in it.