Magnetic Fields of the Outer Planets

The rapidly rotating giant planets of the outer solar system all possess strong dynamo-driven magnetic fields that carve a large cavity in the flowing magnetized solar wind. Each planet brings a unique facet to the study of planetary magnetism. Jupiter possesses the largest planetary magnetic moment, 1.55x10(20) Tm-3, 2x10(4) times larger than the terrestrial magnetic moment whose axis of symmetry is offset about 10A degrees from the rotation axis, a tilt angle very similar to that of the Earth. Saturn has a dipole magnetic moment of 4.6x10(18) Tm-3 or 600 times that of the Earth, but unlike the Earth and Jupiter, the tilt of this magnetic moment is less than 1A degrees to the rotation axis. The other two gas giants, Uranus and Neptune, have unusual magnetic fields as well, not only because of their tilts but also because of the harmonic content of their internal fields. Uranus has two anomalous tilts, of its rotation axis and of its dipole axis. Unlike the other planets, the rotation axis of Uranus is tilted 97.5A degrees to the normal to its orbital plane. Its magnetic dipole moment of 3.9x10(17) Tm-3 is about 50 times the terrestrial moment with a tilt angle of close to 60A degrees to the rotation axis of the planet. In contrast, Neptune with a more normal obliquity has a magnetic moment of 2.2x10(17) Tm-3 or slightly over 25 times the terrestrial moment. The tilt angle of this moment is 47A degrees, smaller than that of Uranus but much larger than those of the Earth, Jupiter and Saturn. These two planets have such high harmonic content in their fields that the single flyby of Voyager was unable to resolve the higher degree coefficients accurately. The four gas giants have no apparent surface features that reflect the motion of the deep interior, so the magnetic field has been used to attempt to provide this information. This approach works very well at Jupiter where there is a significant tilt of the dipole and a long baseline of magnetic field measurements (Pioneer 10 to Galileo). The rotation rate is 870.536A degrees per day corresponding to a (System III) period of 9 h 55 min 26.704 s. At Saturn, it has been much more difficult to determine the equivalent rotation period. The most probable rotation period of the interior is close to 10 h 33 min, but at this writing, the number is still uncertain. For Uranus and Neptune, the magnetic field is better suited for the determination of the planetary rotation period but the baseline is too short. While it is possible that the smaller planetary bodies of the outer solar system, too, have magnetic fields or once had, but the current missions to Vesta, Ceres and Pluto do not include magnetic measurements.