Magnetic Field

Saturn's magnetic field is 35 times weaker than Jupiter's, but it is still 550 times greater than Earth's. Saturn's magnetic pole and its geographic pole (the pole of its rotation) are aligned to within one degree. As shown in the figure on page 116, the planet is surrounded by a magnetosphere, the volume dominated by the planet's own magnetic field. The solar wind strikes the sunward side of the magnetosphere and compresses, creating a shock wave called the bow shock. The bow shock wraps smoothly around the planet, streaming off the far side in an apron. A region called the magnetopause separates the compressed, heated, smoothly flowing solar wind from the oddly shaped magnetosphere. Finally, a thin region called the magne-tosheath separates the bow shock from the magnetopause. On the night side of the planet the solar wind pulls the magnetosphere into a magnetotail, an extension of the field that can be 25 to 100 times the radius of Saturn in length. In its basic structure, Saturn's magnetic structure as described here is similar to that of any of the planets with their own magnetic field.The size of the planet's field and the varying strength of the solar wind determine the size and the shape of each magnetosphere and its attendant structures.

Like Jupiter's, the polarity of Saturn's field is opposite that of the Earth's, that is, compasses on Saturn would point to its south pole.

Saturn's magnetic field lines pass into its south pole and out of its north pole. Saturn's field and Jupiter's field are thought to be produced in the same manner, by convecting currents in its liquid hydrogen layer. The planets also have in common the complexity of their fields: Though both fields are dominated by the dipole component (analogous to a simple bar magnet), there are more complex fields present as well, including quadrupole fields (two positive and two negative poles) and octupole fields (four of each). Unlike any other planet in the solar system, Saturn's magnetic field is almost perfectly aligned with its rotation axis. Since the Earth's field is known to move measurably within human history, the other planets' fields may also be assumed to move. It may simply be a fortuitous moment in the age of the solar system that space observations have caught Saturn's field in this position.

The 15 innermost moons of Saturn orbit either in or through the edges of the planet's magnetosphere. Titan, the outermost of these

Saturn's magnetic field, like those of all the planets, is pulled out into a long tail by the force of the solar wind (compare to the undistorted shapes of magnetic fields shown in the figure on page 29).

moons, moves in and out of the magnetosphere depending on its position in its orbit and the strength of the solar wind. Titan, Enceladus, Tethys, and Dione all lose gas to Saturn's magnetosphere, adding ions that spiral along the magnetic field lines.

Saturn's aurora is also unique in observations of the solar system. When ions from the solar wind or from nearby moons follow magnetic field lines toward the poles of the planet and strike the upper atmosphere, the energy of the accelerated particles is released as light. All other planetary auroras observed have been bright, colored displays like ours on Earth. Saturn, on the other hand, displays both bright and dark auroras. Its north pole has a dark halo caused by the aurora.The process that produces the aurora may also produce unusual atmospheric particles, and these particles may absorb sunlight more efficiently than the rest of the atmosphere, and this excess absorption may explain the dark aurora.

The model for Saturn's interior is relatively simple, because there is so little direct data. As presented here the model cannot explain the density or heat flow of the planet. Much more remains to be discovered. Somewhat more is known about Saturn's shallow surface movements, and far more is known about its moons.

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