The Magnetic Field And Magnetosphere

The nonthermal radio emissions described above are the natural result of trapped charged particles interacting with Jupiter's magnetic field and ionosphere. Interpretation of these observations led to a definition of the basic characteristics of the planet's magnetic field and magnetosphere that was shown to be remarkably accurate by direct exploration of the vicinity of Jupiter by the Pioneer and Voyager spacecraft. The basic magnetic field of the planet is dipolar in nature, generated by a hydro-magnetic dynamo that is driven by convection within the electrically conducting outer layers of Jupiter's interior. The magnetic moment is 19,000 times greater than Earth's, leading to a field strength at the equator of 4.3 gauss, compared with 0.3 gauss at Earth's surface. The axis of the magnetic dipole is offset by a tenth of Jupiter's equatorial radius of 71,500 km (44,400 miles) from the planet's rotational axis, to which it is indeed inclined by 10°. The orientation of the Jovian magnetic field is opposite to the present orientation of Earth's field, such that a terrestrial compass taken to Jupiter would point south.

The magnetic field dominates the region around Jupiter in the shape of an extended teardrop. The round side of the teardrop faces the Sun, where the Jovian field repels the solar wind, forming a bow shock at a distance of about 3 million km (1.9 million miles) from the planet. Opposite the Sun, an immense magneto-tail stretches out to the orbit of Saturn, a distance of 650 million km (404 million miles), which is almost as far as Jupiter's distance from the Sun. These dimensions make Jupiter's magnetosphere the largest permanent structure in the solar system, dwarfing the Sun's diameter of 1.4 million km (870,000 miles). Within this huge region, the most striking activity is generated by the moon Io, whose influence on the decametric radiation is discussed in the section above. An electric current of approximately five million amperes flows in the magnetic flux tube linking Jupiter and Io. This satellite is also the source of a toroidal cloud of ions, or plasma, that surrounds its orbit.

The energy to power this huge magnetosphere comes ultimately from the planet's rotation, which must accordingly be slowing down at an immeasurably small rate. Charged particles such as electrons that are spiraling along the magnetic field lines are forced to move around the planet with the same speed as the field and, hence, with the rotation period of the planet itself. That is why radio astronomers on Earth were able to deduce the System III rotation period long before any spacecraft measured it directly. This trapping of charged particles by the Jovian magnetic field means that the ions shed by Io in its orbit move with the System III period of nearly 10 hours rather than the 42 % hours that Io takes to revolve around Jupiter. Thus, Io's plasma wake precedes the moon in its orbit about Jupiter.

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