Magnetic Field

As shown in the lower color insert on page C-1, Jupiter's huge size, fast spin, and liquid metallic hydrogen interior make a perfect circumstance for a huge magnetic field, and Jupiter does, in fact, have the largest, strongest magnetic field of all the planets. The planet's fast spin results in currents in the vast inner region of metallic hydrogen that reaches from below about 0.8 Jovian radii down to the core. These currents create a magnetic dynamo, much as currents in the outer, liquid iron core on Earth produce its magnetic dynamo.

Sir Joseph Larmour, an Irish physicist and mathematician, first proposed the hypothesis that the Earth's active magnetic field might be explained by the way the moving fluid iron in Earth's outer core mimics electrical currents, and the fact that every electric current has an associated, enveloping magnetic field.The combination of convective currents and a spinning inner and outer core is called the dynamo effect. If the fluid motion is fast, large, and conductive enough, then a magnetic field can not only be created but also carried and deformed by the moving fluid (this is also what happens in the Sun). The inner core rotates faster than the outer core, pulling field lines into itself and twisting them. Fluid upwelling from the boundary with the inner core is twisted by the Coriolis effect, and in turn twists the magnetic field. In these ways, it is thought that the field lines are made extremely complex.The exact patterns of the field at the core are not known. On Jupiter the flowing currents of metallic hydrogen act as an electrical current:There are free electrons and charged particles in the metallic hydrogen, just like the liquid iron on Earth. Jupiter's large internal solid core makes the patterns of convection in the liquid hydrogen more complex, and it creates a magnetic field that is asymmetric and has more than two poles, as discussed below.

Jupiter's magnetic field is tipped by 10 degrees with respect to its rotation axis, similar to the tip of Earth's magnetic field. Jupiter's magnetic field has the opposite polarization than Earth's: Its north magnetic pole is at its south rotational pole. This means that, according to the conventions of physics, magnetic field lines are imagined to run out from the north rotational pole of the planet and into the south rotational pole on Jupiter. Compasses on Jupiter would point to the south pole, rather than the north.The magnetic field on Earth reverses direction periodically, and Earth's north magnetic field has been at its South Pole many times in Earth's history (this is clearly recorded in

Dipolar and Quadrupolar Field Lines

Magnetic and rotational axis

Magnetic and rotational axis

A dipolar planetary magnetic field resembles the field of a bar magnet. Jupiter's magnetic field is dominated by a dipole field but has strong components of more complex fields as well.

the rock record). The Earth may be entering a period of magnetic reversal. Jupiter's magnetic field may well be expected to move and reverse over time, as well.

Jupiter's magnetic moment is 19,000 times stronger than Earth's. This intensity results in a magnetic field around the planet so huge that if it were visible, from Earth it would appear several times larger than the full Moon. At Jupiter's equator, the intensity of its magnetic field is about 10 times Earth's, and Jupiter's field is two or three times stronger at its poles than at its equator. Jupiter's field is also asymmetric. Though people tend to think of magnetic fields just in terms of dipoles, meaning a system with a north and a south pole like the Earth's, there are other, more complex configurations possible for magnetic fields. The next most complex after the dipole is the quadrupole, in which the field has four poles equally spaced around the sphere of the planet. After the quadrupole comes the octupole, which has eight poles. Earth's magnetic field is thought to lose strength in its dipole field and degenerate into quadrupole and octu-pole fields as it reverses, and then to reform into the reversed dipole field. In the case of Jupiter, stronger quadrupole and octupole fields exist continuously along with the major dipole field, so its field is asymmetrical and its field lines more complex than Earth's.

In the direction toward the Sun, Jupiter's magnetic field extends about 60 Jupiter radii, and has a bow shock, a magnetosheath, and a magnetopause, just as Earth's has. The bow shock is caused by the speedy solar plasma wind striking and piling up against the near side of the magnetic field, like the bow wave of a boat.The bow shock exists at a distance of about 75 Jupiter radii from the planet, and across the bow shock, the solar wind is slowed, compressed, and heated.The shocked solar wind is called the magnetosheath.The inner edge of the magnetosheath is called the magnetopause, inside of which is Jupiter's magnetosphere, the volume of its magnetic field. Away from the Sun, the solar wind elongates Jupiter's magnetic field to about 350 Jupiter radii (though this number fluctuates by as much as 100 radii), so long that it almost reaches Saturn's orbit.

Another major difference between Jupiter's and the Earth's magnetic fields is that Jupiter's four innermost moons orbit within Jupiter's magnetic field. Io, the innermost moon, orbits at about 5.9 Jupiter radii and interacts the most with Jupiter's magnetic field. Sulfur dioxide gas (SO) flows off of Io at a rate of about a ton per second (this is a similar production rate to that of an active comet). Ionized gases from Io interact with the magnetic field, creating interactions that produce radio emissions detectable from Earth, as described in the next section.

Ganymede, another of the inner satellites, has its own internal magnetic field, and so it is surrounded by its own magnetosphere, buried within Jupiter's.

On Earth, auroras are created by charged particles from the solar wind that are guided into the atmosphere by the Earth's magnetic field, where they bombard atmospheric molecules, which then emit light to release the extra energy given them by the bombardment. Since Jupiter has a massive magnetic field and an atmosphere, it might be expected that it, also, would have auroral displays, though its main atmospheric constituents are hydrogen and helium while on Earth they are nitrogen and oxygen. In 1979 auroras were first observed on Jupiter by a satellite called the International Ultraviolet Explorer. Jupiter's auroras are now known to be 1,000 times more energetic than Earth's auroras, and also more complex.

On Jupiter, there is an additional source for auroral emissions beyond the solar wind:The ions shed from Io. Ions from Io fly off onto the magnetic field lines of Jupiter's field, emitting visible light, and sometimes creating a perfect arc away from and back to the planet.The more familiar polar auroras on Jupiter consist of a strong oval around each of the magnetic poles, along with a more diffuse cap of auroral emissions that spread over the high latitudes and extend down past the main oval onto the night side of the planet (see image in the upper color insert on page C-2).While auroras on Earth are green, red, and sometimes blue, due to the characteristic light emissions of ionized oxygen and nitrogen, the hydrogen on Jupiter creates pink auroras.

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