JUPITER: MAGNETIC FIELD AND MAGNETOSPHERE

 

C. T. RUSSELL AND J. G. LUHMANN

Originally published in
Encyclopedia of Planetary Sciences, edited by J. H. Shirley and R. W. Fainbridge,
372-373, Chapman and Hall, New York, 1997.

 

Jupiter is the largest planet in the solar system with a radius of over 70 000 km. It rotates most rapidly of all the planets with a period of only 9 h 55 min 29.7 s. It also has the largest magnetic moment (computed as the product of the equatorial surface field and the cube of the planetary radius). Consequently it also has the largest magnetosphere in the solar system, large enough to encompass easily the Sun and the visible corona. If the Jovian magnetosphere were visible from Earth, it would be bigger than the Moon in the night sky. Jupiter is also a powerful emitter of radio waves. Its giant magnetosphere acts both as a trap and an accelerator of energetic charged particles. The most energetic of the trapped electrons radiate at radio frequencies, and it was the radio frequency radiation that led in 1955 to the discovery that Jupiter had a magnetic field (Burke and Franklin, 1955). Jupiter's magnetosphere differs importantly from the Earth's magnetosphere in that its energy is predominantly derived from sources internal to the magnetosphere rather than through its interaction with the solar wind.

 

Planet and Interior

Jupiter's interior is very different from the interiors of the terrestrial planets and may even have important differences from the interior of Saturn because of its much greater mass. The planet consists mainly of hydrogen and helium. The enormous gravitational force exerted by the planet compresses the helium and hydrogen into the liquid state and converts the hydrogen to an electrically conducting metal at depths below about 0.75 Jovian radii (Rj). It is within this electrically conducting metallic hydrogen fluid that the Jovian dynamo is generated. The energy for the dynamo consists in part of primordial heat from the formation of the planet and in part the release of gravitational energy of denser material, drops of liquid helium, settling to the center of the planet. This process is analogous to the terrestrial dynamo power source, which is believed to be the solidification of the inner core.

 

Magnetic Field

Even before the first probes to Jupiter much was known about the Jovian magnetic field from radio measurements. The moment was correctly estimated within a factor of two and the 10o tilt of the dipole moment correctly deduced. In 1973 and 1974 Jupiter was probed by Pioneer 10 and 11 (q.v.; which passed within 2.9 and 1.6 Rj), and again in 1979 when Voyager 1 and 2 flew within 5 and 10 Rj of the center of the planet. The most information about the planetary magnetic field came from Pioneer 11 which not only passed closest to Jupiter but did so in a retrograde sense that increased the range of planetary longitudes probed. These data revealed a magnetic field rich in multiple harmonics (in comparison to that of the Earth), presumably because the Jovian dynamo source region is closer to the surface of the planet. The dipole moment was found to be 1.55 X 1020 T M3, almost 20 000 times that of the Earth, with a tilt of 10o. Table 1 gives the spherical harmonic coefficients of Schmidt normalized Legendre polynomials averaged over the solutions of the fits to the two data sets available from Pioneer 11 (Connerney, 1981; Smith and Gulkis, 1979).

 

Magnetosphere

The immense size of the Jovian magnetosphere is a result of the combination of three factors: (1) the strength of the planetary magnetic field, (2) the low density of the solar wind at 5.2 AU, and (3) the rapid rotation of the planet. If the magnetosphere were a vacuum, this latter effect would not be important. However, the moon Io's volcanically derived atmosphere is lost by sputtering to the magnetosphere. The trapped radiation belt particles collide with the atmospheric particles and knock them out of Io's gravitational sphere of influence and into orbit about Jupiter. There they are ionized by charge exchange, impact ionization and photoionization and spun up into corotation with the planet by the electric field associated with the rotating magnetized Jovian plasma (see Planetary Torus). The velocities associated with this process combined with the high mass loss rate from To are sufficient to distort the magnetic field of Jupiter into a disk, or magnetodisk as sketched in Figure 1. The centrifugal force associated with this magnetodisk stretches the magnetosphere in all directions and increases the forward radius of the magnetosphere to close to 100 Jovian radii at times. Since a Jovian radius is more than ten Earth radii, the linear dimension of the Jovian magnetosphere is about 100 times that of the Earth and its volume a million times bigger.

Fig. 1. Cross-section of the magnetosphere of Jupiter in the noon-midnight meridian. Solid lines represent the direction of the magnetic field. The magnetic field lines point out of the planet in the northern hemisphere, opposite to the present day terrestrial field (I. M. Engle, J. Geophys. Res., 96, p. 7793, 1991, copyright by the American Geophysical Union.)

The sputtering process leads to an interesting feedback process because the particles sputtered, lost to Jupiter and then accelerated, are eventually energized to high energies and return to sputter again and hence maintain the level of the radiation belts. The energy for all this acceleration is derived from the rotational energy of the planet. However, this energy reservoir is so large that no significant change in rotation has occurred due to magnetospheric processes over the age of the planet.

As with other magnetospheres, both intrinsic to the planet and induced by the solar wind interaction, Jupiter has a magnetic tail extending in the antisolar direction. In concert with vast size in the forward direction, the magnetotail is of enormous dimensions in the antisolar direction, stretching (at least) all the way to Saturn's orbit, over 5 AU downstream.

The Jovian magnetosphere is very dynamic. The magnetodisk configuration is much more sensitive to the variations in the solar wind pressure than other magnetospheres, and thus the magnetosphere is constantly in motion. Deep in the interior of the magnetosphere the mass and energy injections associated with the sputtering from the volcanic atmosphere of Io is sensitive to that volcanic activity. Thus the emission of radio waves is not constant but varies with time. Like the Earth, the radiation belts are not permanently trapped on the magnetic field lines but scatter and precipitate into the atmosphere, resulting in auroral emissions from the atmosphere.

The most recent mission to Jupiter is the Galileo mission (q.v.) launched in October 1989, with an arrival in December 1995, following two terrestrial gravity assists. Galileo carries a comprehensive payload that will follow up on the discoveries of the Pioneer and Voyager missions.

 

Acknowledgements

This work was supported in part by the National Aeronautics and Space Administration under research grant NAGW-2573.

 

References

Burke, B. F. and Franklin, K. L. (1955,) 'Observations of variable radio source associated with the planet Jupiter.'J. Geophys. Res., 60, 213-7.

Connerney, J. E. P. (1981) The magnetic field of Jupiter: a generalized inverse approach. J. Geophys. Res., 86, 7679-93.

Engle, I. M. (1991) Idealized Voyager Jovian magnetosphere shape and field. J. Geophys. Res., 96, 7793-7802.

Dessler, A. J. (ed.) (1983) Physics of the Jovian magnetosphere. New York: Cambridge University Press.

Russell, C. T. (1987) Planetary magnetism, in Geomagnetism, Vol. 2 (ed. J. A. Jacob). London: Academic Press, pp. 457-523.

Smith, E. J. and Gulkis, S. (1979) The magnetic field of Jupiter: a comparison of radio astronomy and spacecraft observations. Ann. Rev. Earth Planet Sci., 7, 385-415.


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