SATURN: 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,
718-719, Chapman and Hall, New York, 1997.

 

Saturn is the second largest planet in the solar system with a radius of over 60 000 km. it rotates almost as fast as Jupiter, having a rotational period of 10 h 39 min. Moreover the composition of Saturn is thought to be similar to that of Jupiter, consisting of mainly hydrogen and helium. Nevertheless, Saturn and Jupiter have many differences. First, Saturn's intrinsic magnetic field is much weaker than that of Jupiter. Second. the magnetic field, instead of being rich in harmonics, has a very simple structure, one that is axisymmetric to a very high degree of accuracy. Further, the magnetosphere of Saturn does not have an efficient energizing mechanism deep in the heart of the magnetosphere as does Jupiter. In short, Saturn has a relatively simple magnetic field and a relatively quiescent magnetosphere.

 

Planet and Interior

The composition of Saturn is thought to be similar to that of Jupiter but its smaller radius and hence much smaller mass leads to an interior with a much smaller conducting core. As occurs at Jupiter, the cooling of the Saturn's interior causes condensation of helium within the hydrogen-helium fluid. This process then releases heat due to the change of phase and due to gravitational settling, and provides a heat source for powering convection in the interior of the planet and for powering the magnetic dynamo.

 

Magnetic Field

While Saturn does generate radio waves, these waves are not strong enough to be detected on Earth. Thus until Pioneer 11 reached Saturn in 1979 it was not known whether Saturn had an intrinsic magnetic field. The passage of Pioneer 11 within 1.4 Saturn radii of the center of the planet was soon replicated by Voyager I in 1980, passing within 3.1 Rs, and Voyager 2 in 1981, passing within 2.7 Rs. These spacecraft found it magnetic field quite unlike that at any other planet. To the accuracy that could be obtained, there was no tilt to the rotation axis and the interior magnetic field was perfectly axisymmetric. The magnetic moment, the surface field strength at the equator times the cube of the radius, was also somewhat smaller than expected at 4.6 X 1018 T m3. While this value is 580 times larger than that of the Earth, it is over 30 times smaller than that of Jupiter despite the small (15%) difference in radius. The contribution of the quadrupole moment at Saturn is also small compared to the quadrupole contribution at Jupiter and the Earth. For example, the ratio of the terrestrial quadrupole to dipole moment is 0.14 but at Saturn it is 0.07 (see Earth: magnetic field and magnetosphere).

 

Magnetosphere

Saturn also has an immense magnetosphere whose linear dimension is about one-fifth that of the Jovian magnetosphere. This magnetosphere is more similar to the terrestrial magnetospheres than that of Jupiter. The magnetosphere traps radiation belt particles, and these particles reach levels similar to those of the terrestrial magnetosphere. On their inner edge the radiation belts are terminated by the main (A, B and C) rings of Saturn, which absorb any particles that encounter them. The radiation belt particles also are absorbed if they collide with one of the moons. Hence there are local minima in the energetic particle fluxes at each of the moons. Unlike Jupiter, but like the Earth, there is no internal energy and mass source deep in the Saturnian magnetosphere. However Titan, which orbits just inside the average location of the magnetopause, in the far reaches of the magnetosphere, has an interesting interaction.

Titan (q.v.) is the most gas-rich moon in the solar system, having an atmospheric mass per unit area much greater than even that of the Earth. At its upper levels this atmosphere becomes ionized through charge exchange, impact ionization and photoionization. This newly created plasma adds mass to the magnetospheric plasma, which attempts to circulate in the Saturnian magnetosphere at a velocity similar to that needed to remain stationary with respect to the rotating planet. Since this velocity is much faster than the orbital velocity of Titan, the added mass slows the 'corotating' magnetospheric plasma. The magnetic field of the planet that is effectively frozen to the magnetospheric plasma is then stretched and draped about the planet, forming a slingshot which accelerates the added mass up to corotational speeds. Thus the interaction between the Saturn magnetosphere and the Titan atmosphere resembles the interaction of the solar wind with comets and with Venus (Kivelson and Russell, 1983).

The Saturn magnetosphere, like the other planetary magnetospheres, is an efficient deflector of the solar wind. The solar wind at Saturn flows more rapidly with respect to the velocity of compressional waves than at Jupiter and the terrestrial planets. Thus the shock that forms at Saturn is very intense. Ironically this strength may weaken at least one form of coupling of the solar wind with the magnetosphere, that due to reconnection. However, some aspects of the interaction of the solar wind plasma should be much stronger than at Jupiter or at Earth because of the increased strength of the shock and the scale size of the interaction, which can accelerate charged particles to very high levels.

Saturn is also expected (like Jupiter) to have a very large tail, possibly one that could be dynamic like that of the Earth. However, observations of the tail are quite limited and we must wait until the Cassini mission (q.v.) in the early 21st century for further studies of the magnetic field, magnetosphere and magnetotail, and the answers to many of the questions that the Pioneer and Voyager data have generated.

 

Acknowledgments

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

 

References

Connerney, J. E. P. Acuna, M. H. and Ness, N. F. (1984) The Z3 model of Saturn's magnetic field and the Pioneer 11 vector helium magnetometer observations. J. Geophys. Res., 89, 7541-44.

Kivelson, M. G. and Russell, C. T. (1983) The interaction of flowing plasmas with planetary ionospheres: a Titan-Venus comparison. J. Geophys. Res., 88, 49-57.

Russell, C. T. (1987) Planetary magnetism, in Geomagnetism Vol 2 (ed. J. A. Jacobs). London: Academic Press, pp. 457-23.


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