NEPTUNE: 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,
532, Chapman and Hall, New York, 1997.

Prior to the Voyager 2 encounter with Neptune in August 1989, the planet was thought to possess an ordinary magnetic field and magnetosphere. However, the intrinsic magnetic field of Neptune was equally as bizarre as that of Uranus (see Uranus: Magnetic Field and Magnetosphere), with a symmetry axis inclined by 49^o to the planetary rotation axis. Since the rotation axis itself was almost at right angles to the solar direction, the polar axis of the magnetosphere, called the polar cusp, was alternately nearly aligned with the flow and then nearly at right angles to the flow. At the time of the passage of the Voyager spacecraft, the polar cusp was so oriented relative to the encounter trajectory that Voyager (for the first time at any planet) entered the magnetosphere through the polar cusp (Belcher et al., 1989).

Planet and Interior

Neptune has an equatorial radius of 24 800 km, which is slightly less than the radius of Uranus, a mass of 17 Earth masses (which is slightly greater than the mass of Uranus). and a rotation period of 16.1 h, which is slightly shorter than that of Uranus. Thus we might expect that the internal structures of the two planets would be very similar, and from all accounts they are. Both are inferred to have a large rocky core, a middle layer of ice water and ammonia and an outer envelope of hydrogen and helium, accounting for the outer one-third of the radius of the planet. Unlike Uranus, the rotation axis of the planet is inclined by only 29^o to the orbital plane, similar to the 23.5^o obliquity of the Earth.

Magnetic Field

The magnetic dipole axis of Neptune is tilted at an angle of 47^o to the spin axis of the planet. The extrapolated near-equatorial surface field is 1.42 猥, corresponding to a magnetic moment (equatorial surface field times radius cubed) of 2.16 x 10¹⁷ Tm³ close to 27 times greater than the terrestrial magnetic moment. The quadrupole moment of Neptune is quite large and makes a greater contribution to the surface magnetic field than at any other planet. The octupole and higher moments are essentially undetermined (Connerney, Acuna and Ness, 1991).

Magnetosphere

The Neptunian magnetosphere is perhaps the most quiescent magnetosphere in the solar system. The wave levels and energetic particle fluxes are both very low. No evidence for dynamical phenomena were seen in the Voyager flyby. Otherwise the magnetosphere is very similar to the terrestrial magnetosphere, with a bow shock, magnetopause and magnetotail of similar relative dimensions to those of the Earth. The most forward portion of the magnetopause is estimated to lie on average at about 26 Neptunian radii in front of the planet, and of the bow shock at about 34 Neptune radii.

Acknowledgements

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

References

Belcher, J. W., Bridge, H. S., Bagenal, F. et al., (1989) Plasma observations near Neptune: initial results from Voyager 2. Science, 246, 1478-82.

Connerney, J. E. P., Acuna M. H. and Ness N. F. (1991) The magnetic field of Neptune. J. Geophys. Res., 96, 19023-42.

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