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Originally published in
Encyclopedia of Planetary Sciences, edited by J. H. Shirley and
R. W. Fainbridge,
863-864, Chapman and Hall, New York, 1997.
Prior to the Voyager encounter in January 1986, Uranus was expected to possess an unusual magnetosphere quite unlike that of the Earth, because the spin axis of Uranus is nearly in the plane in which Uranus orbits the Sun (Siscoe, 1975). If the intrinsic magnetic field of Uranus had been nearly aligned with the rotational axis, as the planets previously visited were, the polar axis of the magnetosphere, or the polar cusp as it is called, would have been aligned with the solar wind flow as Voyager flew by the planet. Ironically, the magnetic axis of the intrinsic magnetic field of Uranus was far from spin axis-aligned, so that the solar wind blew nearly perpendicular to the magnetic axis, as it does at Mercury, Earth, Jupiter and Saturn. Thus, while Uranus has an unusual intrinsic magnetic field, the resulting magnetosphere was found to be very Earth-like.
Uranus has an equatorial radius of 25 600 km, less than half that of Saturn. It has a mean density of 1.25 g cm-3and rotates with a period of 17.2 h. Its rotational axis is inclined at an angle of 97.9o to the pole of its orbital plane, in such a direction that, during the Voyager 2 flyby, the rotational axis was pointing nearly toward the Sun. The Uranian rings and satellites therefore orbit in planes almost orthogonal to the ecliptic plane. The interior is believed to consist of three principal layers, a rocky core, a water-ice and ammonia layer, and a gaseous envelope of hydrogen and helium, comprising the outer 30% of the planet.
The magnetic dipole moment of Uranus is tilted at an angle of 59o to the spin axis of the planet. The extrapolated near-equatorial surface field is 23 µT, corresponding to a magnetic moment (equatorial surface field times radius cubed) of 3.9 x 1017 T m3, close to 50 times greater than the terrestrial magnetic moment. The contribution of the quadrupole moment to the surface magnetic field is large, almost comparable to the contribution from the dipole moment. This relative contribution is in fact only exceeded by that of Neptune. The higher moments of the field are essentially unconstrained by the Voyager observations.
The Uranian magnetosphere is very similar to the terrestrial magnetosphere. There is a bow shock that deflects the supersonic flow of the solar wind in front of the magnetospheric cavity and a magnetic tail extending far downstream. The forward part of the magnetosphere extends to approximately 25 planetary radii and the bow shock to about 33 planetary radii. Evidence from the energetic particles fluxes and wave amplitudes in the magnetosphere indicate that it is somewhat less active than that of the Earth. However, there is some evidence for magnetospheric dynamics, or a magnetospheric 'substorm,' in the energetic particle data (Mauk et al., 1997; Sittler, Ogilvie and Selesniek, 1987).
This work was supported in part by the National Aeronautics and Space Administration under research grant NAGW-2573.
Connerney, J. E. P. and Ness, N. F. (1997) The magnetic field of Uranus. J. Geophys. Res., 92, 15329-36.
Mauk, B. H., Krimigis, S. J., Keath, E. P. et al. (1987) The hot plasma and radiation environment of the Uranian magnetosphere. J. Geophys. Res., 92, 15283-308.
Russell, C. T. (1987) Planetary magnetism, in Geomagnetism Vol. 2 (ed. J. A. Jacobs). New York: Academic Press, 457-23.
Siscoe, G. L. (1975) Particle and field environment of Uranus. Icarus, 24,311.
Sittler, E. C., Ogilvie, K. W. and Selesniek, R. (1987) Survey of electrons in the Uranian magnetosphere: Voyager 2 observations. J. Geophys. Res., 92, 15263-81.
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