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Images courtesy of Wikipedia [magnetic field diagram] and NASA, ESA, L. Sromovsky and P. Fry (University of Wisconsin), H. Hammel (Space Science Institute), and K. Rages (SETI Institute) [Uranus photo].
The Magnetic Poles of Uranus
The planet Uranus has an odd magnetic field. The planet's magnetic poles are nowhere near the geographic poles (as defined by the spin axis) of Uranus.
A portion of the magnetic field of Uranus is a dipole field, like the field around a bar magnet. The dipole field of Uranus is tilted 59° away from the planet's spin axis. For comparison, Earth's magnetic field is tilted, too... but only by 11°. Furthermore, the "imaginary bar magnet" of the Uranian dipole field doesn't even pass through the planet's center. It misses the center of Uranus by nearly one-third of the planet's radius.
The magnetic field of Uranus is not, however, a simple dipole field. Instead, it is a complex blend of dipole ("two poles") and quadrupole ("four poles") magnetic field elements. As a result of this strange and complex field geometry, the magnetic field at the "surface" of Uranus is extremely variable. On Earth, the magnetic field has a strength of about 0.3 gauss anywhere on the planet's surface. The field at the "surface" of Uranus is as low as 0.1 gauss in some parts of the southern hemisphere, and as high as 1.1 gauss at some places in the northern hemisphere.
Why is the Uranian magnetic field so odd? Scientists aren't sure, but they do have a theory. The magnetic fields of some planets are generated by swirling motions of electrically conducting fluids (molten iron at Earth and Mercury, liquid metallic hydrogen at Jupiter and Saturn) in the planet's core. However, the swirling, conducting fluid on Uranus may be a salty "ocean" within the planet... but not at the core. Neptune, which is similar to Uranus in many ways, also has an unusual field. Astronomers believe the magnetic fields of these two "ice giants" are produced in a similar fashion which is different from the other planets.
Not only is the Uranian magnetic field tilted... the planet's spin axis is also tilted, by a whopping 98°. You might expect, with such a large tilt of both the spin axis and the magnetic dipole axis, that the magnetosphere of Uranus would be very different from those of other planets. Strangely enough, it is not. Scientists are studying this, trying to determine why the planet's tilt and lopsided magnetic field produce such a "normal" magnetosphere. Uranus, like other planets that have both a magnetosphere and an atmosphere, produces auroral light shows in its atmosphere above the magnetic poles.
