Tilted magnetic fields
. The magnetic fields of Uranus and Neptune can be represented
by a simple bar magnet, or dipole, embedded in the planet, but with a
magnetic axis that is tilted with respect to the rotation axis. For
Uranus the tilt is about 60 degrees; Neptune has a tilt of 47 degrees.
In contrast, the magnetic axes of Jupiter, Saturn and Earth are much
more nearly aligned with their rotation axes. The arrow of the rotation
axis points from the geographic south towards geographic north, and the
magnetic axis similarly points from magnetic south to magnetic north. On
Uranus and Neptune a terrestrial compass would point toward the southern
hemisphere of the planet, while on Earth it points toward the geographic
north pole. In addition to dipole part of their magnetic field, Uranus
and Neptune have a large additional component known as the quadrupole
one. A method of visualizing this is to imagine that the dipole has a
magnetic center that is offset radially from the center of the planet.
As shown here, the equivalent offset for Uranus is almost a third of the
planet's radius, and there is a larger offset for Neptune of nearly half
its radius. But such off-center dipoles are only useful as a picture of
what the external field looks like and do not help in understanding how
it is produced deep down.
Copyright 2010, Professor Kenneth R. Lang, Tufts University