Straightening the magnetic tilts of planets.

Straightening the magnetic tilts of planets

The axis of Earth's magnetic field tilts about 11 [degrees] off line from the axis on which the planet rotates. Studying other planets, scientists have interpreted data from the Voyager and Pioneer spacecraft as indicating that the difference between the axis of rotation and the magnetic field axis is about 10 [degrees] at Jupiter, 1 [degrees] at Saturn, 60 [degrees] at Uranus and 47 [degrees] at Neptune.

But why should planetary magnetic fields tilt at all?

Two scientists now propose an explanation that they say may account for all the observed tilts in magnetic fields. Syun-Ichi Akasofu of the University of Alaska in Fairbanks and Takao Saito of Tohoku University in Sendai, Japan, suggest that inside these planets, as well as in any others that produce their own magnetic fields, the rotational and magnetic-field axes are actually aligned. Only above a planet's surface, which is where passing spacecraft make their measurements, do the two axes diverge.

The researchers base their theory -- which they plan to present in detail May 30 at a meeting of the American Geophysical Union in Baltimore -- on the magnetic behavior of the sun.

The basic magnetic fields of the sun and planets are called dipoles, essentially resembling a bar magnet that has a north pole at one end and a south pole at the other. Instruments on Earth can measure the tilt of the axis of the sun's magnetic field not only far from the sun but also at the photosphere, its visible surface.

Such observations have shown that the sun's rotation axis and its magnetic axis coincide, both in the photosphere and above it, Akasofu and Saito note. In addition, other dipoles occur in the photosphere. But beyond a distance of about 2.5 solar radii, the axis of the solar magnetic field changes throughout the 11-year cycle of solar activity.

The researchers liken the photosphere's multiple dipoles to what may exist within planets. Planetary magnetic fields also look like dipoles as measured at the region above the surface, but Akasofu says there is no way to measure the magnetic field of the interior.

At present, Akasofu says, scientists have only one surviving theory for the origin of planetary magnetic fields. Called the dynamo theory, it evokes a comparison between a planet's magnetic axis and the shaft of an electric motor. Akasofu notes, however, that since a planet's rotation is such an important source of energy for its dynamo, the observed large tilts of planetary magnetic fields with respect to their rotation axes pose "a great puzzle."

There are at least two possible ways to explain those tilts, the researchers say. One is that the main dipole field is indeed tilted, which would mean that the dynamo theory needs marked revision to explain the tilts already observed at the planets of the solar system. On the other hand, although the main dipole may be aligned with the inner portion of the rotation axis, there may be other dipolar fields within a planet that result in a large dipole tilt as measured from outside.

Akasofu and Saito propose that the magnetic field as measured outside a planet may arise from two components. One is the central dipole, which goes through the planet's core and aligns with the rotation axis. The other consists of a few other dipolar fields at the outermost portion of the core. The combined field of the central dipole and of the core-surface dipoles, they say, may produce a field outside the planet that shows a large tilt.