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WHAT GIVES the Sun and its planets magnetic fields? Accepted theory
says that they arise from the movement of liquid in their cores, which conduct
electricity. As the planets rotate, the movement of the charged particles
produces a magnetic field, as in a simple dynamo. Now two researchers have
challenged this theory with a mathematical model that is much less complicated.

Syun-Ichi Akasofu, of the University of Alaska Fairbanks Geophysical
Institute, claims that ‘the basic physics could be simple, and be explained
by a simple model’. But other researchers disagree, saying that this simpler
model adds nothing to the understanding of the physical processes that lead
to solar and planetary magnetism.

Akasofu and his colleague Takao Saito of Tokohu University, Japan, presented
their ideas at a meeting of the American Geophysical Union in Baltimore
last month. They based their model on data from the Sun over a sunspot cycle,
from 1976 to 1987, during which time the Sun’s magnetic field changed from
parallel to perpendicular to its poles.

Their model for planetary fields, including that of the Earth, consists
of a dipole field, one which behaves as if there were a bar magnet aligned
parallel to the axis of rotation and several variable dipoles on the outside
of each planet’s core.

The impetus for Akasofu and Saito’s work has come from the data that
the Voyager spacecraft sent back from the outer planets. On the Earth, the
magnetic poles are inclined at about 11 degrees away from the geographic
poles. But the magnetic axis of Neptune is inclined at 47 degrees from its
rotation axis, and that of Uranus at 60 degrees. As Akasofu said, ‘Let’s
not worry about 11 degrees. It’s the substantially inclined angles that
we have to explain.’

Akasofu is not satisfied with theories that produce such fields through
a dynamo inclined to the axes of rotation. He says, ‘I cannot think of any
dynamo that would do this.’ He and Saito have used the field of the Sun
as an analogy for that of a planet, treating the field at 2.5 solar radii
as if it were a planetary field. The field at the Sun’s surface, the photosphere,
then represents magnetism on the outside of the planet’s core.

Akasofu described their modelling using the Sun’s magnetic field in
the years from 1976 to 1987. ‘We assumed that there was an axial dipole,
and tried to fit the rest of the variations with less than three smaller
dipoles on the surface. We reproduced the observed field pretty well.’

The Sun’s field changes from being aligned parallel to its poles at
the minimum of sunspot activity, to perpendicular at the peak of the sunspot
cycle. Akasofu and Saito found that the field at peak sunspot activity could
be formed by the combination of a relatively weak axial dipole and a number
of stronger dipoles aligned east-west on the photosphere, close to the Sun’s
equator. The dipoles on the photosphere rotate as the core rotates, altering
position and strength to reproduce the changes observed in a sunspot cycle.

But other researchers question the value of this modelling. Kathy Whaler,
of the University of Leeds, feels that these ideas do not add to the physical
understanding of the processes involved. ‘You can describe any planet’s
magnetic field in this way. It is just a different mathematical representation,
with the same number of free parameters.’ In addition, she says, there is
‘no physical mechanism’ to generate dipoles near the surface of the core.

But Whaler agrees with Akasofu and Saito that the magnetic fields of
the gas planets Neptune and Uranus remain a puzzle: ‘It still leaves the
problem of how to generate the highly inclined fields.’