In 1971, while a postgraduate student at the University of East Anglia, I
discovered a new type of instability which can affect the motion of a
rotating, electrically-conducting fluid. This instability arises if the
magnetic field increases sufficiently rapidly with distance from the
rotation axis, and can occur even if the actual strength of the magnetic
field is quite small. Non-axisymmetric waves then grow in amplitude, and
propagate slowly, relative to the rotating fluid itself, around the
rotation axis.
The applications of such work lie within the liquid core of the Earth,
where the Earth's magnetic field originates. Such `field-gradient'
instabilities, as they have come to be called, may, for instance, limit
the growth of the magnetic field by the so-called `dynamo' process.
J. Fluid Mech. Vol 52, pp 529-541, 1972