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Figure 8.26. The Parker-Levy dynamo. (a) Rotation of an electrically conducting
fluid outer core results in the stretching of the magnetic-dipole field lines; they are
wound into a toroidal field. Toroidal magnetic fields cannot be detected at the
Earth's surface because of the intervening insulating mantle. (b) The Coriolis force
acting on the convecting fluid gives rise to spiralling motions (as in Fig. 8.25). The
motion has opposite polarity in the northern and southern hemispheres. Such
cyclonic motions are analogous to atmospheric cyclones and anticyclones. (c) The
toroidal field lines shown in (a) are further deformed into loops by the spiralling
motions shown in (b). These loops tend to rotate into longitudinal planes and so
effectively regenerate the original dipole field. (From Levy (1976). Reproduced with
permission from the Annual Reviews of Earth and Planetary Sciences ,Vol. 5, c
1976
by Annual Reviews Inc.)
field or to changing boundary conditions, or to the influence of the inner core
on the flow. It seems from palaeomagnetic measurements that, during a reversal,
the magnitude of the field diminished to about 10% of its normal value, and
the path followed by the north magnetic pole was a complex wandering from
north to south rather than a simple line of longitude from north to south. The
length of time necessary to complete a reversal is short, approximately 5000 yr
or less. Since the inner core is a conductor, there is an electromagnetic coupling
between the outer core and the inner core. This is likely to be the main coupling
between the inner core and the outer core because the major viscosity contrast
there (Table 8.3) means that viscous coupling will be weak. There may, however,
be some coupling resulting from topography on the boundary if it is not smooth.
The electromagnetic coupling between the inner and outer core means that the
inner core has a stabilizing effect on the geodynamo: field reversals will take
place only when fluctuations exceed a threshold value. Models of the geodynamo
have often ignored the fact that the inner core is a conductor.
Three-dimensional dynamic computer simulations of a rotating electrically
conducting fluid outer shell surrounding a solid inner sphere have produced a
magnetic field that underwent reversal and had a differential rotation of the solid
inner sphere. However, such numerical simulations cannot be directly applied to
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