Geoscience Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 8.27.
A sequence of images of a numerical dynamo model during a reversal
of the magnetic field. Columns (a)-(d) show images for every 3000 yr. Top row, map
view of the radial field at the earth surface. Middle row, map view of the radial field
at the core-mantle boundary. Orange, outward field; blue, inward field. Note that the
magnitude of the surface field is dispayed magnified by a factor of ten. Bottom row,
longitudinally averaged magnetic field through the core. Outer circle, core-mantle
boundary; inner circle, inner core. The right-hand half of each plot shows contours
of the toroidal field direction and intensity (red lines, eastwards; blue lines,
westwards). The left-hand half of each plot shows magnetic-field lines for the
poloidal field (red lines, anticlockwise; blue lines, clockwise). Colour version Plate
16. (Reprinted with permission from
Nature
(Glatzmaier
et al
.,
Nature
,
401
, 885-90)
Copyright 1999 Macmillan Magazines Ltd.)
the Earth yet, since realistic values for all parameters
8
are not yet computationally
achievable. Figure 8.27 shows stages of a magnetic reversal in progress in a
numerical dynamo model. Increasing computer power should mean that, in the
next decade, simulations of realistic core models that can aid our understand-
ing of the geodynamo and the dynamics of the inner and outer core may be
achievable.
The liquid outer core has a viscosity of about 10
−
3
Pa s (comparable to that
of water at room temperature and pressure) while the viscosity of the inner
core is about 10
13
±
3
Pa s (Table 8.3). The flow velocity in the outer core is
about 10
4
myr
−
1
and locally even faster flows occur over short distances. These
8
For the core the Rossby number (
Ro
), the ratio of the inertial forces to the Coriolis forces, is about
10
−
8
. The Ekman number (
E
), the ratio of the viscous forces to the Coriolis forces, is
10
−
9
. The
Roberts number (
q
), the ratio of thermal diffusivity to magnetic diffusivity, is
10
−
5
.
∼
