Global Positioning System Reference
In-Depth Information
The largest component of this magnetic field is a dipole (like the field
produced by a bar magnet) that is aligned along the earth's rotation axis.
There are other, non-dipole components of the field, however, and these
(along with irregularities of the core, mantle, and crust) greatly complicate
the observed field patterns. Add in the intrinsic complexity of the physics
(magnetohydrodynamics), and you can see why a detailed understanding
was slow in coming.
The basic model is that of a dynamo—in fact, of two coupled dynamos.
The dynamics of such a system are chaotic, just like the earth's magnetic
field. The polarity of the twin-dynamo model flips chaotically
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—that is,
the north and south poles suddenly swap positions, and then swap back
again in an irregular and never-ending sequence. This is very reminiscent
of the well-known reversal of the earth's magnetic field, which has been
detected via geological records. It occurs every few hundred thousand
years but in a seemingly random fashion: the last reversal was 780,000
years ago.
21
Each reversal takes a few thousand years to complete—a blink
of an eye, geologically—and during a reversal there is no reliable field
orientation. One feature of the dynamo model is that it can explain key
features not only of the chaotic magnetism of our planet, but also of the
regular 11-year cycle of magnetic polarity flips that occur in the sun, which
is a very di√erent physical system.
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on the physical configuration, the induced magnetic field and electrical current can then be
self-sustaining—even if the initial magnetic field were to disappear—so long as an energy
source exists to maintain convective flow in the outer core. (Perhaps natural radioactivity
generates heat that leads to convective flow in the outer core.) The self-excited dynamo
model discussed in the text is a simple example of this idea. Our knowledge of the earth's
interior is imprecise, however. None of the core's constituent matter can be measured
directly, so its nature is inferred. In particular, we do not know much about the conductivity
and other magnetohydrodynamic properties of matter at the high temperatures and very
high pressures that exist in the earth's interior. See Kearey et al. (2009, pp. 74-77) for a
recent accessible account of this complex phenomenon.
20. Chaotic events appear to be random, but they are really a consequence of the
underlying dynamics. The equations that govern meteorology are similarly chaotic, which
is why detailed, long-range weather forecasting is so hard to do. Predictions can be made a
little way into the future, given accurate data of the current state of the weather, but they
become worse as we look further into the future. The same situation exists with geo-
magnetic field predictions.
21. The five previous reversals were at 990,000 years ago and at 1.07 million, 1.19
million, 1.20 million, and 1.77 million years ago. Polarity flip simulations can often be seen
online.
22. Technical and semitechnical accounts of the physics of geomagnetism can be found
in Bu√et (2000), Carrigan and Gubbins (1979), Kibble and Berkshire (1985, p. 265),
