Public message 3284  SCIENCE Area 19:52  Wednesday   4-Sep-91
From: HENRY SHAW
  To: MIKE DAVEY
  Re: EARTH'S MAGNETIC FIELD 2

...continued from previous

It has been known since shortly after Gauss first measured the strength of
the field in ~1830 that the magnetic dipole moment of the earth is
decreasing. This decrease has averaged about 0.05%/yr since 1830.  There
are, however, significant fluctuations about this average and in about 1960,
the rate of decrease increased significantly (see Fraser-Smith, A. C., 1987,
Centered and eccentric geomagnetic dipoles and their poles, 1600-1985.
Reviews of Geophysics, v25, pp.1-16.).

The magnetic field of the earth is *not* a simple dipole (the field
associated with, say, an "ideal" bar magnet).  Though the dipole field is the
dominant one today at the surface, the whole field is a bumpy, lumpy sort
of thing.  Mathematically, one can describe the total field by a sum of
functions known as spherical harmonics.  [Conceptually, this is similar to
describing the motions of a vibrating guitar string;  there is the fundamen-
tal frequency and there are also higher frequency "harmonic" vibrations.]
As the dipole field of the earth decays, higher-order "bumps" in the field
grow, which implies that the total energy stored in the field has remained
approximately constant (Verosub, K., and Cox, A., 1971, Changes in the
total magnetic energy external to the earth's core, Jour. Geomagnetism and
Geoelectricity, v23, pp. 235-42).  The field we see at the surface of the
earth is probably the result of a the sum over the fields produced by a
number of convective cells in the outer core.  Each of which produces a
poloidal field.   As the size, shape, and location of each of these cells
changes over time, the net field we see at the surface undergoes corre-
sponding changes.

There is ample evidence that the present time-variation of the field (the
decreasing dipole moment) has not always been the case.  Over short
timescales (thousands of years), one can determine the strength of the
paleo-field by measuring the strength of the remanent magnetism of baked
clay pots and bricks and comparing that to the intensity produced in the
same samples by heating and cooling them in magnetic fields of varying
strength in the laboratory.  Because of the "bumpiness" of the field over
the surface of the earth at any given time, one cannot infer the strength of
the dipole from these types of measurements made at a single site;  one
needs data from around the world.

Such data have been obtained on artifacts dating back more than 12,000
years, and the resulting curve, consisting of 1167 individual determinations
for the variations in the intensity of the dipole field shows that the dipole
moment has decreased for the past 2000 years, extending the historical
record. The decay over the past 1000 years has been particularly fast.
From 6000 to 3000 years before present, however, there was a significant
INCREASE in the strength of the dipole (McElhinny, M., and Senanayake,
W. E., 1982, Variations in the geomagnetic dipole 1:  the past 50,000 years.
J. Geomag. Geoelect. v34, pp.39-51.).

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