Re: Earth's Magnetic Field

[brian whatcott <inet@INTELLISYS.NET>]

At 14:59 7/11/99 -0700, Leigh wrote:
> There are many publications, and multipole decompositions, and numerical
> simulations, and... The sort of understanding I seek is of a much less
> indirect sort. I guess I'm a simple fellow. I figure that something well
> understood ought to be accessible to me.[] If Dr. Glatzmeier understands the
> Earth's magnetic phenomena was he able to get you to understand it? If
> so, can you now relate your understanding to the rest of us?
>
> I'm afraid that is my ultimate test of physical understanding.
>
> Leigh

With much the same question in mind, I asked workers in several Surveys
and an Insitute for their understanding. Here is the first response,
from David Barraclough of the UK Geological Survey in Edinburgh:

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From: "David Barraclough" <DRBAR@wpo.nerc.ac.uk>
To: <inet@intellisys.net>
Subject: Mechanism for Geomagnetic Field -Reply



We know that over 90 percent of the main geomagnetic field originates
within the body of the Earth and that this field varies on time scales
spanning the range from decades to millions of years.  Below a few
tens of kilometres depth temperatures within the Earth are too high
for permanent magnets to exist.  Seismological studies show that the
interior of the Earth below the outer crust consists of a mantle
(which is made of rock-like material but flows very slowly over
geological time-scale), an outer core that is fluid and is largely
composed of iron with a few percent of (probably) silicon, and an
inner core with a similar composition to the outer core but, because
of the extremely high pressures) solid.

Consideration of these facts points to the fluid outer core as the
most likely place to look for the source of the main geomagnetic
field.  Because the material in the outer core is fluid, and therefore
almost certainly in motion, and an electrical conductor, we have two
of the requirements for a dynamo.  The third requirement (a magnetic
field) is likely to have existed when the core formed (as it does now)
in the form of the magnetic field of the Sun.  Present views are that
the fluid motions in the outer core were such that they were able to
amplify this small, pre-existing ambient field to values typical of
the present-day geomagnetic field and have subsequently been such as
to sustain this field with its present-day polarity or in the reversed
polarity that is known from studies of fossilised rock magnetism.

Physical models (admittedly very simplified) have demonstrated that
these ideas are plausible.  Computer simulations using much  more
realistic mathematical models of the dynamo process have recently
shown that the detailed behaviour of the geomagnetic field (in
particular the polarity reversals over geological time-scales) can be
reproduced.

Reasonably non-mathematical introductions to the dynamo theory of the
origin of the geomagnetic field can be found in the following two
texts:
"The Magnetic Field of the Earth: Paleomagnetism, the Core and the
Deep Mantle" by R. T. Merrill, M. W. McElhinny & P. L. McFadden
(Academic Press, 1996) and "Introduction to Geomagnetism" by W. D.
Parkinson (Scottish Academic Press, 1983).

I hope this very brief sketch is of some use.

Yours sincerely

David R Barraclough

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Regards
Brian



brian whatcott <inet@intellisys.net>
Altus OK