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Re: Earth's Magnetic Field

Thank you for the excellent information and SciAM reference. You've given me
a great start!

Appreciatively,
bob


In a message dated 7/10/99 10:36:18 PM, billb@ESKIMO.COM writes:

<< On Fri, 9 Jul 1999, Bob Yeend wrote:

I am looking for information about the origin of the earth's magnetic field;
i.e. what causes it.

Anyone have a good explanation / resource?

There was a very good article in Scientific American on current research
within the last two years or so. If I recall correctly, the field is
produced by what is essentially a large number of "self-excited homopolar
generator" regions in the liquid nickle-iron core of the Earth. The
generators are powered by convection forces, and the patterns of flow are
influenced by the Earth's rotation.

A simple model of a "self-excited homopolar generator" consists of a
rotating conductive disk, a pair of sliding carbon brushes which make
contact with the rim of the disk and its center, as well as a "stator"
electromagnet coil connected to the brushes. That's it. If the disk is
forcibly rotated, and if an external magnetic field is applied, then the
disk will generate a current. The circuit includes the electromagnet,
therefor the current created by the disk ends up generating a magnetic
field which connects the coil to the disk. Once this field appears, the
externally-applied field can be removed. No, it's not perpetual motion,
instead it's more like the magnetic analog of a Wimshurst machine. In a
Wimshurst machine, charge-separation is produced by induction, and it
takes e-field to make e-field. When we first start turning the crank, the
machine "decides", and it can spontaneously produce either polarity. The
HPG (homopolar generator) exhibits a similar effect: it runs by magnetic
induction, it takes b-field to make b-field, and when the conductor-disk
is spun, there's no telling which way the current will go when it appears.

A simple real-world self-excited HPG is very simple. It consists of a
metal disk with spiral slots cut in it. If brushes connected by a
shorting-bar are provided, and if the disk is spun fast enough, then the
disk creates its own "stator" field because the spiral slots force a
spiralling (coil-like) current as well as a radial current. Very weird,
no?

How to visualize what happens in the Earth? I'm not certain, but here's
what I see: think of a roiling blob of liquid iron which is penetrated by
circles of b-field flux and by circles of electric current. As the iron
stirs itself, the flows of charge are dragged along as the metal moves.
Rings of electric current are stretched and folded, and new rings of
b-field are brought into being within the volume. A bit of the field
leaks out and allows our compasses and dip-needles to operate.


I remember the diagrams in the SciAm article depicted a complex tangle of
fluid vortex-threads in the liquid outer core of the Earth. The whole
mess is like a "substance" composed of millions of interacting Homopolar
Generators. As long as the liquid iron keeps roiling, enormous fields and
electric currents will be generated. The article showed that the b-field
flux mostly connects in circles within the core, and only a bit of
fringing fields leak out of the iron. Compasses see the average of the
patches of leaking fields. Depending on how their computer simulations of
the core were tuned, the researchers could produce spontaneous pole-flips
and low-field "stutters" just like the Earth apparently has produced over
the aeons.



Speaking of hundreds of KG of liquid mercury for "rotating pool" telescope
mirrors... it might be interesting to stir a few cubic meters of liquid
iron violently. The "HPG" effect should begin to operate, and intense
electric currents and associated b-fields would build up within the metal
(and act as friction via "electromagnetic braking," so the stirring would
take some work.)


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William J. Beaty SCIENCE HOBBYIST website
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