Re: magnetic field lines like these?

[William Beaty <billb@eskimo.com>]

On Wed, 6 May 1998, LUDWIK KOWALSKI wrote:

> How do NS reversals occur? Does B goes to zero everywhere and then starts
> growing in the opposite direction? Or is the magnetic moment turning without
> a significant change in magnitude? Perhaps Uranus is in the middle of a
> reversion in which the magnitude is more or less constant.

I recall a SciAm article about this from a few years back.  Some people
were working on a numerical simulation of liquid convection and electric
current in the Earth's core, and they found that part of the natural
behavior of their simulation was that the field would periodically vanish,
only to reappear in reversed direction.  It looked like a Chaos type of
thing, with a bunch of interacting vortex threads, and with the field
reversals being a natural "emergent property" of the dynamical system. 

I could imagine that if equal populations of electric current loops having
opposite rotations could randomly arise in the core, then the external
field could randomly vanish.  The field would form closed loops inside the
core, like two horseshoe-magnets with opposite poles butted together.

About the earlier "superconductor" stuff I mentioned.  It's not that the
iron core is a perfect conductor, instead the persistent currents arise
because the size of the current loops give enormous inductance.  Long-term
currents come about from low "R" and high "L", and whereas
superconductivity is a low-R phenomenon, the Earth-core currents are a
high-L effect.  I guess this is analogous to 1-farad capacitors which
behave like batteries, but with high-value, single-turn inductors being
the "battery" and storing currents for decades. If a miniature model of
earth's core "generator" was constructed, the currents would decay in a
fraction of a second unless the simulated convection was sped up
enormously.  Either that, or use superconductive materials to compensate
for the reduced inductance of the small current loops in the tiny model. 

Aside: if all conductors in a standard DC PM generator were replaced with
superconductors, and if the generator output was shorted, what would
happen as the generator shaft was continuously cranked?  The current in
the shorted loop would rise continuously, but since magnetic flux cannot
"cut" through a superconductor, I think the shape and motions of the
fields in the generator, especially near the commutator, would look
verrrrry interesting.  The superconducting generator must be grabbing
"hanks" of flux from the permanent magnet and then stuffing them into its
shorted output loop.  A sort of "flux pump", with the superconducting
output loop acting as the "storage bottle".  If I could visualize this, I
think I would have an intuitive understanding of the Earth's core
generator effect.


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William J. Beaty                                  SCIENCE HOBBYIST website
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