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Re: Earth's Magnetic Field

On Mon, 12 Jul 1999, paul o johnson wrote:

But I still have a problem even with that explanation. David states that the
Earth's fluid outer core is almost certainly in motion. In what frame of
reference is this motion? With respect to the solid inner core? The solid
surface?

The computer simulations in that SciAm article depict the motion as a big
nasty mess of 3D vortices. Imagine a hot-tub with several turbulent water
jets. Imagine a bunch of smoke-rings which are colliding and
interweaving.

The CM of the Earth-Moon system? In using the sun's small magnetic field
at the Earth to power the Earth's interior dynamo, David implies that
the fluid core's motion is with respect to the sun. Well, is this "third
requirement" motion (relative motion of the fluid core in the sun's
magnetic field) due to the Earth's rotational motion, it's revolutionary
motion, the vector sum of both, or some other?

I think he was referring to the "triggering" mechanism which initially
jump-started the dynamo. Once the dynamo runs, it becomes self-supporting
and needs no external field from the sun. If there was no initial
magnetic field at all, then the dynamo might never get started. A
Wimshurst electrostatic generator gives a good analogy: if there are no
ambient e-fields present, then the Wimshurst generator will never develop
a high voltage no matter how long we turn the crank. But if a small
e-field hits the device, it causes a breaking of symmetry and the machine
can then ramp up its voltage exponentially until it is limited by corona
leakage. A shorted-out self-exciting dynamo is similar: we can crank it
forever, but if there is zero ambient b-field, then the generator won't
run. If a small ambient b-field is present, then the generator can create
a small current, which makes a bigger b-field, which generates a larger
current, and the current grows exponentially until it is limited by
resistive losses.




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