[thedoc (OP)]

Wilf James asked the Naked Scientists:
   
I have heard and read about the Earth's magnetism from many sources. Generally, it is accounted for by variations on the theme of the "Moving Metal Theory". In spite of the fact that it is still (apparently) a theory, I have come across numerous instances where the speakers and writers treat the theory as an established fact.

I think that the moving metal theory for the Earth's magnetism as described in Wikipedia and other places is flawed. Every explanation I have found conflicts with Lenz's Law.

The general outline of the theory presupposes that there is an intial magnetic field that induces a current in the moving metal in the Earth's outer fluid core. Lenz's Law says that the magnetic field of an induced current is of the opposite polarity to the magnetic field that induced the current. If the two magnetic fields are combined in any way they will tend to cancel each other.

There is a magnetic field created by the ring current around the Earth. It acts nore or less through the Earth's centre of rotation. If this magnetic field  was like the magnetic field of a magnetron magnet, the magnetic field strength around any circular parth around the Earth's centre of rotation would be unvarying. Only a changing magnetic field can induce a current in a conductor. The ring current magnetic field is slightly off centre so there will be small variations in the magnetic field strength of the ring curreent magnetic field around circular paths around the Earth's spin axis. These small variations will induce currents whose magnetic fields will oppose the ring current magnetic field somewhat.

The ring current magnetic field creates a magnetic south pole near the geographic north pole. The north poles of navigation compasses are attracted to a magnetic south pole.

The present ring current is indirectly caused by the emissions of positively charged particles from the sun in the solar wind. This means that the sun is continually building up an ever stronger negative static charge. Sooner or later the negative charge on the sun could inhibit the emission of positively charged particles. Then the sun could begin to emit negatively charged particles in the solar wind. A negative solar wind would reverse the direction of the ring current and produce a magnetic field that is opposite to that we have now.
The reversal of the polarity of the sun's emissions is consistent with the laws applying to static electricity.
What laws (if any) apply to a magnetic field reversal for the moving metal theory.?

Is there any way that the moving metal theory can be tested empirically to prove that it is a true explanation for the Earth's magnetic field?
Is there any way to show empirically that the ring current magnetic field is not the true cause of the Earth's magnetic field?

Thanks for reading this.

Wilf James BSc.

What do you think?

[evan_au]

As you say, if the solar wind consisted mostly of particles of one polarity, it would soon become self-limiting due to electrostatic attraction.

However, the solar wind consists mostly of protons (positive) and electrons (negative), so overall the Sun remains electrically neutral.

The Sun does exhibit periodic reversals about every 11 years, but they are reversals of the Sun's magnetic field, rather than the Sun's electric field. It is thought that the source of the Sun's magnetic field is a combination of magnetism, electric currents, differential rotation and temperature differences as you might find inside the Earth - only on a much shorter timescale, and without any liquid metal (the Sun is composed of a conductive plasma). This conductive fluid resists movement of the magnetic field through the fluid by Lenz's law, but the differential rotation twists the magnetic field lines.

The Ring Current around the Earth obeys Lenz's law - it generates a field that opposes the Earth's magnetic field. Presumably, when the Earth's magnetic field reverses, this current will also reverse.

Whatever the initial magnetic field which formed the "seed" for Earth's magnetic field, it is quickly distorted, because the moving metal theory requires differential rotation between different layers of the Earth's core. Evidence for differential rotation has been observed by measuring transit times for earthquake waves; differential rotation on the Sun is visible at different lattitudes.

There has been experimental verification of the moving metal theory in the lab, and some confirmation via computer modelling.