[Baron Munchausen]

Originally posted by Kamog:
I never understood this idea of the earth's magnetic poles moving around. How can the magnetic field, which was stable for a long period of time, suddenly reverse polarity? If the magnetic field is produced by the flow of molten material inside the earth, then it takes a tremendous amount of energy to change the direction of flow for so much mass. Where does this energy come from? Also, I thought that a flow of material will not produce a magnetic field unless there's an electrical current flowing through it. I mean, if you take a piece of iron and move it around where there's no magnetic field, it's not going to produce its own magnetic field, would it?

It will not be 'sudden' except from a geologic standpoint. It will take centuries. And yes, the earth is very large but that does not mean it has to be immutable. It does not rotate in a completely stable fashion either. It has a 'wobble' known as 'precession of the equinox' so who is to say that the currents of the interior cannot also 'wobble' somehow or other? The continents themselves are moving, bending, breaking, and getting shunted into the interior of the planet. Stability is an illusion.

What powers the earth's magnetic field is an interesting question. As far as I know, only an electric current can generate a magnetic field. So, somehow or other the earth's core is generating electricity. You'd have to ask some geologist who specializes in the dynamics of the 'deep' parts of the earth (dunno what the name for that specialty would be) for the prevailing theory on how that happens.

Geomagnetism FAQ from NOAA:
http://www.ngdc.noaa.gov/seg/potfld/faqgeom.shtml

Edit:

http://hyperphysics.phy-astr.gsu.edu.../magearth.html

Quote:

Magnetic Field of the Earth

The earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the earth. The problem with that picture is that the Curie temperature of iron is about 770 C . The earth's core is hotter than that and therefore not magnetic. So how did the earth get its magnetic field?

Magnetic fields surround electric currents, so we surmise that circulating electic currents in the Earth's molten metalic core are the origin of the magnetic field. A current loop gives a field similar to that of the earth.

The earth's magnetic field is attributed to a dynamo effect of circulating electric current, but it is not constant in direction. Rock specimens of different age in similar locations have different directions of permanent magnetization. Evidence for 171 magnetic field reversals during the past 71 million years has been reported.

Although the details of the dynamo effect are not known in detail, the rotation of the Earth plays a part in generating the currents which are presumed to be the source of the magnetic field. Mariner 2 found that Venus does not have such a magnetic field although its core iron content must be similar to that of the Earth. Venus's rotation period of 243 Earth days is just too slow to produce the dynamo effect.

Interaction of the terrestrial magnetic field with particles from the solar wind sets up the conditions for the aurora phenomena near the poles.

The Dynamo Effect

The simple question "how does the Earth get its magnetic field?" does not have a simple answer. It does seem clear that the generation of the magnetic field is linked to the rotation of the earth, since Venus with a similar iron-core composition but a 243 Earth-day rotation period does not have a measurable magnetic field. It certainly seems plausible that it depends upon the rotation of the fluid metallic iron which makes up a large portion of the interior, and the rotating conductor model leads to the term "dynamo effect" or "geodynamo", evoking the image of an electric generator.

Convection drives the outer-core fluid and it circulates relative to the earth. This means the electrically conducting material moves relative to the earth's magnetic field. If it can obtain a charge by some interaction like friction between layers, an effective current loop could be produced. The magnetic field of a current loop could sustain the magnetic dipole type magnetic field of the earth. Large-scale computer models are approaching a realistic simulation of such a geodynamo.

Endquote ---

The original has some diagrams and links to further information.

[ January 03, 2004, 22:20: Message edited by: Baron Munchausen ]