|MadSci Network: Earth Sciences|
The Earth's magnetic field is mainly generated in the Earth's core. This is a large metallic (Fe, Ni, S) fluid body surrounding another metallic (Fe, Ni, mainly) solid body. This is a perfect situation for a dynamo, as the metallic fluid revolves around the solid core, generating electric currents and a magnetic field (that is weak if compared with magnets we know in the practice).
Some observations of rocks in the oceanic ridge (the undersea mountains that cross the oceans, separating the continents) showed that some presented a magnetic polarity (because they can maintain the magnetic field of the time they were generated) compatible with the field we observe today on Earth; but a large extent of rock presents a field that has the inverse polarity.
When the number of observations became large enough to obtain a pattern, it was clear that large extents of rock present "normal" polarity, and large adjacent extents present "reverse" polarity. As the rocks retain the magnetic parameters of the Earth's magnetic field in the age they when they were formed, we can conclude that the Earth's magnetic field switches the poles sometimes.
To explain this, some dynamo models were proposed. One of these models suggests that if we have two coupled dynamos, rotating with different velocities and opposing one another, we can generate a magnetic field that can be stable for a time in one polarity, and that can reverse polarity after some time (depending upon the rotation, dynamo material, etc.).
There's not a defined pattern of reversion in Earth's magnetic field history. We have times of intense reversing, and times of stability, and these times could not give us some clue of periodicity.
In the other planets of the solar system , we have little knowledge of their magnetic field histories before now. If the planet has a fluid core (some planets had in past, but have now solidified) in a condition similar to Earth, we may have this inversion occurring, but, as far as I know, nothing about this has been observed yet.
Eder C. Molina
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