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A compass can tell me the directions of the Earth's North and South poles? What is it about the Earth that produces this "polarity" such that a compass can pick it up?

The first thing that jumped into my head was the Earth's rotation, but if that is the explanation, why have I heard from people that the Earth's polarity can switch every million years or so?

enter image description here

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  • $\begingroup$ A better question would be: why are the Earth's magnetic poles near its axis of rotation? If you consider the two questions similar enough, edit your question to this format, or ask it as a new question, or let me know and I will ask myself. $\endgroup$ – Pavel V. Apr 16 '14 at 10:45
  • $\begingroup$ The alignment of geographic and magnetic axis varies inside the solar system. See sketches in saturn.jpl.nasa.gov/files/Planetary_Magnetics.pdf $\endgroup$ – tobias47n9e Apr 16 '14 at 10:56
  • $\begingroup$ @PavelV., I believe your question is separate to the one I asked. Feel free to ask this question yourself. $\endgroup$ – Kenshin Apr 16 '14 at 10:57
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Well, firstly it's important to recognise that the poles are merely the extremities of the shape of a magnetic field - the earth's magnetic field. All magnetic fields have polarities as such.

However, you're asking why the field itself even exists, I gather. In this case it's generated by electric currents in the conductive molten iron (and other metals) in the core of the earth, as a result of convection currents generated by escaping heat. This is known as a geodynamo, and is one way of generating a magnetic field. However, exactly why this occurs is still under investigation.

More is available in this Wiki piece - specifically on the physical origins of the earth's magnetic field.

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There is an alternative theory that the Earth's magnetic field is due to ocean currents since the sea contains (charged) dissolved salts. The flipping of the poles would then presumably be due to a major change in the currents. From what I have read there is a great deal of uncertainty about this subject.

Physics world article

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  • $\begingroup$ Do you have a reference, an article in which that theory have been mentioned? $\endgroup$ – plannapus Jun 6 '14 at 10:02
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    $\begingroup$ physicsworld.com/cws/article/news/2009/jun/14/… $\endgroup$ – mistermarko Jun 6 '14 at 10:09
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    $\begingroup$ Do you have an actual Journal Source, one peer reviewed by experts? $\endgroup$ – Neo Jun 6 '14 at 20:46
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    $\begingroup$ No, this article does not say the source of the magnetic field is due to ocean currents. Rather it says "slight drifting of the magnetic field over years-to-centuries" is due to electrically charged particles and ocean currents. He goes on to speculate that the flipping of the poles could be due to the ocean, but maintains that the primary magnetic field of the Earth is due to the geodynamo effect. $\endgroup$ – farrenthorpe Jun 6 '14 at 22:07
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    $\begingroup$ The mass of the oceans is insignificant compared to the mass of the iron in the core. $\endgroup$ – vsz Jan 15 '15 at 20:43

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