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I was wondering why the earth's magnetic poles are so close to the rotational poles. The magnetic north pole is currently about 500km from the true north pole. That is pretty close in the scale of the planet.

  • Is this purely by coincidence or is there a correlation?

  • In any sphere of magnetized liquid that is rotating do the magnetic poles align themselves with the axis of rotation?

  • Or perhaps the opposite, does the rotation align itself with the magnetic poles?

  • How about a solid magnet.

  • If so, why?

I can see that an external magnetic force would cause the axis to align, so I'll specify the absence of any such force. Unless there's some such force that acts on earth that I'm not aware of.

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Tom, it is no coincidence that the magnetic north pole is close to the geographic north pole. To the best of our knowledge, that has been true for a few hundred million years, aside from relatively brief intervals where the polarity reverses.

The main source of the magnetic is the molten iron alloy in the Earth’s outer core (NOT the mantle). It convects vigorously, driven mostly by heat flow as the Earth cools. It is also highly conducting, so a magnetic field will set up currents in it, and a mechanism called the dynamo effect will provide a feedback loop that keeps the currents going. The rotation of the Earth tends to organize the flow, and that is the reason the field tends to be nearly parallel to the axis.

Now for that “magnetized” fluid. I imagine you wanted an Earth analogue, which would be a conducting fluid. Such an experiment has actually been done many times (see, for example, this video). The rotating fluid will not be magnetized unless an external field is applied, at least for a short while. And without a source of buoyancy (such as heat flow) to drive convection, the current will rapidly die away as soon as the field is removed. Mars used to have a magnetic field, but it long ago cooled to the point that it couldn’t sustain a dynamo.

The solar wind has a strong effect on the Earth’s magnetic field in space, but only a very small effect at the surface. The flow of molten metals is indeed far from uniform, and sources of asymmetry include topography and compositional heterogeneity at the surface of the inner core and the core-mantle boundary.

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Consider a sphere, filled with fluid as you said, but it the fluid itself is not magnetised directly. Rather, there a flow of conducting molten material throughout the mantle, caused by the rotation of the earth,the coriolis force and a couple of other factors. This flowing fluid current, is the source of earth's magnetic field, a geodynamo model. Ideally speaking, had every flow pattern been uniform, and had there been no solar winds, there is no apparent reason for the magnetic poles to deviate from the rotation axis.

1) However, the flow of molten metals inside earth is far from uniform, varying in speed and current density in various places.

2)The continuos bombardment of the magnetic field by energetic particles from the sun(solar wind) has given rise to high assymetry in the magnetic field. The magnetic poles are still drifting, and will reverse in the future, as it has been doing many times before(evidence from seabeds and rocks tell this).

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  • $\begingroup$ there is no apparent reason for the magnetic poles to deviate from the rotation axis. This suggests that without the irregularities the poles and axis would align. I can see how they get out of alignment. I'm asking why they "would" align without an outside magnetic force. $\endgroup$ – Tom Nov 22 '16 at 18:53
  • $\begingroup$ Without an external force, the flow of the molten metals will be tangential, along non intersecting planes perpendicular to the axis of rotation(imagine the rotating sphere for yourself and see which direction the flow will be). Then, isnt it obvious that the magnetic field of a current carrying loop is perpendicular and passing through its center? Add up all such loops, and consider the superposition of their magnetic fields. It will be in the direction of axis of rotation. This much should be high school physics. $\endgroup$ – Lelouch Nov 23 '16 at 0:07
  • $\begingroup$ Didn't take physics in high school :P $\endgroup$ – Tom Nov 23 '16 at 0:35

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