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Is there any reason why Earth's magnetic poles are so close to its axis of rotation, or is it a coincidence? Why does the Earth have a somewhat uniform magnetic field with two poles? If you take a bunch of random particles and put them together, why should a pole form on each side of this collection?

Is it in practice possible to create a device capable of canceling the Earth's magnetic field in a region the size of the North Sea? This could help us control the wandering of certain unwanted crabs.

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Maybe you can use a directed magnetic pulse to re-magnetize the crabs instead? –  endolith Nov 22 '11 at 4:23
    
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If you take a bunch of random particles and put them together, why should a pole form on each side of this collection?

Some particles already have a magnetic field. Many particles are polar, such that they will orient themselves in a magnetic field. If you jumble them all together, they will self align, and eventually one strong field will be externally detectable even though their individual fields were small and unorganized at the start.

Perform this experiment: Drop a bunch of magnetic powder and dirt into a bag. Shake vigorously. What is the resulting clump's magnetic signature?

Is it in practice possible to create a device capable of canceling the earth's magnetic field in a region the size of the north sea?

No. What you want is a Helmholtz coil, adjusted electronically to react to the earth's changing field.

However, the area of the field required, even though it would be relatively low magnetic force, would require entirely too much energy to be practical. Further, an ideal Helmholtz coil, where the field is uniformly 0 everywhere inside the coils, requires essentially a cubic structure. The North Sea is 970 KM long, and thus the coils would need to be 970KM in diameter, vertically oriented, buried a significant portion of that depth into the ground on either side of the north sea.

Further, it would really mess up the compasses of people traveling anywhere near the coils, not to mention other animals that appear to depend on magnetic fields, such as some migrating birds.

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I keep referring to wiki articles, and the earth magnetic field one is quite ok.

So the answer is that the rotation of the earth plays a role, but it is not as simple as one to one. The dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales.

Now about the crabs, and the north sea: though in principle one can change the magnetic field in small regions changing the field on such magnitudes is not possible.

If you described the problem with magnetic fields and the crabs, maybe a solution could be designed generating strong magnetic fields to attract them to special points where they can be harvested/eliminated. Strong magnetic fields, orders of magnitude stronger than the earth can be made without too much expense.

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Or you could just add small refrigerator magnets to each crab? –  Martin Beckett Mar 23 '11 at 18:29
    
of course I mean attract them as the light attracts moths. presumably these crabs orient themselves with the magnetic field. –  anna v Mar 23 '11 at 19:40
    
I thought the plan was just to annoy the crabs! I imagine a crab walking around in circles (sideways?) would be fairly amusing –  Martin Beckett Mar 23 '11 at 19:42
    
@MartinBeckett: You can introduce ferromagnetic grains into lobster statocysts and then cause them to do flips by bringing a magnet nearby, they think gravity has reversed. –  endolith Mar 25 at 16:03
    
@endolith - does this mean science finally has a way to make crabs walk forward? –  Martin Beckett Mar 26 at 23:26
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The magnetic poles are apparently defined by the movement of material deep inside the earth's liquid core.

Since the core is liquid, there are Coriolis effects which align the currents in the core. The effect is similar to that seen in the earth's atmosphere, but with a far longer time scale. So while the structure of the flows in the earth's core are unknown, we expect that they will have a structure vaguely reminiscent of the earth's atmosphere. See the wikipedia article on "prevailing winds".

Thus I suspect that a literature search will reveal that the earth's magnetic north pole is approximately aligned parallel (or anti-parallel) with the planet's rotation axis because this is a symmetry of the forces that create the magnetic field.

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I read something on deviation of core/mantle boundary from perfect sphere not long ago (New Scientist online or such). This may be reason for the deviaton from perfect alignement. –  Georg Mar 24 '11 at 10:21
    
I'm guessing that, like the weather above ground, it's a chaotic environment and dances around a lot. But the thing that is bothering me, regarding the above argument, is one could also argue that there's no reason for north to be preferred to south, and therefore, by symmetry, there should be no magnetic field at all. (I guess I have to resort to some symmetry breaking argument.) –  Carl Brannen Mar 25 '11 at 2:05
    
if You calculate the mean field over long times (>millions of years) the earth has no field in fact. :=) The field switches direction every 300 000 years. –  Georg Mar 25 '11 at 9:26
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