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what do you mean by 'straight' ?
it does not go exactly from pole to pole, and has reversed a number of times in the history of the earth
I know that its not aligned with the geographic poles but do we know that it is straight as a line and not curved?
If you are imagining a physical pole that can be bent, then that's not accurate.
We don't know what actually created the magnetic field of the earth, but it is hypothesized to be caused by electric charge in the earth's core.
as the core of the earth rotates the moving charge creates a magnetic field, just like an electromagnet.
here is a diagram of sorts:|dw:1329925793919:dw|this is a circular wire carrying an electric current I. A current produces a magnetic field B that curls around the wire.
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here's my screwy attempt to draw in the induced B-field:|dw:1329925959495:dw|so the molten core of the earth behaves like a circular current as it spins, which induces a uni-directional B-field. Nor 'straightness' required
So earth acts like a coil wrapped around some kind of core?
yeah, more or less
the outer core is like a liquid shell of electric charge, and as it spins it acts like a circular coil of current, so it creates a B-field perpendicular to the plane of rotation