The strong magnetic fields in neutron stars are supposed to come from magnetic flux conservation. If we have:
PhiB=intBdS=const
where PhiB is the magnetic field flux, B is the magnetic field strength, and dS is the elemental closed surface; then, this integral is constant through the surface.
If we consider the star surface over which take the integral, than
S=4piR2
where R is the star radius. This can be translated, altogether with the magnetic flux conservation law, as:
Bf=Bi(fracRiRf)2
where i and f are the indices for initial and final stages.
We know that the star implodes from a whatever star size to sim10 km. So the radii ratio is huge. You just need a starting magnetic field of 10−100 G, to get a final magnetic field of the order of 1012 G, that is typical in neutron stars.
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