Would Earth's magnetic field be strong assuming all the iron from Earth's core were cooled down and then ordered in such a way that the magnetic fields of each individual atom aligns in such a way to create a magnetic field. Would it be stronger than Jupiter's magnetic field or would it even get larger and be the strongest magnetic field?
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If the Earth's core were a ferromagnet, the magnetic field at the Earth's surface would be a couple of hundred times stronger than the magnetic field on Jupiter's surface, but at a radius corresponding to surface of Jupiter, it would be slightly weaker than Jupiter's surface field.
It is impossible for all the magnetic moments of atoms in the Earth's core to align in single magnetic domain. In the absence of an externally applied field, atoms in a ferromagnetic are aligned by short-range quantum forces, but when a domain gets big enough long-range classical magnetic forces overcome the quantum forces and the domain splits into smaller oppositely aligned domains. The maximum size of a spherical iron domain is about 10 nm. (e.g. See slide 35 of these lectures)
The surface magnetic field you can get from a chunk of magnetized iron is hence limited by its maximum remanence of about a tesla (=10000 gauss). Dipole magnetic fields fall off as $1/r^3$, so if the Earth's core were a 1 tesla ferromagnet with a radius of about 3500 km, the magnetic field at the Earth's surface (with radius about 6400 km) would be about 1600 gauss. At a distance corresponding to Jupiter's radius of about 71000 km, the field would be about 1 gauss, somewhat smaller than Jupiter's actual surface magnetic field of about 4 gauss at its equator and 14 gauss at its North pole.
In practice - if that makes any sense when talking about Earth-core-sized permanent magnets - the field would likely be less. The solidified iron/nickel material of the Earth's core might not have high enough coercivity to hold its maximum remanence.
David Bailey
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