Earth's magnetic field

Summary

Earth's magnetic field is the magnetic field produced in and around the Earth, mainly by electric currents in the liquid iron outer core. Smaller parts come from magnetised crustal rocks and from currents in the ionosphere and magnetosphere. Near the surface it is roughly the field of a tilted dipole at the Earth's centre, with an intensity of about $25\,000$ to $65\,000$ nanotesla. In the source-free region between the surface and the ionosphere it is the gradient of a harmonic potential. This allows its description by spherical harmonics in Gauss's analysis. It changes slowly in time by secular variation and reverses its polarity at irregular intervals over geological time.