The spectrum of variations is associated with all these current flows, and magnetic storms. Besides their effect on communications, power grids, etc, they provide an excitation source for studying electrical conductivity. Magnetosphere well above this, to many earth radii.involves the solar wind, field lines, and complex physics of their interaction. Paleomagnetism and magnetic prospecting both measure things dependent on rock magnetic properties. A quick look at rock magnetism.
Magnetic properties arise from single unpaired electron in the outermost shell.
Completely filled shells have no magnetic moment, but an unfilled shell produces a magnetic moment and a spin moment, since there is only 1 electron in the shell. The magnetic properties of materials depend on such unpaired shells, plus the effects of crystal structure and ordering of the atoms. On the interior of a magnetically susceptible material the magnetic B field has two sources. The other is the magnetization M which is parallel to B in an isotropic material, but anisotropic materials may have M and B not parallel. In reality k is almost always less than 1, usually much less, and may be positive or negative for earth materials.