Earth acts as though it were a huge dipole magnet with the positive and negative poles near the North and South Poles. This does not mean that Earth is literally a dipole magnet—there are too many variations in the field—but that the best fit for a model of the field is two poles of a magnet, rather than a quadrupole, or other shape. The magnetic field of the earth allows magnetic compasses to work, making navigation much easier. It also molds the configuration of Van Allen belts, bands of high-energy charged particles around the earth's atmosphere.

Most of Earth's magnetic field (90%) occurs below the surface and possibly exists because Earth's core doesn't move at the same rate as the earth's mantle (the layer between the earth's core and its crust). The external 10% of the field is generated by movement of ions in the upper...