Earth's magnetic field (and the surface magnetic field) is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole (see Magnetic North Pole) and the other magnetic field N pole near the Earth's geographic south pole (see Magnetic South Pole). The cause of the field can be explained by dynamo theory. Magnetic fields extend infinitely, though they are weaker further from their source. The Earth's magnetic field, which effectively extends several tens of thousands of kilometres into space, is called the magnetosphere.