1. n. [Geophysics]

Earth’s main magnetic field generated in the Earth’s fluid outer core by a self-exciting dynamo process. Approximately 95% of the total magnetic field measured at Earth’s surface comes from this main field, a significant portion of which may be described as the field of a dipole placed at the Earth’s center and tilted approximately 11° from the Earth’s rotational axis. The magnitude of the main magnetic field is nearly 60,000 nT near the Earth’s poles and about 30,000 nT near the equator. However, there are significant nondipole contributions to the main magnetic field that complicate its mathematical and graphical representation, including that the relative strengths of nondipole components change. As additional complications, the main field varies slowly because of changes within the Earth’s core and the magnetic dipole axis pole position itself wanders over time.

See related terms: dipole field,  nondipole field,  crustal magnetic field,  external disturbance field,  local magnetic interference