Under quiet solar and geomagnetic activity conditions, the magnetic field measured at the Earth’s surface is primarily (>90%) due to a magneto-hydrodynamic dynamo, operating in the liquid outer core of the Earth. The secular (or time) variation of this field operates on a scale of months to centuries, or more, with position dependent amplitude of anywhere between zero and up to a few hundred nT year-1. The core field morphology is closely dipolar, at least far from the Earth, and is inclined to the Earth’s rotation axis by around 11 degrees at the present time (see Figure E-1).
Superimposed on this core field is the static magnetic field of geological sources in the lithosphere and upper mantle. Typically the field from these crustal rocks decays rapidly away from the source. For example, in low Earth orbit, the crustal signature is probably no more than about 20 nT, decaying rapidly with altitude.
Traditionally the combination of the core and crustal field is referred to as the ‘main field’. The mean “main field” as measured at the Earth’s surface is depicted in Figure E-1.