Earth's magnetic field strength

Earth's field is generated by convecting liquid iron in the outer core, a self-sustaining dynamo, and at the surface it is weak: about half a gauss, some five hundred times feebler than a refrigerator magnet held at arm's length. It is also untidy. The intensity runs from roughly 22 μT over the South Atlantic Anomaly to 65 μT near the magnetic poles, the dip angle varies from horizontal at the magnetic equator to vertical at the poles, and the whole pattern drifts — the north magnetic pole has been sprinting from Canada toward Siberia at tens of kilometres a year.

Carl Friedrich Gauss made the first absolute measurement of the field's strength in 1832, expressing it in millimetres, grams and seconds, which is where the whole idea of a coherent system of physical units began. Today the field is mapped by the IGRF model and by satellites such as ESA's Swarm. Any single number for it is a typical value only: navigation, magnetometer surveys and MRI shielding all need the local, dated field rather than a global average.