Geomagnetic secular variation

Summary

Geomagnetic secular variation is the slow change of the internal part of Earth's magnetic field over years to centuries. It comes from the flow of liquid iron in the outer core, which carries and stretches the magnetic field lines. One well-known feature is the westward drift of parts of the field, of about $0.2$ degrees of longitude per year. Others are the decrease of the dipole strength since the first absolute measurements in the 1830s, and the motion of the magnetic poles. It is described by the time derivatives of the Gauss coefficients, and it limits how long any field model stays accurate.