The Earth’s magnetic field shows variability on all timescales, ranging from nanoseconds to millions of years. Most transient variations are of external origin, reflecting interactions between the solar wind and the Earth’s atmosphere. Longer-term changes, called secular variations, are of internal origin. The average annual field has been closely defined by satellite observations. The intensity of the field varies from about 30 μ‎T near the equator to 60 μ‎T near the observed geomagnetic poles at 73° N 100° W and 68° S 143° E. Most of the field (80%) can be accounted for mathematically by a single microgeocentric dipole inclined at 11.3° to the Earth’s rotation pole, known as the geomagnetic dipole, with a magnetic moment of 8.01 × 10²² A/m². The remaining field, the micro-non-dipole field, forms 12 main areas, varying by ±1.5 μ‎T. An improved mathematical fit to the observed field was obtained using an inclined dipole offset from the geocentre by 340 km. At present the pattern of the field shows a tendency to drift westwards at about 0.2°/year, but this is not considered to be persistent on archaeological timescales. The field shows an ability to reverse polarity on a geologic timescale, with three polarity changes per million years during the last 60 million years, but long periods, of about 50 million years, of constant polarity are also known. The geomagnetic field is generally attributed to fluid motions within the outer core; these carry magnetic lines of force with them (magnetohydrodynamics), creating a coupled self-exciting dynamo that allows polarity reversals.