The Earth's magnetic field shows variability on all time-scales, 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 for 1980. Previous annual patterns were based on geomagnetic observatories, supplemented by field surveys and corrected to some particular time on the basis of observed secular variation in the preceding years. 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. During this century, 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 time-scales. The field shows an ability to reverse polarity on a geologic time-scale, 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.
Subjects: Meteorology and Climatology.