Dipole Moment Variation

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The present geomagnetic field is approximately that expected from a dipole located at the center of the Earth and tilted by about relative to the rotation axis. The magnetic moment associated with such a dipole can be inferred from measurements of magnetic field strength. Systematic measurements of relative field strength revealing latitudinal variations were made by De Rossel on the D'Entrecasteaux expedition (1791–1794), but the moment was not evaluated directly until the 1830s when Gauss (q.v.) carried out the first spherical harmonic analysis using absolute measurements of field strength made at geomagnetic observatories. Although the dipole moment has decreased by about 10% since then, the current value of is close to the average for the past 7 ka . A broad range of geomagnetic and paleomagnetic observations indicates that both these values are higher than the longer‐term average, but probably not anomalously high, and that the current rate of change seems not atypical for Earth