A 2,500 year record of the New Zealand geomagnetic field and some models of the global secular variation

Abstract

At the end of the sixteenth century William Gilbert published what is generally considered to be the first modern scientific treatise,'De Magnete' of 1600 AD. In this work, he compared the Earth's magnetic field with that of a uniformly magnetized sphere (Rikitake and Honkura, 1985). Much earlier, the ancient Egyptians knew of magnetic repulsion and the ancient Greeks certainly knew of magnetic rocks or 'lodestones'(Hesse, 1961). By the first century AD the Chinese had become aware that the Earth behaves as a magnet. Later, in the 8th Century AD the Chinese first defined the declination, or the angle that the Earth's magnetic field makes with the geographic north and, accordingly, the first declination measurements were made in about 720 AD by the Buddhist astronomer I-Hsing (Tarling, 1983). At least 9 declination measurements dating from 720 AD to about 1280 AD are known, but it took until the year 1510 for the first European declination measurements to be made. Georg Hartmann, then a vicar of Nurenburg, made these measurements in Rome (Rikitake and Honkura, 1985). Magnetic inclination, or the angle the geomagnetic vector makes with the horizontal, was probably discovered in 1544 by Georg Hartmann (Rikitake and Honkura, 1985). However, this work was not widely known and the first historically important work on inclination was due to an Englishman, Robert Norman in 1576 (Tarling, 1983). Secular variation, the time variation in direction and intensity of the geomagnetic field over the Earth's surface began to be studied in detail during the 17th century. Henry Gellibrand, a Professor of astronomy at Gresham College, in 1839, noted changes in declination measurements made in London over the previous half-century, although he ascribed these changes to inaccuracies in the earlier values. He noted a declination measurement of 11.3° East made by William Borough in 1580 and a further measurement of 6.0° East in 1582 by Edmund Gunter, and compared them with his own value of 4.1° made in 1634. Apparently, he did not realise the significance of the variation (Hellmann, 1896). The most striking feature of the time variation of the surface field is the apparent slow drift westwards of its main features, and this was first recognised by Halley (Halley, 1683). Since then, westward drift has had a variety of interpretations including the bulk motion of core fluid relative to the mantle (Bullard, 1950) and to magnetohydrodynamic waves in the core which propagate westwards (Hide, 1966).