geomagnetism: main field, secular variation, and westward drift

geomagnetism: main field, secular variation, and westward drift The magnetic field that originates in the Earth's outer core (see magnetic field (origin of the Earth's internal field)) is known as the main field. Roughly 90 per cent of the main field at the Earth's surface can be approximated as a field produced by a magnetic dipole; hence that part the main field is known as the dipole field. The part that cannot be expressed in this way is known as the non-dipole field. At the Earth's surface, the intensity of the main field varies from roughly 24 000 to 66 000 nanoteslas (nT), from the magnetic equator to near the geomagnetic poles, respectively. (The term geomagnetic is generally reserved for a mathematical dipole model of the main field, whereas the term magnetic usually represents local field conditions and includes the non-dipole field and the effect of geological sources.) Observations of changing magnetic declination made over a period of 50 years in London convinced H. Gellibrand (c. 1634) that the main field changes not only in space, but also over time. These temporal changes in the characteristics of the main field (for example, intensity, inclination, and declination) are known as the secular variation (or time variation) of the main field. The Earth's rotation plays a significant role in maintaining the orientation of the geomagnetic dipole close to the axis of rotation (see magnetic field (origin of the Earth's internal field)). In the past 400 years, based on historical magnetic charts, the dipole axis has moved westward at a rate of about 0.80° per year. However, the most remarkable westward drift is displayed by some features of the non-dipole field. Historical magnetic charts and archaeomagnetic data (especially the archaeomagnetic inclination data that systematically deviate from that of the dipole field) going back about 2000 years show that some features of the non-dipole part of the field move westward at rates of between 0.3° and 0.6° per year. The remaining features of the non-dipole field are more stationary. However, both parts of the non-dipole field change in amplitude over time. The dipole field also varies in strength with time. The strength of the ancient field is inferred from the use of various intensity measurement techniques on archaeomagnetic samples (such as pottery, bricks, and kilns) and sequentially formed rock units (for example, lake sediments). A large number of such studies show that the strength of the Earth's field for the past 10 000 years has varied from about 0.8 to 1.5 times the strength of the present geomagnetic field. (The dipole moment is presently about 8 × 10²² Am².) However, measurements going back further to roughly 50 000 years exhibit a much smaller value for the field strength (about half the present value), excluding perhaps a few episodes when the magnetic field was extremely strong (reaching about 5–6 times its present strength). In general, all these different types of secular variations, their rates and their peculiarities, constrain models of fluid motions in the outer core and the generation of the Earth's magnetic field.

Dhananjay Ravat

Bibliography

Merrill, R. T. and and McElhinny, M. W. (1983) The Earth's magnetic field: its history, origin and planetary perspective. Academic Press, London.