of measured magnetic anomaly, showing a ringing pattern that is somewhat similar to the cartoon in (a). Magnetometry is used for more


detailed mapping, particularly near the surface, including in the fields of mineral exploration, archaeology, waste management and other environmental studies. Te magnetic field of the Earth is very


different from the gravity field. Its strength is approximately double (60,000 nTesla) at the poles compared to Equator (30,000 nTesla), so it is common to describe the Earth’s magnetic field as a dipole. Exactly how the magnetic field is created is still a research topic, but there is general agreement that it is a product of circulating currents in the iron-rich outer core of the Earth. Te earth’s magnetic field is not quite


Figure 1.78: Estimated locations for the north magnetic pole since 1831 to 2001. (Source: Wikipedia.)


parallel to its axis, and there is a difference at present of approximately 12 degrees. Tis means that the earth’s magnetic poles do not match the earth’s north and south poles. Te magnetic poles are not fixed and the magnetic north pole is gradually changing its position, as can be seen in Figure 1.78. Tis movement is getting faster (note that the pole changed a lot between 1994 and 2001 when compared with


previous periods). Te strength of the Earth’s magnetic field is decreasing; over the past 100 years, this decrease has been measured to be approximately 6%. If one extrapolates this trend then the earth’s magnetic dipole moment will be zero about 1,500 years from now. Geological observations suggest that such an extrapolation is not necessarily very realistic, since the earth has had many minima in the magnetic field without a corresponding reversal. Te last reversal occurred approximately 780,000 years ago. By measuring the magnetisation of clay minerals from Roman pottery, it is estimated that the earth’s magnetic dipole moment was twice as strong in Roman times as it is today. Te area around the earth that makes


up the dominant part of the earth’s magnetic field is called the magnetosphere.


It protects us from charged particles from the sun, but not completely. Many high-energy particles from the sun are hardly influenced by the magnetic field and therefore it is a common view that the radiation risk on Earth will not be affected greatly as the Earth’s magnetic field weakens over time. Te most important shield against high-energy particles from space is the atmosphere, equivalent to a 4m-thick concrete wall.


Figure 1.79: Simplified sketch of Earth showing the inner and outer core, mantle, crust and atmosphere. Unlike a classic bar magnet, the matter governing Earth’s magnetic field moves around. Geophysicists are pretty sure that the reason Earth has a magnetic field is because its solid iron core is surrounded by a fluid ocean of hot, liquid metal. The flow of liquid iron in Earth’s core creates electric currents, which in turn creates the magnetic field. Kelvinsong (Own work) [CC BY-SA 3.0 (http:// creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons.


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