chapter3 - Chapter 3 The Magnetic Field of the Earth Introduction Studies of the geomagnetic field have a long history in particular because of its

chapter3 - Chapter 3 The Magnetic Field of the Earth...

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Chapter 3 The Magnetic Field of the Earth Introduction Studies of the geomagnetic field have a long history, in particular because of its importance for navigation . The geomagnetic field and its variations over time are our most direct ways to study the dynamics of the core . The variations with time of the geomagnetic field, the secular variations, are the basis for the science of paleomagnetism , and several major discoveries in the late fifties gave important new impulses to the concept of plate tectonics. Magnetism also plays a major role in exploration geophysics in the search for ore deposits. Because of its use as a navigation tool, the study of the magnetic field has a very long history, and probably goes back to the 12 C when it was first exploited by the Chinese. It was not until 1600 that Gilbert postulated that the Earth is, in fact, a gigantic magnet. The origin of the Earth’s field has, however, remained enigmatic for another 300 years after Gilbert’s manifesto ’ De Magnete ’. It was also known early on that the field was not constant in time, and the secular variation is well recorded so that a very useful historical record of the variations in strength and, in particular, in direction is available for research. The first (known) map of declination was published by Halley (yes, the one of the comet) in 1701 (the ’chart of the lines of equal magnetic variation’, also known as the ’ Tabula Nautica ’). The source of the main field and the cause of the secular variation remained a mystery since the rapid fluctuations seemed to be at odds with the rigidity of the Earth, and until early this century an external origin of the field was seriously considered. In a breakthrough (1838) Gauss was able to prove that almost the entire field has to be of internal origin . Gauss used spherical harmonics and showed that the coefficients of the field expansion, which he determined by fitting the surface harmonics to the available magnetic data at that time (a small number of magnetic field measurements at intervals of about 30 along several parallels - lines of constant latitude), were almost identical to the coefficients for a field due to a magnetized sphere or to a dipole. In fact, he also showed from a spectral analysis that the best fit to the observed field was obtained if the dipole was not purely axial but made an angle of about 11 with the Earth’s rotation axis. An outstanding issue remained: what causes the internal field? It was clear that the temperatures in the interior of the Earth are probably much too high to sustain permanent magnetization. A major leap in the understanding of the origin of the field came in the first decade of this century when Oldham (1906) and Gutenberg (1912) demonstrated the existence of a (outer) core with a very low viscosity since it did not seem to allow shear wave propagation ( rigidity =0). So the rigidity problem was solved. From the cosmic abundance of metallic iron it was inferred that metallic iron 67
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68 CHAPTER 3. THE MAGNETIC FIELD OF THE EARTH
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  • Fall '13
  • BertrandI.Halperin
  • Physics, Laplace, The Land, Magnetic Field, International Gravity Field, geomagnetism

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