The Recent Geomagnetic Field and its Variations
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The Recent Geomagnetic Field and its Variations Hermann L¨uhr, Monika Korte and Mioara Mandea
2.1 Introduction The Earth’s magnetic field is the sum of several contributions. These are internal ones, i.e. the core field, also known as the main field, and the crustal or lithospheric field, and external ones originating from ionospheric and magnetospheric currents. The core field is by far the most prominent part. The lithospheric field has its origin in remanent and induced magnetization of the crust and upper mantle. Due to its local and regional variability, the individual features are referred to as magnetic anomalies. Strongly magnetized material or large geological bodies create some outstanding anomalies. A well-known example is the Kursk anomaly, in Ukraine, with peak amplitudes of some 3,000 nT in aeromagnetic data at 500 m altitude, and extending over several 100 km. Weaker anomalies of a few tens to hundreds of nT are present almost everywhere on Earth. These internal parts of the geomagnetic field can well be described mathematically by a spherical harmonic expansion . This is the representation of the magnetic field potential as a series of multipoles: n = 1 represents the dipole contribution, n = 2 that from a quadrupole, n = 3 from an octupole and so on (see Sect. 2.3.1 for more details). The degrees thus are a measure of spatial wavelength and the field can be analyzed by plotting the power, the squared magnetic field strength, B2 , averaged over the globe as a function of spherical harmonic degree. Figure 2.1 shows such a spatial spectrum of the recent magnetic field power at the Earth’s surface. The approximate corresponding wavelength can be calculated by dividing the circumference of the Earth (ca. 40,000 km) by the given spherical harmonic degree. Such a spectrum shows two branches with very different slopes. On a logarithmic scale the two parts can reasonably well be approximated by linear fits if the dipole is excluded. The transition from one slope to the other occurs between degrees 13 and 15. Hermann L¨uhr GeoForschungsZentrum Potsdam Telegrafenberg 14473 Potsdam Germany Monika Korte GeoForschungsZentrum Potsdam Telegrafenberg 14473 Potsdam Germany Mioara Mandea GeoForschungsZentrum Potsdam Telegrafenberg 14473 Potsdam Germany
K.-H. Glaßmeier et al. (eds.), Geomagnetic Field Variations, Advances in Geophysical and Environmental Mechanics and Mathematics, c Springer-Verlag Berlin Heidelberg 2009
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Lithosphere
n 2 m 2 Fig. 2.1 Main and lithospheric field spatial spectrum, Rn = (n + 1) ∑nm=0 ((gm n ) + (hn ) ) with the notation of Equation 2.4, at the Earth’s surface. Spatial wavelength in kilometre is obtained by dividing the circumference of the Earth by spherical harmonic degree n
At smaller degrees the spectrum is dominated by the field generated in the core. The dipole (degree 1) is quite outstanding. The power of the degrees 2 to about 13 falls off with a slope that is consistent with a source at about 3,000 km below the surface. This distance coinc
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