Hydromagnetic waves in Earth's core and their influence on ...

6 Chapter 1 — IntroductionThe orig<strong>in</strong> of the geomagnetic field <strong>in</strong> Earth’s liquid <strong>core</strong> has been confirmed by theelegant study of Hide <strong>and</strong> Mal<strong>in</strong> (1981). They were able to estimate the radius of the<strong>in</strong>ternal source region us<strong>in</strong>g geomagnetic secular variation observations. Their resultagreed very well with the seismologically determ<strong>in</strong>ed outer <strong>core</strong> radius. This studyalso suggested that Earth’s mantle must be a rather good electrical <strong>in</strong>sulator, provid<strong>in</strong>gjustification for consider<strong>in</strong>g Earth’s magnetic field <strong>in</strong> the mantle as a potential field.Further evidence suggest<strong>in</strong>g that the mantle is a good electrical <strong>in</strong>sulator comes highpressure m<strong>in</strong>eral physics experiments which imply that the electrical conductivity islikely to be less than 10 Sm −1 (Shankl<strong>and</strong> et al., 1993) above the m<strong>in</strong>eralogically dist<strong>in</strong>ctD ′′ layer present at the base of the mantle.Adopt<strong>in</strong>g this assumption that the mantle is to first order an electrical <strong>in</strong>sulator, it ispossible (with suitable regularisation; for a discussion of this concept see Parker (1994))to downward cont<strong>in</strong>ue the geomagnetic field to the edge of its generation region at the<strong>core</strong> surface. Figure 1.4 shows a contour map of the radial magnetic field B r at the<strong>core</strong> surface <strong>in</strong> 2000 A.D. obta<strong>in</strong>ed by downward cont<strong>in</strong>u<strong>in</strong>g a model constra<strong>in</strong>ed byobservations made by the Ørsted satellite. Red colours represent regions where radialmagnetic field exits the <strong>core</strong>, blue colours represent regions where radial magnetic fieldenters the <strong>core</strong>; the <strong>in</strong>tensity of the colour <strong>in</strong>dicates the concentration of field l<strong>in</strong>es.9008007006005004003002001000-100-200-300-400-500-600-700-800-900µΤ2000Figure 1.4: Radial magnetic field B r at the <strong>core</strong> surface <strong>in</strong> 2000 A.D.Us<strong>in</strong>g data from the Ørsted satellite, Jackson (2003) constructed this high resolutionimage of B r at the <strong>core</strong> surface us<strong>in</strong>g a maximum entropy norm as regularisation when<strong>in</strong>vert<strong>in</strong>g to f<strong>in</strong>d a geomagnetic field model at the <strong>core</strong> surface. The map projection isAitoff equal-area projection <strong>and</strong> the scale is <strong>in</strong> µT.The magnetic field at the <strong>core</strong> surface is considerably more complicated than that atEarth’s surface, with more small scale structure visible <strong>and</strong> the field amplitude vary<strong>in</strong>gbetween ±1×10 6 nT compared to ± 55000nT at Earth’s surface. At high latitudes <strong>in</strong> bothhemispheres (under Canada, Siberia <strong>and</strong> under the antipodal sites close to Antarctica)high amplitude field concentrations are observed. Close to the geographical equator aseries of high amplitude positive B r features are observed stretch<strong>in</strong>g from under the