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

79 Chapter 3 — Historical fieldmeasurement methods that would produce such a spatially <strong>and</strong> temporally coherentsignal on these long time scales, imply<strong>in</strong>g that this must be a genu<strong>in</strong>e mode of magneticfield evolution at the <strong>core</strong> surface.Other signals identified <strong>in</strong> ˜B r <strong>in</strong>clude a weaker m=7 signal found at low to mid latitudes<strong>in</strong> the northern hemisphere <strong>and</strong> a m=2, 3 signal that is strongest at mid latitudes <strong>in</strong> thesouthern hemisphere. The m=7 signal is observed most clearly near to latitude 30 ◦ N<strong>in</strong> the 20th century though it is <strong>in</strong>itially located at lower latitudes <strong>and</strong> slowly driftsnorthward. It has frequency f=0.004 cpyr (a period of around 250 years) <strong>and</strong> moveswestward at an average speed of around 12 km yr −1 . The m=2, 3 signal is observedmost clearly between 20 ◦ S <strong>and</strong> 40 ◦ S <strong>and</strong> has a frequency of 0.0042 cpyr (a period of 235years) <strong>and</strong> moves rapidly westward at an average speed of 30 km yr −1 . It is strongesttowards the end of the historical record (especially after 1800), <strong>and</strong> at longitudes 150 ◦ E to90 ◦ W. It was further found that although the highest amplitude, spatially <strong>and</strong> temporallycoherent patterns of field evolution were located <strong>in</strong> the Atlantic hemisphere, weakerpatterns with similar form were also present <strong>in</strong> the Pacific hemisphere.It was not possible to detect either dispersive behaviour or any systematic geographictrends <strong>in</strong> azimuthal speeds of field features. These are properties associated with hydromagneticwave propagation mechanisms <strong>in</strong> rapidly rotat<strong>in</strong>g fluids (see chapter 6). Thenegative results obta<strong>in</strong>ed therefore suggest that the azimuthal motion of the wave-likepatterns <strong>in</strong> B r may be more consistent with advection by azimuthal flow than withhydromagnetic wave propagation <strong>in</strong> the <strong>core</strong>. Wave propagation as a mechanism forazimuthal magnetic field motion cannot be ruled out on the basis of the results of thischapter, but there is no strong evidence <strong>in</strong> its favour.