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

115 Chapter 5 — Dynamo model outputshould however be noted that this example may not be Earth-like because its mean zonalflows are rather weak s<strong>in</strong>ce convection is only marg<strong>in</strong>ally supercritical <strong>and</strong> because it hasno-slip boundary conditions at a relatively large Ekman number so viscous suppressionof zonal flows will be significant. It should f<strong>in</strong>ally be noted that (i) the magnitude ofu φ with<strong>in</strong> the wave flow is considerable larger than the azimuthal speed at which thepattern is mov<strong>in</strong>g <strong>and</strong> (ii) the magnetic field concentrations mov<strong>in</strong>g with the wave do notpulsate <strong>in</strong> amplitude. This is consistent with the predictions of the k<strong>in</strong>ematic analysisof the effects of wave flows on a magnetic field presented <strong>in</strong> chapter 8.5.2.3 Discussion of results from DYN1Compar<strong>in</strong>g the evolution of B r at the outer boundary <strong>in</strong> DYN1 to that observed at the<strong>core</strong> surface <strong>in</strong> gufm1, the most obvious similarity is the presence of concentrations ofB r at low latitudes that drift westward. There are however several rather importantdifferences. First, there is only a s<strong>in</strong>gle pair of drift<strong>in</strong>g B r concentrations <strong>in</strong> DYN1,while a coherent pattern of concentrations is observed <strong>in</strong> gufm1. Secondly, <strong>in</strong> DYN1the perturbation <strong>in</strong> B r is E A while that observed <strong>in</strong> gufm1 is E S . Thirdly, the B rconcentrations <strong>in</strong> DYN1 are reversed flux patches, while those seen <strong>in</strong> gufm1 are thesame polarity as the dom<strong>in</strong>ant (dipolar) field <strong>in</strong> that hemisphere.None of these differences appears to be <strong>in</strong>surmountable. Consider<strong>in</strong>g the first problem,there are (at least weak) <strong>in</strong>dications of concentrations <strong>in</strong> ˜B r associated with the pathsof all downwell<strong>in</strong>gs observed <strong>in</strong> figure 5.6b. Consider<strong>in</strong>g the second problem, if an E Sflow of the type suggested by DYN1 is the orig<strong>in</strong> of azimuthally mov<strong>in</strong>g concentrations<strong>in</strong> B r <strong>and</strong> it acts on an E S background magnetic field at low latitudes (rather thanthe E A fields present <strong>in</strong> DYN1) then E S perturbations <strong>in</strong> B r could be produced asseen <strong>in</strong> geomagnetic field models. Regard<strong>in</strong>g the third problem, s<strong>in</strong>ce the mechanism offormation of magnetic field concentrations seems to <strong>in</strong>volve focus<strong>in</strong>g of magnetic fieldl<strong>in</strong>es by convergence at downwell<strong>in</strong>gs, the reversed polarity is likely a consequence of thepolarity of the <strong>in</strong>itial B r (see animation A5.1). If the wave flow acted on normal <strong>in</strong>itialB r then the result<strong>in</strong>g field concentrations would be of normal polarity (as is observed <strong>in</strong>DYN2) rather than the observed reversed polarity, so this difference does not appear tobe crucial.Regard<strong>in</strong>g the hemispherical conf<strong>in</strong>ement of field evolution patterns, it is <strong>in</strong>terest<strong>in</strong>g tonote that the azimuthally mov<strong>in</strong>g field features found <strong>in</strong> DYN1 were most prom<strong>in</strong>ent<strong>in</strong> one hemisphere than the other, even though the underly<strong>in</strong>g wave flow was global.This appears to have been because the background field was more suitable for creat<strong>in</strong>gfield concentrations <strong>in</strong> one hemisphere. The breakdown of the azimuthally mov<strong>in</strong>g fieldconcentrations appears to have been the result of nonl<strong>in</strong>ear <strong>in</strong>teractions between thewave flow <strong>and</strong> other flow structures, as well as due to the effects of magnetic diffusion.