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

8 Chapter 1 — IntroductionPoirier, 1994).1.3.3 Energy sources <strong>and</strong> <strong>core</strong> motionsThe liquid iron <strong>in</strong> Earth’s <strong>core</strong> is thought to be undergo<strong>in</strong>g vigorous motion. On shorttime scales of hours to days disturbances driven by <strong>core</strong>-mantle coupl<strong>in</strong>g <strong>and</strong> tidal forc<strong>in</strong>gwill produce a broad spectrum of <strong>in</strong>ternal gravity <strong>and</strong> <strong>in</strong>ertial <strong>waves</strong> (Aldridge, 2003). Ithas been suggested that precessional <strong>and</strong> tidal stra<strong>in</strong>s could produce <strong>in</strong>stabilities lead<strong>in</strong>gto large amplitude flows <strong>in</strong> the <strong>core</strong> (Malkus, 1994). On longer time scales the presenceof a variety of energy sources suggests that convection is occurr<strong>in</strong>g <strong>in</strong> the outer <strong>core</strong>.These energy sources <strong>in</strong>clude secular cool<strong>in</strong>g of the <strong>core</strong> <strong>in</strong> the aftermath of planetaryformation, release of latent heat <strong>and</strong> buoyant light material at the <strong>in</strong>ner-<strong>core</strong> boundaryas the <strong>in</strong>ner <strong>core</strong> solidifies, <strong>and</strong> radioactive heat<strong>in</strong>g (see, for example, Gubb<strong>in</strong>s et al.(2003; 2004); Roberts et al. (2003)).Due to the very low viscosity of liquid iron <strong>in</strong> the <strong>core</strong>, vigorous convective motionsare expected to be dom<strong>in</strong>ated by the <strong>in</strong>fluence of the Coriolis forces (a consequence ofplanetary rotation) <strong>and</strong> the strong magnetic fields present there. Nonl<strong>in</strong>ear <strong>in</strong>teractionsbetween flow, magnetic <strong>and</strong> buoyancy fields will produce complex, time-dependent flowpatterns. The presence of magnetic fields <strong>in</strong> an electrically conduct<strong>in</strong>g fluid also permitsa class of fluid motions known as hydromagnetic <strong>waves</strong>. These arise because the distortionof magnetic fields by fluid motion produces a magnetic (Lorentz) force oppos<strong>in</strong>gthat distortion. In comb<strong>in</strong>ation with fluid <strong>in</strong>ertia this magnetic restor<strong>in</strong>g force leads totransverse wave motions (Alfvén, 1942). The form of these hydromagnetic <strong>waves</strong> will bestrongly <strong>in</strong>fluenced by rapid rotation (Lehnert, 1954), unstable stratification (Brag<strong>in</strong>sky,1964), <strong>and</strong> spherical geometry (Hide, 1966) all of which are thought to be important <strong>in</strong>Earth’s outer <strong>core</strong>.1.3.4 The geodynamo mechanismThe generation of the geomagnetic field <strong>in</strong> Earth’s <strong>core</strong> is now fairly well understoodthanks <strong>in</strong> particular to recent advances <strong>in</strong> computational modell<strong>in</strong>g (see, for example,Roberts <strong>and</strong> Glatzmaier (2000a)). The high temperatures <strong>in</strong> the <strong>core</strong> <strong>and</strong> the timedependenceof the observed field rule out an explanation <strong>in</strong> terms of permanent magnetism.Instead, a mechanism produc<strong>in</strong>g a rapidly evolv<strong>in</strong>g, self-susta<strong>in</strong><strong>in</strong>g, magneticfield is required. A dynamo explanation is the only reasonable c<strong>and</strong>idate for such amechanism (see, for example, Gubb<strong>in</strong>s <strong>and</strong> Masters (1979)). A dynamo is a system thatconverts k<strong>in</strong>etic energy <strong>in</strong>to magnetic energy by motional <strong>in</strong>duction given some <strong>in</strong>itialseed magnetic field (Roberts, 1994). Motions of electrically conduct<strong>in</strong>g fluid <strong>in</strong> Earth’s<strong>core</strong> mov<strong>in</strong>g through a weak pre-exist<strong>in</strong>g magnetic field (e.g. the <strong>in</strong>ter-planetary magneticfield) will generate electrical currents. These electrical currents produce magnetic