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

3 Chapter 1 — IntroductionA.D. to 1990 A.D. us<strong>in</strong>g all suitable observations from this <strong>in</strong>terval (see appendix A).The geomagnetic field is also recorded <strong>in</strong>directly <strong>in</strong> magnetised rocks (both igneous <strong>and</strong>sedimentary) as they form, <strong>and</strong> <strong>in</strong> pottery <strong>and</strong> ceramics as these are fired (see, forexample, McElh<strong>in</strong>ny <strong>and</strong> McFadden (2000)). Such paleomagnetic <strong>and</strong> archeomagneticobservations allow the behaviour of Earth’s magnetic field to be studied over time scalesof thous<strong>and</strong>s to millions of years. Unfortunately, these sources can usually only provide<strong>in</strong>formation on the gross features of the geomagnetic field, because of the small numbersof records that exist at any particular time <strong>and</strong> the large uncerta<strong>in</strong>ties <strong>in</strong>herent <strong>in</strong> themeasurements. Such observations do however <strong>in</strong>dicate that Earth’s global magnetic fieldhas been present for at least the past 3.5 billion years (Hale, 1987; Layer et al., 1996)<strong>and</strong> that it has been predom<strong>in</strong>antly dipolar for much of that time.1.2.2 Geomagnetic secular variationThe geomagnetic field varies on wide range of time scales. At the highest frequency endof the spectrum are changes over seconds to days caused by the dynamic <strong>in</strong>teractionbetween the geomagnetic field <strong>and</strong> the solar w<strong>in</strong>d far above Earth’s surface. These rapidexternal variations take place about a relatively slowly vary<strong>in</strong>g ma<strong>in</strong> (<strong>in</strong>ternal) field thatis the focus of attention <strong>in</strong> this thesis.On time scales of about a year, abrupt changes <strong>in</strong> the second time derivative of the fieldare observed <strong>in</strong> geomagnetic observatory records. These geomagnetic secular variationimpulses or jerks (Courtillot et al., 1978; Courtillot <strong>and</strong> Le Mouël, 1984) are of globalextent (Alex<strong>and</strong>rescu et al., 1996), <strong>and</strong> have been shown to have <strong>their</strong> orig<strong>in</strong> <strong>in</strong>side theEarth (Mal<strong>in</strong> <strong>and</strong> Hodder, 1982). Geomagnetic jerks are correlated with changes <strong>in</strong> thelength of day (Holme <strong>and</strong> deViron, 2005) <strong>and</strong> can be expla<strong>in</strong>ed as a superposition of rigid,axisymmetric motions of the fluid outer <strong>core</strong> known as torsional oscillations (Bloxhamet al., 2002).On time scales of centuries to millennia, undoubtedly the most obvious change <strong>in</strong> thegeomagnetic field is an east-west (azimuthal) motion of contours of D. This aspect ofgeomagnetic secular variation is commonly referred to as ‘the westward drift’ because theazimuthal motion observed s<strong>in</strong>ce the advent of systematic observations has been <strong>in</strong> thewestward direction. The westward drift was noted by Halley (1692) who felt it providedevidence for motions deep with<strong>in</strong> the Earth. Later this drift was quantified by Bullardet al. (1950) <strong>and</strong> Vest<strong>in</strong>e <strong>and</strong> Kahle (1968) who estimated its speed to be ∼ 0.2 degreeslongitude per year. Bloxham et al. (1989) showed that the westward drift is not globallyuniform, but is <strong>in</strong>stead associated with the motion of particular field concentrationsfound primarily <strong>in</strong> the Atlantic hemisphere. Ward<strong>in</strong>ski (2005) has recently shown thatfrom 1980 A.D. to 2000 A.D. the westward drift had an average value of 0.11 ◦ yr −1 ,be<strong>in</strong>g significantly faster at low latitudes (0.22 ◦ yr −1 ). The westward drift is most easily