Earliest Swiss magnetic observation: degrees East by ZWINGER

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1 Earliest Swiss magnetic observation: degrees East by ZWINGER Likely THEODORE ZWINGER at BASEL Theodor Zwinger III. (* 26. August 1658 in Basel; 22. April 1724 ebenda) war ein Schweizer Mediziner und Hochschullehrer. 67

2 Geodynamo Some questions Why is the north pole near the geographical pole? Why does the field reverse on occasions? Why is the observed time between reversals variable? What is the energy source of the field? 68

3 Geodynamo How is Earth s magnetic field generated? 1906 Oldham discovers Earth s core 1919 Sir Joseph Larmor - self-excited dynamo Currents in core generate the field - but what generates the currents? 69

4 Geodynamo B field Self-excitation: disk-dynamo E field 70

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6 Bullard Disk Dynamo (see FocusTerra) 72

7 Geodynamo A mechanical device such as the disc dynamo is very different from a spherical body such as the core The process has been shown to work in the laboratory - pumped sodium (Natrium) Can the same process work in the core? 73

8 Karlsruhe dynamo experiment 74

9 Geodynamo Convection Energy sources Core Magnetic Field Core Fluid Flow 75

10 The State of Earth s Core 76

11 Geodynamo Convection Convection is an instability in a fluid that develops as a more efficient way of transporting heat than conduction In a pan of water heated from below, convection sets in very quickly (with a modest temperature difference) The material motion carries with it heat, from the bottom of the pan to the top Convection in rapidly rotating systems is very different from in non-rotating systems 77

12 Convection Geodynamo Physical Processes 78

13 Geodynamo Energy sources Heat associated with cooling of the Earth Freezing of inner core - liberates latent heat Freezing of inner core - releases lighter elements (lower density) Radioactivity? E.g Potassium 79

14 The crystallisation of the inner core Let us assume that the inner core has been growing over the entire age of the earth at a constant rate Age ~ years Radius 1280 km Geodynamo It grows at a rate of ( )/( ) ~ 0.3 mm/year Roughly 1 million kilograms of iron are plated onto the inner core every second 80

15 Heat diffusion Decay time scaling The heat diffusion equation: If it takes one day to defrost a frozen chicken, how long does it take to defrost a Woolly Mammoth? (Extinct at end of ice age but preserved in glaciers ) 81

16 Heat diffusion The heat diffusion equation: 82

17 Heat diffusion 83

18 Heat diffusion 84

19 Heat diffusion 85

20 Heat diffusion 27 tonne (27,000 kilo) block of ice was air lifted from Siberia How big is the block (ice density~10 3 kg/m 3 )? How long to melt it? Air lift February 2000 Professor Watkins in a somewhat awkward moment 86

21 Magnetic diffusion As the electrical conductivity increases, so the diffusivity decreases and the decay time becomes longer and longer; this is because the electrical currents continue to circulate longer in a good conductor 87

22 Magnetic diffusion 88

23 Magnetic diffusion 89

24 Magnetic reversals Figure 2 The duration of the directional changes during polarity transitions varies as a function of site latitude. The estimates obtained using the CSD method are taken as the best estimate. However, because this method may in some cases (particularly in lowsedimentation-rate records) overestimate the durations, the estimates obtained using the VGP ^ 458 cut-off method are also shown as lower limits. Results for the Brunhes Matuyama reversal are indicated by solid squares, the Upper Jaramillo by open circles, the Lower Jaramillo by open squares and the Upper Olduvai by solid circles. Figure 1 Comparison of the transition-zone thickness in the lower Jaramillo record from ODP Hole 758A (ref. 27) determined using the three methods described in the text. The estimated thickness of the transition zone varies almost by a factor of two, depending on how the transition zone is defined. 90

25 Magnetic field at the core surface The field at the core surface is different in morphology to the field at the Earth s surface We obtain the field at the core surface by a process known as downward continuation We see concentrations of flux at high latitudes, and surprisingly little flux at the poles Using historical measurements of the field, we can see how the field has evolved in time over the last 400 years 91

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28 Laser Sheet The experiment, E 10-5

29 A rapidlyrota<ng fluid acts as if it has rigidity along the rota<on axis the Proudman- Taylor theorem

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31 Geodynamo Convection Because of the spherical geometry, heat needs to be transported from the centre of the core to the surrounding mantle The rotation provides a preferred axis which leads to a very special form of convection 97

32 Geodynamo 98

33 Geodynamo High latitude flux patches are suggestive of the convection pattern in the core 99

34 Numerical dynamo models Glatzmaier & Roberts (1996) 100

35 Geodynamo 101

36 Geodynamo Reversals Still very little is understood about reversals Argument from the underlying equations Eq 1 - Heat transport (involves velocity and temperature) - doesn t involve the magnetic field Eq 2 - Newton s law governing momentum transfer (called the Navier-Stokes equation in fluid dynamics) - involves the magnetic field in a term that varies like B 2 Eq 3 - Magnetic field evolution (Faraday & Ampere) - is an equation in which B appears in every term linearly - generically ab=gb+cb 102

37 History of Magnetic Field Merrill, McElhinny & McFadden (1996) 103