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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.
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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?
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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?
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Geodynamo B field Self-excitation: disk-dynamo E field
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Bullard Disk Dynamo (see FocusTerra)
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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?
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Karlsruhe dynamo experiment
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Geodynamo Convection Energy sources Core Magnetic Field Core Fluid
Flow
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The State of Earth’s Core
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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
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Geodynamo Physical Processes Convection
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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
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Geodynamo 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 It grows at a rate of (1280 103)/(4.5 109) ~ 0.3 mm/year Roughly 1 million kilograms of iron are plated onto the inner core every second
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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)
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Heat diffusion The heat diffusion equation:
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Heat diffusion ‘
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Heat diffusion
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Heat diffusion
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Heat diffusion 27 tonne (27,000 kilo) block of ice was air lifted from Siberia How big is the block (ice density~103kg/m3)? How long to melt it? Professor Watkins in a somewhat awkward moment Air lift February 2000
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currents continue to circulate longer in a good conductor
Magnetic diffusion Point out that 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
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Magnetic diffusion
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Magnetic diffusion
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Magnetic reversals
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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
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The experiment, E≈10-5 Laser Sheet
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A rapidly-rotating fluid acts as if it has rigidity along the rotation axis – the Proudman-Taylor theorem
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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
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Geodynamo
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Geodynamo High latitude flux patches are suggestive of the convection pattern in the core
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Numerical dynamo models
Glatzmaier & Roberts (1996)
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Geodynamo
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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 B2 Eq 3 - Magnetic field evolution (Faraday & Ampere) is an equation in which B appears in every term linearly generically aB=gB+cB
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History of Magnetic Field
Merrill, McElhinny & McFadden (1996)
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