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.

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?

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?

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

6. Bullard Disk Dynamo (see FocusTerra)

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?

8. Karlsruhe dynamo experiment

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

10. The State of Earth’s Core

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

12. Geodynamo
Physical Processes
Convection

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

14. 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

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)

16. Heat diffusion
The heat diffusion equation:

17. Heat diffusion ‘

18. Heat diffusion

19. Heat diffusion

20. 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

21. 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

22. Magnetic diffusion

23. Magnetic diffusion

24. Magnetic reversals

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

28. The experiment, E≈10-5
Laser
Sheet

29. A rapidly-rotating fluid acts as if it has rigidity along the rotation axis – the Proudman-Taylor theorem

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

32. Geodynamo

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

34. Numerical dynamo models
Glatzmaier & Roberts (1996)

35. Geodynamo

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 B2
Eq 3 - Magnetic field evolution (Faraday & Ampere)
is an equation in which B appears in every term linearly
generically aB=gB+cB

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