1. 30140 Geophysics Part 2: Solid Earth Physics
Lecture 8: Physics of Magnetism and Geomagnetism

2. Topics of the second part of the course #
 Date
 Topic
Chapter in Lowrie
 Instructor 1
5/2
Earth System
Jens Olaf Pepke Pedersen 2
12/2 3
19/2 4
26/2 5
5/3
Solid Earth Overview  1
Chris Finlay 6
12/3
Gravity  2
Nils Olsen 7
19/3 8
26/3
Geomagnetism  5 9
2/4
Geoelectricity  4 10
9/4
16/4 Easter holidays 11
23/4
Seismology  3
Klaus Mosegaard 12
30/4 13
7/5

3. 4.0 A brief history of Geomagnetism
100 A.D. Magnetic compass invented
by the Chinese
1180 Compass described by Guyot de Provins
and Alexander Neckam in Europe
1600 William Gilbert's De Magnete (right):
`Earth itself is a great magnet’
1634 Henry Gellibrand realizes that
the geomagnetic field changes in time

4. 4.0 A brief history of Geomagnetism
1701 Edmund Halley publishes the
first geomagnetic field map (right)
1770 James Cook documents the
Earth’s magnetic field in the Pacific

5. 4.0 A brief history of Geomagnetism
1820 Hans Christian Ørsted shows electric currents
produce magnetic fields
Carl Friedrich Gauss (right) measures
absolute magnetic intensity and makes first
global mathematic model of geomagnetic field
1906 Bernard Brunhes finds evidence of
geomagnetic reversals
1955 Invention of proton magnetometer allows
very accurate intensity determination
1980 MAGSAT satellite allows true global coverage of field measurements
- Post 1999: Era of continuous satellite observations

6. 4.1 Physics of Magnetism What is a magnetic field?
Observation: Nearby electric currents exert forces on each other
Force on an element dl1 carrying current I1 due to another element dl2 carrying a current I2 is:
Focusing on the force on dl1 and integrating over the elements dl2 we can write this as
where we have defined the magnetic flux density (or magnetic field) to be
B describes the force exerted on an
electrical current (moving charged particle)
by another electrical current.

7. 4.1 Physics of Magnetism Forces on currents and charged particles
(Credit: Lowrie, 2007)
Biot-Savart Law
Lorenz force
Units of B : N A-1 m-1 = Tesla (T) nT= 10-9 T

8. 4.1 Physics of Magnetism Magnetic field due to a long wire
(Credit: Lowrie, 2007)

9. 4.1 Physics of Magnetism Elementary sources of magnetic fields
(Credit: Lowrie, 2007)
Short bar magnet
Small current loop
Uniformly magnetized sphere

10. 4.1 Physics of Magnetism Magnetic potential and field due to a dipole
Consider a pair of magnetic poles:
(Credit: Lowrie, 2007)

11. 4.1 Physics of Magnetism Magnetized material
Magnetization is the vector sum of all
magnetic moments in the material
Each magnetic moment mi is associated with a current loop at an atomic scale
The total magnetic field inside a magnetized material is

12. 4.1 Physics of Magnetism Magnetized materials
(Credit: Lowrie, 2007)

13. 4.1 Physics of Magnetism Types of magnetization
(Credit: Lowrie, 2007)
k=C/T : Curie
Law
Paramagnetic
Paramagnetic
above T=θ:
Curie-Weiss Law

14. 4.1 Physics of Magnetism Types of magnetization
(Credit: Lowrie, 2007)

15. 4.1 Physics of Magnetism Thermo-remanent magnetization
(Credit: Lowrie, 2007)

16. 4.1 Physics of Magnetism Depositional remanent magnetization
(Credit: Lowrie, 2007)

17. 4.2 Geomagnetism Measured Field Components
(Credit: Lowrie, 2007)

18. 4.2 Geomagnetism Magnetic field sources: Internal & External
Credit: ESA

19. 4.2 Geomagnetism External current systems: Magnetosphere

20. 4.2 Geomagnetism External current systems: Ionosphere

21. 4.2 Geomagnetism Example: Measured External Field Variations
- Regular daily variations due to solar quiet (Sq) current driven by solar heating of the ionosphere
(Credit: Lowrie, 2007)
- Irregular variations (e.g. geomagnetic storms) due to enhanced solar activity modulating magnetospheric currents (e.g. ring current amplitude and location)

22. 4.2 Geomagnetism Spherical Harmonic Description of Potential
(Credit: Lowrie, 2007)
n=m
General case:
n ≠ m
m=0

23. 4.2 Geomagnetism Spherical harmonic spectra
(Credit: Lowrie, 2007)

24. 4.2 Geomagnetism Simplest model : An axial dipole
Inclination and field intensity are simply
related to the site latitude: Br F
(Units = 103 nT)

25. 4.2 Geomagnetism Total Intensity - F from IGRF (2010)
IGRF = International Geomagnetic Reference Field, up to SH degree n=13)

26. 4.2 Geomagnetism Inclination - I from IGRF (2010)

27. 4.2 Geomagnetism Evolution of Declination - D

28. 4.2 Geomagnetism Evolution of Declination - D

29. 4.2 Geomagnetism Evolution of Declination - D

30. 4.2 Geomagnetism Declination – D from IGRF (2010)

31. 4.2 Geomagnetism Axial Dipole Decay since 1840

32. Next week: Geoelectricity
Instructor: Prof. Nils Olsen
Preparation: Read Chapter 4.3 [p ], Lowrie (Geoelectricity)
For , do Exercises 1-4 and 7-9 in Lowrie Chapter 5.10, pp360. We shall discuss in class.