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1DTU Space, Technical University of Denmark 30140 Geophysics Part 2: Solid Earth Physics Lecture 8: Physics of Magnetism and Geomagnetism.

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Presentation on theme: "1DTU Space, Technical University of Denmark 30140 Geophysics Part 2: Solid Earth Physics Lecture 8: Physics of Magnetism and Geomagnetism."— Presentation transcript:

1 1DTU Space, Technical University of Denmark Geophysics Part 2: Solid Earth Physics Lecture 8: Physics of Magnetism and Geomagnetism

2 2DTU Space, Technical University of Denmark Topics of the second part of the course # Date TopicChapter in Lowrie Instructor 15/2Earth System Jens Olaf Pepke Pedersen 212/2Earth System Jens Olaf Pepke Pedersen 319/2Earth System Jens Olaf Pepke Pedersen 426/2Earth System Jens Olaf Pepke Pedersen 55/3Solid Earth Overview 1Chris Finlay 612/3Gravity 2Nils Olsen 719/3Gravity 2Nils Olsen 826/3Geomagnetism 5Chris Finlay 92/4Geoelectricity 4Nils Olsen 109/4Geomagnetism 5Chris Finlay 16/4 Easter holidays 1123/4Seismology 3Klaus Mosegaard 1230/4Seismology 3Klaus Mosegaard 137/5Seismology 3Klaus Mosegaard

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

4 4DTU Space, Technical University of Denmark 4.0 A brief history of Geomagnetism Edmund Halley publishes the first geomagnetic field map (right) James Cook documents the Earth’s magnetic field in the Pacific

5 5DTU Space, Technical University of Denmark 4.0 A brief history of Geomagnetism 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 Bernard Brunhes finds evidence of geomagnetic reversals Invention of proton magnetometer allows very accurate intensity determination MAGSAT satellite allows true global coverage of field measurements - Post 1999: Era of continuous satellite observations

6 6DTU Space, Technical University of Denmark 4.1 Physics of Magnetism What is a magnetic field? -Observation: Nearby electric currents exert forces on each other -Force on an element dl 1 carrying current I 1 due to another element dl 2 carrying a current I 2 is: -Focusing on the force on dl 1 and integrating over the elements dl 2 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 7DTU Space, Technical University of Denmark 4.1 Physics of Magnetism Forces on currents and charged particles Biot-Savart LawLorenz force Units of B : N A -1 m -1 = Tesla (T). 1nT= T (Credit: Lowrie, 2007)

8 8DTU Space, Technical University of Denmark 4.1 Physics of Magnetism Magnetic field due to a long wire (Credit: Lowrie, 2007)

9 9DTU Space, Technical University of Denmark 4.1 Physics of Magnetism Elementary sources of magnetic fields Short bar magnetSmall current loopUniformly magnetized sphere (Credit: Lowrie, 2007)

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

11 11DTU Space, Technical University of Denmark 4.1 Physics of Magnetism Magnetized material -Magnetization is the vector sum of all magnetic moments in the material -Each magnetic moment m i is associated with a current loop at an atomic scale -The total magnetic field inside a magnetized material is

12 12DTU Space, Technical University of Denmark 4.1 Physics of Magnetism Magnetized materials (Credit: Lowrie, 2007)

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

14 14DTU Space, Technical University of Denmark 4.1 Physics of Magnetism Types of magnetization (Credit: Lowrie, 2007)

15 15DTU Space, Technical University of Denmark 4.1 Physics of Magnetism Thermo-remanent magnetization (Credit: Lowrie, 2007)

16 16DTU Space, Technical University of Denmark 4.1 Physics of Magnetism Depositional remanent magnetization (Credit: Lowrie, 2007)

17 17DTU Space, Technical University of Denmark 4.2 Geomagnetism Measured Field Components (Credit: Lowrie, 2007)

18 18DTU Space, Technical University of Denmark 4.2 Geomagnetism Magnetic field sources: Internal & External Credit: ESA

19 19DTU Space, Technical University of Denmark 4.2 Geomagnetism External current systems: Magnetosphere

20 20DTU Space, Technical University of Denmark 4.2 Geomagnetism External current systems: Ionosphere

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

22 22DTU Space, Technical University of Denmark 4.2 Geomagnetism Spherical Harmonic Description of Potential n=m m=0 General case: n ≠ m (Credit: Lowrie, 2007)

23 23DTU Space, Technical University of Denmark 4.2 Geomagnetism Spherical harmonic spectra (Credit: Lowrie, 2007)

24 24DTU Space, Technical University of Denmark 4.2 Geomagnetism Simplest model : An axial dipole BrBr F (Units = 10 3 nT) Inclination and field intensity are simply related to the site latitude:

25 25DTU Space, Technical University of Denmark 4.2 Geomagnetism Total Intensity - F from IGRF (2010) IGRF = International Geomagnetic Reference Field, up to SH degree n=13)

26 26DTU Space, Technical University of Denmark 4.2 Geomagnetism Inclination - I from IGRF (2010)

27 27DTU Space, Technical University of Denmark 4.2 Geomagnetism Evolution of Declination - D

28 28DTU Space, Technical University of Denmark 4.2 Geomagnetism Evolution of Declination - D

29 29DTU Space, Technical University of Denmark 4.2 Geomagnetism Evolution of Declination - D

30 30DTU Space, Technical University of Denmark 4.2 Geomagnetism Declination – D from IGRF (2010)

31 31DTU Space, Technical University of Denmark 4.2 Geomagnetism Axial Dipole Decay since 1840

32 32DTU Space, Technical University of Denmark 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.


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