Presentation on theme: "30140 Geophysics Part 2: Solid Earth Physics"— Presentation transcript:
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 TopicChapter in Lowrie Instructor15/2Earth SystemJens Olaf Pepke Pedersen212/2319/2426/255/3Solid Earth Overview 1Chris Finlay612/3Gravity 2Nils Olsen719/3826/3Geomagnetism 592/4Geoelectricity 4109/416/4 Easter holidays1123/4Seismology 3Klaus Mosegaard1230/4137/5
3 4.0 A brief history of Geomagnetism 100 A.D. Magnetic compass inventedby the Chinese1180 Compass described by Guyot de Provinsand Alexander Neckam in Europe1600 William Gilbert's De Magnete (right):`Earth itself is a great magnet’1634 Henry Gellibrand realizes thatthe geomagnetic field changes in time
4 4.0 A brief history of Geomagnetism 1701 Edmund Halley publishes thefirst geomagnetic field map (right)1770 James Cook documents theEarth’s magnetic field in the Pacific
5 4.0 A brief history of Geomagnetism 1820 Hans Christian Ørsted shows electric currentsproduce magnetic fieldsCarl Friedrich Gauss (right) measuresabsolute magnetic intensity and makes firstglobal mathematic model of geomagnetic field1906 Bernard Brunhes finds evidence ofgeomagnetic reversals1955 Invention of proton magnetometer allowsvery accurate intensity determination1980 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 otherForce 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 aswhere we have defined the magnetic flux density (or magnetic field) to beB describes the force exerted on anelectrical 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 LawLorenz forceUnits 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 magnetSmall current loopUniformly 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 allmagnetic moments in the materialEach magnetic moment mi is associated with a current loop at an atomic scaleThe 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 : CurieLawParamagneticParamagneticabove 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)
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)
32 Next week: Geoelectricity Instructor: Prof. Nils OlsenPreparation: 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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