1dtu space, technical university of denmark 30140 geophysics part 2: solid earth physics lecture 8:...

1 DTU Space, Technical University of Denmark

30140 GeophysicsPart 2: Solid Earth Physics

Lecture 8: Physics of Magnetism and Geomagnetism


Topics of the second part of the course

# Date Topic Chapter in Lowrie Instructor

1 5/2 Earth System Jens Olaf Pepke Pedersen2 12/2 Earth System Jens Olaf Pepke Pedersen3 19/2 Earth System Jens Olaf Pepke Pedersen4 26/2 Earth System Jens Olaf Pepke Pedersen5 5/3 Solid Earth Overview 1 Chris Finlay6 12/3 Gravity 2 Nils Olsen7 19/3 Gravity 2 Nils Olsen8 26/3 Geomagnetism 5 Chris Finlay9 2/4 Geoelectricity 4 Nils Olsen10 9/4 Geomagnetism 5 Chris Finlay 16/4 Easter holidays

11 23/4 Seismology 3 Klaus Mosegaard12 30/4 Seismology 3 Klaus Mosegaard13 7/5 Seismology 3 Klaus Mosegaard


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


4.0 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


4.0 A brief history of Geomagnetism- 1820 Hans Christian Ørsted shows electric currents

produce magnetic fields

- 1832-1840 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


4.1 Physics of MagnetismWhat 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.


4.1 Physics of MagnetismForces on currents and charged particles

Biot-Savart Law Lorenz force

Units of B : N A-1 m-1 = Tesla (T). 1nT= 10-9 T

(Credit: Lowrie, 2007)


4.1 Physics of MagnetismMagnetic field due to a long wire



4.1 Physics of MagnetismElementary sources of magnetic fields

Short bar magnet Small current loop Uniformly magnetized sphere



4.1 Physics of MagnetismMagnetic potential and field due to a dipole

Consider a pair of magnetic poles:



4.1 Physics of MagnetismMagnetized 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


4.1 Physics of MagnetismMagnetized materials



4.1 Physics of MagnetismTypes of magnetization

k=C/T : Curie

LawParamagnetic

Paramagnetic

above T=θ:

Curie-Weiss Law



4.1 Physics of MagnetismTypes of magnetization



4.1 Physics of MagnetismThermo-remanent magnetization



4.1 Physics of MagnetismDepositional remanent magnetization



4.2 GeomagnetismMeasured Field Components



4.2 GeomagnetismMagnetic field sources: Internal & External

Credit: ESA


4.2 Geomagnetism External current systems: Magnetosphere


4.2 Geomagnetism External current systems: Ionosphere


4.2 GeomagnetismExample: 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)



4.2 GeomagnetismSpherical Harmonic Description of Potential

n=m

m=0General case:

n ≠ m



4.2 GeomagnetismSpherical harmonic spectra



4.2 GeomagnetismSimplest model : An axial dipole

Br F

(Units = 103 nT)

Inclination and field intensity are simply

related to the site latitude:


4.2 GeomagnetismTotal Intensity - F from IGRF (2010)

IGRF = International Geomagnetic Reference Field, up to SH degree n=13)


4.2 GeomagnetismInclination - I from IGRF (2010)


4.2 GeomagnetismEvolution of Declination - D


4.2 GeomagnetismDeclination – D from IGRF (2010)


4.2 GeomagnetismAxial Dipole Decay since 1840


Next week: Geoelectricity

• Instructor: Prof. Nils Olsen

• Preparation: Read Chapter 4.3 [p.252-276], Lowrie (Geoelectricity)

• For 09.04.14, do Exercises 1-4 and 7-9 in Lowrie Chapter 5.10, pp360. We shall discuss in class.

1dtu space, technical university of denmark 30140 geophysics part 2: solid earth physics lecture 8:...

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