De Verborgen Attractie van de Aarde

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Origin and significance of the geomagnetic field
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Origin and significance of the geomagnetic field

• Cor Langereis
• Paleomagnetic Laboratory
• Fort Hoofddijk
• Department of Earth Sciences
• Utrecht University

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Origin and significance of the geomagnetic field

• The basics
• Magnetism the basics a bit of history
• Earths magnetic field an introduction
• Geomagnetism present historical field
• Paleomagnetism the ancient field
• Intensity of the field through time

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Magnetism the very basics

• - Magnetic field H
• - Magnetic moment m
• - Magnetic induction B
• - Magnetic susceptibility ?
• - Magnetisation M
• - Magnetic dipoles
• - Magnetic permeability m

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Magnetic field H
H i / l i / 2? r A/m
Tauxe, 2005
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Magnetic moment m (dipole)
m i O i ? r2 Am2
Tauxe, 2005
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Magnetic induction B
B m H Tesla
Tauxe, 2005
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Magnetostatic energy
Tauxe, 2005
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Magnetization
Remanent magnetization Mr
Induced magnetization Mi c H
Total magnetization in matter (e.g. rocks)
Mtot Mi Mr
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Magnetism a bit of history
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Magnetic declination in China
Compass via Arabs to Europe
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Petrus Peregrinus (1269)Epistola de Magnete
Discovered dipole properties of lodestone first
scientific treatise ever written !
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Petrus Peregrinus Medal of the MPRG Division
Pierre de Maricourt AKA Petrus Peregrinus
(pilgrim) 1269 AD Siege of Lucera (Apulia) by
armies of Charles dAnjou Epistola de Magnete
Castle of Lucera
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Records (ship logs) used for historical
observationsof the field
Navigation (charts)
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The Earth is a big magnet ...

• De Magnete, Magneticisque
• Corporibus, et de Magno
• Magnete Tellure
• Gilbert (1600)

First known physical property of the Earth
(Newton discovered gravity in 1672)
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19th centuryNatural remanent magnetization
(NRM) directions from ancient rockslike a tape
recording ...
Paleomagnetism
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Gauss (1838) mathematical description of the
geomagnetic field
Course Geopotential Fields
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Recent history ...

• Bernard Brunhes (1906)
• field reversals

• Joseph Larmor (1919)
• dynamo process
• 19-fifties
• Proof of continental drift
• 19-sixties
• Birth of plate tectonics

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External FieldMagnetic Storms Space Weather
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Earths external magnetic field
The external magnetic field Space Weather
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External Fields Solar Variation
1 of the internal field
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Main Elements of the Field
Tauxe, 2005
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Earths Dipole Field
Simple bar magnet (dipole)
Not so simple dynamo process in fluid outer core
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Dipole ... and non-dipole field
Tauxe, 2005
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Poles

• Geographic
• vs.
• Magnetic
• vs.
• Geomagnetic

Butler, 1992
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Magnetic geomagneticpolesNorthern Hemisphere
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Magnetic geomagnetic poles Southern Hemisphere
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Secular variation of the field
Borssele, in 1615
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Secular variation declination
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Secular variationinclination
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Geomagnetism present historical field
DI 20
DD 65
3000 yr
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Secular Variation observatories
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Secular Variation observatories
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Secular variation aeromagnetics
Sabaka et al., 2004
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Secular variation satellites
Hulot et al.,, 2002
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IGRF International Geomagnetic Reference Field
Tauxe, 2005
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Magnetic directional data
Tauxe, 2005
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Virtual Geomagnetic Poles (VGP)
Tauxe, 2005
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Virtual Geomagnetic Poles (VGP)
Tauxe, 2005
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Secular Variation

• Averaged over time
• Geomagnetic Pole
• coincides with
• Rotation Axis
• Geocentric
• Axial
• Dipole
• Hypothesis (GAD)

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Jackson, Jonkers Walker, 2000
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Jackson, Jonkers Walker, 2000
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Is the field decaying ?
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Is the field decaying ?
Consequences for the external magnetic field
Space Weather
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Paleomagnetism the ancient field

• Present historical field excellent - good
• Archeomagnetic field (0-7 kyr) reasonable
• Last 5 Myr well OK
• Back in time gets worse and worse
• Archean field still mostly a mystery
• Data quantity quality decrease exponentially
  with time,
• but it is not hopeless ( said the russian
  optimist)

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Archeomagnetism

• German secular variation
• master curve
• (800 BC to Present)

Schnepp Lanos, 2005
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Archeomagnetism
Korte et al., 2005a,b
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Archeomagnetism
Korte et al., 2005
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Secular Variation

• Averaged over time
• Geomagnetic Pole
• coincides with
• Rotation Axis
• Geocentric
• Axial
• Dipole
• hypothesis (GAD)

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Paleomagnetic Pole Average VGP

• Di,Ii
• VGPi
• VGPave, a95

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Excursions of the field
Excursions are associated with low dipole
intensity
Langereis et al., 1997
Guyodo Valet, 1999
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Reversals of the field
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Reversal time scale a barcode

• Seconds - years
• external fields
• 300-500 yr
• outer core convection
• 3000-5000 yr
• inner core diffusion
• gt 10 kyr
• CMB mantle processes

• 100 yr 10 kyr
• secular variation
• excursions
• reversals
• 10 kyr 1 Myr
• polarity (sub)chrons
• 10-100 Myr
• superchrons

Butler, 1992
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Marine magnetic anomalies
World crustal age
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Intensity of the field through time

• Like paleomagnetic directions, intensity data
  quantity quality decrease exponentially with
  time

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Paleointensity different techniques
Valet, 2003

• All techniques are time-consuming and have a low
  success rate (15)

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Relation with cosmogenic isotopes
Valet, 2003

• Cosmogenic isotopes (e.g. 14C,10Be,36Cl) are
  formed by cosmic radiation and solar wind, and
  have a short half-life.
• They are an independent measure of geomagnetic
  field strength

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Scarce data before 300 Ma
Biggin et al., 2008
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Intensity of the field through time
From lifeless to living
Tarduno et al., 2006

• What was the field like during the period from
  lifeless to living ?
• What is the influence of the growth of the solid
  inner core ?

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Origin of the geomagnetic field

• Geodynamo powered by chemical convection in the
  fluid outer core.
• The role of the inner core (and its growth) is
  still subject to debate

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Assignment 1 geomagnetic field

• Handout
• Lisa Tauxe course notes (link)
• Programs (website)