Paleomagnetisch Laboratorium Fort Hoofddijk

Magnetic noise Fort Hoofddijk compared to other Uithof building night Fort Hoofddijk Uithof building

Magnetism: a bit of history

Petrus Peregrinus (1269) Epistola de Magnete Discovered dipole properties of lodestone: first scientific treatise ever written !

Pierre de Maricourt AKA Petrus Peregrinus (pilgrim) 1269 AD: Siege of Lucera (Apulia) by armies of Charles d’Anjou Epistola de Magnete Castle of Lucera

What is a Magnet ? The Earth …. N S

De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure Gilbert (1600) The Earth is a Big Magnet... First known physical property of the Earth (Newton discovered gravity in 1672) William Gilbert (1600):

Carl Friedrich Gauss (1838): Mathematical description of the Earth’s magnetic field The Earth is a big magnet

Gauss (1838): mathematical description of the geomagnetic field

Records (ship logs) used for historical observations of the field Navigation (charts)

Jackson, Jonkers & Walker, 2000 Historical Observations: Navigation

Jackson, Jonkers & Walker, 2000

Earth Magnetic Field: Inclination

Plate Tectonics

Plate Tectonics: Fossils

~305 Ma

Plate Tectonics: Rock types and Mountain belts

Plate Tectonics: Apparent Polar Wander Paths d 1

North America and Eurasia: Atlantic Ocean as today North America rotated to make a single APWP, so a single pole Apparent Polar Wander Path (APWP)

Polarity reversals a dipole reversing at irregular times

Bernard Brunhes (1906): field reversals Motonori Matuyama (1926): Reversely magnetized rocks Some more history...

Sediments (DSDP) & biostratigraphy Lavas & K/Ar dating GPTS: Geomagnetic Polarity Time Scale Alexey Khramov (1962): Magnetostratigraphy

Some more history... Harry Hess (1960): Ocean floor spreading Vine and Matthews (1963): Magnetic anomalies Lawrence Morley (1963): Magnetic anomalies

Geomagnetic time scales

The Earth is a BIG magnet and protects us … a magnetic shield

External Fields: Solar Variation 1‰ of the internal field

Magnetic Storms Beautiful auroras ….

Magnetic Storms … but also damage (powerplants)

Magnetic Storms Magnetic storms have impact: - on satellites & planes - on global telecommunication - on pipelines Natural gas pipeline (Finland) I > 10 Ampere

Magnetic Storms: the biggest August 28 th – September 2 nd Coronal Mass Ejection - Auroras seen in Hawaii, Cuba, Italy - Field reduced by 1600 nT (~4% of present-day field) March 13 th – Coronal Mass Ejection - Auroras seen in Texas - Quebec: in 90 seconds 6 million people were without electricity Ice cores show evidence that events of similar intensity recur at an average rate of approximately once per 500 years. ‘Solar SuperStorm’

Origin and significance of the geomagnetic field

Let’s start simplest: ‘A Bar Magnet’ at the Centre of the Earth Also called a dipole …. Origin of the Field

Poles Geographic vs. Magnetic vs. Geomagnetic Butler, 1992

Origin of the Field

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

Paleomagnetism 19 th century: Natural remanent magnetization (NRM) directions from ancient rocks like a tape recording... ‘rock music’ of the Earth

 D ~65°  I ~20° ~3000 yr Archeomagnetic Observations

Secular variation of the field Borssele, in 1615

Secular variation: declination

Magnetic & geomagnetic poles Northern Hemisphere

Magnetic & geomagnetic poles Southern Hemisphere

NRC

Is the field decaying ? Consequences for the external magnetic field: ‘Space Weather’

Is the Field Decaying ?

Next Class: Fysics of magnetism Background reading for Wednesday! Essentials chapter 1