Dynamic Earth - Lecture 17: Earth's Magnetic Field

Lodestone compass

• discovered in China ~1000 AD

• Refers to a mineral that is a natural magnet (ex. magnetite)

• used for geomancy long before it was used for navigation

William Gilbert

• Personal physician to Queen Elizabeth I

• Published De Magnete in 1600, a 6-volume treatise documenting his experiments with magnetism

• Proposed that Earth behaves like a giant bar magnet with the magnetic axis approximately North-South

Who confirmed the idea of an internally generated field?

C.F. Gauss

Magnetic dipole

• a bar magnet generates a magnetic dipole

• produced by a pair of magnetic poles of equal strength, but opposite sign, a small distance apart

  

How many dimensions in Earth’s magnetic field?

3-dimensional

Three possible measurements of Earth’s magnetic field

1. Inclination = angle measured from horizontal (angle at which it points into Earth) - also called “dip angle”

2. Declination = angle measured from geographic north

3. Intensity = magnetic field strength

How is the field generated?

CANNOT actually be generated by a permanent bar magnet at its center because high temps destroy magnetism

• When ferromagnetic materials exceed a certain temperature (known as their Curie temperature) they lose their permanent magnetism

Maxwell’s equations showed that… (2)

1. A changing magnetic field creates an electric field

2. A changing electric field creates a magnetic field

What generates a dipole magnetic field?

A current loop

• an electrical current flowing in a loop

Dynamo

A machine for converting mechanical energy to electrical current flow (and magnetic field)

• dynamo action in earth’s core generates geomagnetic field

• requires movement of electrically conductive liquid Fe in the core

What causes the motion of liquid Fe in the outer core?

1. Thermal convection in the outer core

2. compositional convection in the outer core

1. Thermal convection in the outer core

• Caused by heat released from the inner core to the outer core

• Some heat is released due to freezing of the inner core

• Some radioactive heating may also contribute

2. Compositional convection in the outer core

• Inner core is made of solid Fe and is growing due to freezing of the liquid outer core (as Earth cools over time)

• Pure Fe is freezing out and grows the inner core while the lighter elements remain in the outer core

• Due to their low density, light elements rise in the outer core creating convection cells

  

What is the dominant cause of convection today?

compositional convection

Is the geomagnetic field dynamic or static?

Dynamic - unlike bar magnet

What is the source of the field in the Earth’s outer core?

Motion of molten metal generates field in a process called geodynamo

3 parts of Earth’s total magnetic field

1. Main (internally generated) Magnetic Field - 99.2%

2. External Magnetic Field - 0.6%

3. Induced Magnetic Field - 0.2%

Solar wind

Charged particles ejected from the sun

Magnetosphere

A zone where charged particles are affected by Earth’s magnetic field

Magnetotail

Portion of magnetosphere that is pushed away from the sun by the solar wind