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L13.1 : The Earth As a Magnet
L13 Earth’s Magnetic Field
The Earth is a huge magnet
Lecture outline:
The Earth as a magnet
The field is approximately a
magnetic dipole (like a bar
magnet)
Strength of the Earth’s Magnetic Field
Origin of the Earth’s Magnetic Field
Solar Wind and Modification of Field
The south pole of the dipole
is located in the northern
hemisphere, and vice-versa
Radiation Belts
Aurora
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L13.2 : The Earth as a Magnet
Magnetic dipole moment
points down (northern to
Southern hemisphere),
magnitude
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L13.3 : Strength of Earth’s
Magnetic Field
Sydney NSW : Latitude 33:55:00 S 151:10:00 E
Magnetic Declination 12.5O Inclination –64.5O
Dipole axis is inclined at an
angle
to rotation
axis
Geomagnetic north pole is (currently) in NW
Greenland. This varies significantly over time.
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L13.4 : Origin of Earth’s
Magnetic Field
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L13.5 : Fluid Dynamo
There is no detailed
explanation of the origin
of the field.
Faraday’s fluid dynamo
experiment from Waterloo
Bridge in London (1831).
Likely to be caused by
current loops in the
molten outer core (a
“geomagnetic dynamo”)
The experiment failed (effect
too small) but the principle
was sound:
Wires
An electrically conducting fluid moving through a
magnetic field will generate a current in a closed
electric circuit (which has some parts moving with
respect to the fluid and some not).
Field reverses direction
irregularly - on average
every million years.
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L13.6 : Studying the History
of the Earth’s Magnetic Field
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L13.7 : The Solar Wind
Plasma of material
ejected from Sun
through coronal
discharge.
Solidified magma on
e.g. the sea-bed carries
information on the
magnetic field from
past times “frozen in”
at the time it solidified.
Composition reflects that of
Sun – mostly protons and
electrons, about 5% He ions,
some heavier nuclei.
There is evidence that
the field has flipped
some 170 times in the
past 70 million years.
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Modifies the Earth’s
magnetic field in a rather
complicated way.
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L13.8 : Van Allen Belts
L13.9 : Magnetic Bottle
Inner belt includes protons
of energy 10 – 100 MeV
resulting from cosmic ray
collisions with atmosphere.
The trapping of particles is a natural example of the magnetic
bottle principle.
Outer belt contains mostly
particles originating from
the Sun, energies closer
to 1 MeV
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End regions of constricted field lines
effectively act as a mirror.
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L13.10 : Aurora
Believed to be caused by
currents flowing from the
earth to space and back.
Electrons collide with gas in the upper atmosphere (O2 particularly)
to produce light typically at red and green wavelengths.
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