ADV ADV 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 School of Physics- 2004 1 ADV School of Physics- 2004 ADV L13.2 : The Earth as a Magnet Magnetic dipole moment points down (northern to Southern hemisphere), magnitude 2 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. School of Physics- 2004 ADV 3 L13.4 : Origin of Earth’s Magnetic Field School of Physics- 2004 ADV 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. School of Physics- 2004 4 5 School of Physics- 2004 6 ADV L13.6 : Studying the History of the Earth’s Magnetic Field ADV 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. School of Physics- 2004 Modifies the Earth’s magnetic field in a rather complicated way. 7 ADV School of Physics- 2004 8 ADV 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 School of Physics- 2004 End regions of constricted field lines effectively act as a mirror. 9 ADV 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. School of Physics- 2004 11 School of Physics- 2004 10