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Magnetism The Earth’s magnetic field
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Magnetism
The Earth’s magnetic field
Why does the Earth have a magnetic field?
SEED Expert José Navarro writes:
The Earth's core is mostly iron and nickel, both of which can be magnetic metals. The inner core is solid and the outer core is liquid.
The Earth's magnetism doesn't appear to come from the metal itself. Current scientific theories attribute the earth's magnetism to convection currents that move the liquid metal in the outer core, approximately 3,000 kilometers beneath the Earth’s surface. These might be creating electric currents or in some way orienting the local magnetization of the metal.
The Earth’s magnetic field is mostly dipolar – which means that it has one magnetic North pole and one magnetic South pole, similar to a bar (e.g. fridge) magnet.
Magnetic North and South are not the same as geographic North and South. This is called "Magnetic Declination". The amount of difference depends on your location on Earth and it changes gradually over time. When using a compass to read a map, you need to know and account for the magnetic declination. You can see maps and compute magnetic declination for any location in the world at the Geomagnetism pages of the US National Geophysical Data Center. For example:
For Rio de Janeiro in 2005, magnetic declination is 18° 52' W changing by 0° 5' W/year
For Moscow in 2005, magnetic declination is 9° 36' E changing by 0° 6' E/year
In addition to moving around geographically, the strength of the magnetic field also changes over time. Geologists can get information about the magnetic field in ancient times from rocks, particularly those beneath our oceans. It has been found that the magnetic field sometimes suffers a large decrease in strength then becomes strong again. This is called an “excursion”. At other times, there has been a magnetic reversal, in which the magnetic North pole is transformed into a South pole and the South pole becomes a North pole. In the last 10 million years, there have been and average of between 4 and 5 reversals every million years. At other times in Earth's history there have been much longer periods when no reversals occurred.
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