The Earth's magnetic field is like that produced by a large bar magnet. However, the interior of the Earth is not permanently magnetized. The magnetic field may be explained by the dynamo theory. In this theory, the magnetic field is produced by rotation of Earth's fluid metallic core.
The strength of the magnetic field is not uniform over the surface of the Earth. Variations in the magnetic field (anomalies) are related to the nature of the rocks of the Earth's crust. Most anomalies in the oceans can be related to either oceanic ridges or deep-sea trenches.
Explorer 1, the United States' first successful satellite, was launched on Jan. 31, 1958. Data from Explorer 1 led to the first discovery using an artificial satellite, the Van Allen Belts.
Charged particles such as protons or electrons, may be trapped by the Earth's magnetic field. These trapped particles that surround the Earth are called the Van Allen Radiation Belts, after their discoverer, Dr. James Van Allen.
Clouds of ionized (electrically charged) gases from the solar wind become trapped in the magnetosphere to form the aurorae or northern lights (aurora borealis) and southern lights (aurora australis). The aurorae generally occur at altitudes above 100 kilometers (60 miles) in rings about 17 degrees from the magnetic poles. These images show spectacular views of Earth's aurorae taken by astronauts aboard the Space Shuttle.