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The magnetosphere is a comet-shaped region of Earth's outer atmosphere in which the behavior of Earth's magnetosphere. Illustration by Hans & Cassidy. Courtesy of Gale Group. charged particles is strongly influenced by magnetic and ionic phenomena. The term was first introduced by the British astronomer Thomas Gold in 1959 although speculation about the existence of such a region goes back to the early 1930s in the studies of Sydney Chapman and V.C.A. Ferraro.

The magnetosphere exists because of the interaction between Earth's own magnetic field and the solar wind, a rapidly moving plasma consisting of protons and electrons expelled from the sun's surface. The magnetos phere's distinctive shape is a consequence of the fact that the solar wind is deflected by Earth's magnetic field in a manner somewhat similar to the way in which a rock deflects the flow of a stream of water.

The forward (sun-facing) edge of the magnetosphere is located at a distance of about ten Earth radii (about 40,365 mi/65,000 km) from the Earth's surface. At this distance, the pressure of particles escaping from Earth's atmosphere is equal to the pressure of the solar wind. An equilibrium layer with a thickness of about 62 mi (100 km) in this region is known as the magnetopause. The magnetopause completely surrounds the magnetosphere like a thin envelope. Forward of the magnetopause in the direction of the Sun is the magnetosheath, a region in which Earth's magnetic field is highly turbulent.

The magnetosphere extends much farther from Earth on the side away from the Sun (the "night" side) because both the solar wind and particles escaping from Earth's atmosphere are moving in the same direction. It appears that the magnetopause in this direction may be located at a distance of a few thousand Earth radii.

The internal structure of the magnetopause is highly complex. The reason for this complexity is that three distinct factors—the solar wind, Earth's magnetic field, and the Sun's magnetic field—are constantly interacting with each other. This interaction causes the development of distinct regions within the magnetosphere. For example, the night side of the magnetosphere appears to be subdivided into two regions by a thin layer of plasma called the plasma sheet.

Certain familiar astronomical phenomena are related to the magnetosphere. For example, particles excited by the interaction between the solar wind and the magnetosphere may eventually collide with and ionize particles in the upper atmosphere. When these ionized particles return to their ground state, they give off energy that may appear in the form of auroras (aurora borealis or aurora australis).

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