Magnetosphere

A magnetosphere is the region of space surrounding an astronomical object in which charged particles are controlled by that object's magnetic field. The magnetic field near the surface of many astronomical objects resembles that of a dipole. The field lines farther away from the surface can be significantly distorted by the flow of electrically conducting plasma emitted from a nearby star (e.g. the solar wind from the Sun).

Study of Earth's magnetosphere began in 1600, when William Gilbert discovered that the magnetic field on the surface of Earth resembled that on a terrella, a small, magnetized sphere. In the 1940s, Walter M. Elsasser proposed the model of dynamo theory, which attributes Earth's magnetic field to the motion of Earth's iron outer core. Through the use of magnetometers, scientists were able to study the variations in Earth's magnetic field as functions of both time and latitude and longitude. Beginning in the late 1940s, rockets were used to study cosmic rays. In 1958, Explorer 1, the first of the Explorer series of space missions, was launched to study the intensity of cosmic rays above the atmosphere and measure the fluctuations in this activity. This mission observed the existence of the Van Allen radiation belt (located in the inner region of Earth's magnetosphere), with the Explorer 3 mission later that year definitively proving its existence. Also in 1958, Eugene Parker proposed the idea of the solar wind. The term 'magnetosphere' was proposed by Thomas Gold in 1959. The Explorer 12 mission (1961) led to the observation by Cahill and Amazeen in 1963 of a sudden decrease in the strength of the magnetic field near the noon meridian, later named the magnetopause. In 1983, the International Cometary Explorer observed the magnetotail, or the distant magnetic field.

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