Geoscience Reference
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the Sun than does the Earth. The solar wind is very tenuous but it is sufficient to blow out
the tails of comets as they come closer in to the heart of the solar system, so the tails always
point away from the Sun. It also features in imaginative proposals for propelling spacecraft
with vast gossamer-thin solar sails.
2. Diagram of the Earth's magnetic envelope, the magnetosphere, swept back into a
comet-like structure by the solar wind. Arrows show the directions of electrical cur-
rents.
The Earth is sheltered from the solar wind by its magnetic field, the magnetosphere. Be-
cause the solar wind is electrically charged it represents an electrical current, which cannot
cross magnetic field lines. Instead, it compresses the Earth's magnetosphere on the sun-
ward side, like the bow wave of a ship at sea, and stretches it out into a long tail down-wind
which reaches almost as far as the orbit of the Moon. Charged particles caught within the
magnetosphere build up in belts between the field lines where they are forced to spiral,
generating radiation. These radiation belts were first spotted in 1958 when James van Allen
flew the first Geiger counter in space on board the American Explorer 1 satellite. They are
areas to be avoided by spacecraft hoping for a long life and would be lethal to unprotected
astronauts.
Where the Earth's magnetic field lines dive down towards the poles, solar wind particles
can enter the atmosphere, sending atoms ricocheting downwards to produce spectacular au-
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