ESA Science & Technology11-Jul-2005 14:23:58

Background Science

Last Update:  24 Feb 2005

The regions of near-Earth space

The largely empty space between the planets is dominated by the solar wind - a stream of electrically charged particles (mainly electrons and protons) which are ejected at supersonic speeds by the Sun. Fortunately, the Earth's magnetic field is strong enough to protect our planet from this solar gale, usually preventing it from hitting the atmosphere or surface. However, a number of recognisable layers and boundaries can be observed in near-Earth space where the solar wind and Earth's magnetic field meet. These regions will be explored in great detail by Cluster.

The first sign of any interaction between the solar wind and Earth's magnetic field is a shock wave in space on the Sun-facing side of the Earth. This bow shock is rather like the sonic boom caused when a supersonic aircraft slows down and passes through the sound barrier. The Earth's bow shock is created when the supersonic solar wind is suddenly slowed as it approaches the planet's magnetic shield.

Artist's impression showing the
main regions/boundaries of the
Earth's magnetosphere that will be
studied by Cluster

The area of space under the protection of Earth's magnetic field is known as the magnetosphere. Like an island in a stream, Earth is able to divert the flow of particles around it. As they sweep around the planet, the magnetosphere is moulded into a tadpole shape, with a blunt head on the sunward side of the Earth, and a long tail (the magnetotail) which stretches millions of kilometres downwind, that is away from the Sun.

The edge of the magnetosphere is known as the magnetopause. When the Sun is more active, the increased pressure from the solar wind squashes the magnetosphere. At such times, the magnetopause is forced much closer to the Earth until it is only 35  000 kilometres above the planet.

However, there are two weak points in Earth's defences. These cusps occur above the planet's north and south magnetic poles. Particles from the solar wind which leak into the magnetosphere spiral down towards the Earth along magnetic field lines. When they strike atoms in the upper atmosphere, they cause the shimmering sheets of colour known as the auroras (northern and southern lights).

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