MAGNETOSPHERE

region in the atmosphere where magnetic phenomena and the high atmospheric conductivity caused by ionization are important in determining the behaviour of charged particles. The Earth, in contrast to Mars and Venus, has a significant surface magnetic field (approximately 0.5 gauss), which, like its gravitational field, becomes weaker as the distance from the centre of the Earth increases. In the direction of the Sun, at approximately 10 Earth radii (almost 65,000 km, or 40,000 miles), the magnetic field is so weak that the pressure associated with particles escaping from the Earth's gravity is comparable to the opposing pressure associated with the solar windthe flux mainly of protons and electrons escaping from the Sun's gravitational field. This equilibrium region, with a characteristic thickness of 100 km (60 miles), is called the magnetopause and marks the outer boundary of the magnetosphere. The lower boundary of the magnetosphere is several hundred kilometres above the Earth's surface. On the night side, or the side away from the Sun, the forces associated with the magnetic field and the solar wind are parallel, and thus the magnetosphere extends a considerable distance, possibly even several astronomical units (one astronomical unit is the average distance between the Earth and the Sun, about 1.5 108 km). In the direction perpendicular to the solar wind, the random motion of the solar-wind particles exerts a small pressure on the magnetic field, constricting the magnetosphere slightly. The net result is that the shape of the magnetosphere in gross terms is similar to that of a comet, with the Earth located near the nucleus, or head, of the comet and the magnetospheric tail trailing out well beyond the Earth, away from the Sun. Between 10 and 13 Earth radii toward the Sun, there exists the magnetosheath, a region of magnetic turbulence in which both the magnitude and direction of Earth's magnetic field vary erratically. This disturbed region is thought to be caused by the production of magnetohydrodynamic shock waves, which in turn are caused by high-velocity solar-wind particles. Ahead of this bow shock boundary, toward the Sun, is the undisturbed solar wind.