<P> The solar wind exerts a pressure, and if it could reach Earth's atmosphere it would erode it . However, it is kept away by the pressure of the Earth's magnetic field . The magnetopause, the area where the pressures balance, is the boundary of the magnetosphere . Despite its name, the magnetosphere is asymmetric, with the sunward side being about 10 Earth radii out but the other side stretching out in a magnetotail that extends beyond 200 Earth radii . Sunward of the magnetopause is the bow shock, the area where the solar wind slows abruptly . </P> <P> Inside the magnetosphere is the plasmasphere, a donut - shaped region containing low - energy charged particles, or plasma . This region begins at a height of 60 km, extends up to 3 or 4 Earth radii, and includes the ionosphere . This region rotates with the Earth . There are also two concentric tire - shaped regions, called the Van Allen radiation belts, with high - energy ions (energies from 0.1 to 10 million electron volts (MeV)). The inner belt is 1--2 Earth radii out while the outer belt is at 4--7 Earth radii . The plasmasphere and Van Allen belts have partial overlap, with the extent of overlap varying greatly with solar activity . </P> <P> As well as deflecting the solar wind, the Earth's magnetic field deflects cosmic rays, high - energy charged particles that are mostly from outside the Solar system . (Many cosmic rays are kept out of the Solar system by the Sun's magnetosphere, or heliosphere .) By contrast, astronauts on the Moon risk exposure to radiation . Anyone who had been on the Moon's surface during a particularly violent solar eruption in 2005 would have received a lethal dose . </P> <P> Some of the charged particles do get into the magnetosphere . These spiral around field lines, bouncing back and forth between the poles several times per second . In addition, positive ions slowly drift westward and negative ions drift eastward, giving rise to a ring current . This current reduces the magnetic field at the Earth's surface . Particles that penetrate the ionosphere and collide with the atoms there give rise to the lights of the aurorae and also emit X-rays . </P>

How does the earth's magnetic field protect us from cosmic rays
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