The magnetosphere

One of the early surprises of our unmanned space program was the discovery of belts of charged particles high above the Earth' surface. The particles are a source of low frequency synchrotron radiation. The radiation overwhelmed detectors on early spacecraft, making it appear as if no par-

Rotation Magnetic axis axis

ticles were present. However, James van Allen correctly interpreted the strange result as indicating the presence of large numbers of charged particles. We call these belts of charged particles the van Allen radiation belts (Fig. 23.22)

Fig 23.22.

Van Allen radiation belts and the Earth's magnetosphere. (a) The radiation belts are groups of trapped particles, concentrating into two bands, each with the shape of a doughnut that has been hollowed along the inner rim. (b) The Earth's magnetic field deflects the charged particles of the solar wind.The protected region is the magnetosphere, and the boundary is called the magnetopause. Just outside the magnetopause is a shock wave that looks like the bow wave when a ship plows through the water [NASA].

These particles are trapped by the Earth's magnetic field, and stay in spiral paths around the field lines. The region where there are large number of charged particles trapped by the Earth's magnetic field is called the magnetosphere. The region dominated by charged particles is also called the ionosphere. When we discussed solar activity (Chapter 6), we said that charged particles will follow helical paths around magnetic field lines. This is because the force on the particles is perpendicular to both the field lines and the velocity of the particle. This means that there can be no force along the field lines. The component of the velocity along the field, the drift speed, stays fixed, as the particles execute circular motion perpendicular to the field lines.

The situation is different if the particles are moving from a region of a weaker magnetic field to one of a stronger magnetic field, as illustrated in Fig. 23.23. The stronger field is represented by the field lines becoming closer together. We divide the magnetic field into two components,

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