Discovered by artificial satellites launched in the late 1950s, Earth's magnetosphere extends far above the atmosphere. Simply put, the magnetosphere is the region around a planet that is influenced by that planet's magnetic field. Sketched in Figure 7.8, Earth's magnetic field is similar in overall structure to the field of a gigantic bar magnet and completely surrounds our planet. The magnetic field lines, which indicate the strength and direction of the field at any point in space, run from south to north, as indicated by the white arrowheads in the figure.

Figure 7.8 Earth's magnetic field resembles that of an enormous bar magnet situated inside our planet. The arrows on the field lines indicate the direction in which a compass needle would point.

The north and south magnetic poles, where the magnetic field lines intersect Earth's surface vertically, are roughly aligned with Earth's spin axis. Neither pole is fixed relative to our planet, however—both drift at a rate of some 10 km per year—nor are the poles symmetrically placed. At present, Earth's magnetic north pole lies in northern Canada, at a latitude of about 80° N, almost due north of the center of North America; the magnetic south pole lies at a latitude of about 60° S, just off the coast of Antarctica south of Adelaide, Australia.

Earth's magnetosphere contains two doughnut-shaped zones of high-energy charged particles, one located about 3000 km and the other 20,000 km above Earth's surface. These zones are named the Van Allen belts, after the American physicist whose instruments on board one of the first satellites first detected them. We call them "belts" because they are most pronounced near Earth's equator and because they completely surround the planet. Figure 7.9 shows how these invisible regions envelop Earth except near the North and South Poles.

Figure 7.9 High above Earth's atmosphere, the magnetosphere (lightly shaded blue area) contains at least two doughnut-shaped regions (heavily shaded violet areas) of magnetically trapped charged particles. These are the Van Allen belts.

The particles that make up the Van Allen belts originate in the solar wind. Traveling through space, neutral particles and electromagnetic radiation are unaffected by Earth's magnetism, but electrically charged particles are strongly influenced. As illustrated in Figure 7.10, a magnetic field exerts a force on a moving charged particle, causing the particle to spiral around the magnetic field lines. In this way, charged particles—mainly electrons and protons—from the solar wind can become trapped by Earth's magnetism. Earth's magnetic field exerts electromagnetic control over these particles, herding them into the Van Allen belts. The outer belt contains mostly electrons; the much heavier protons accumulate in the inner belt.

Figure 7.10 A charged particle in a magnetic field spirals around the field lines. Thus, charged particles tend to become "trapped" by strong magnetic fields.

We could never survive unprotected in the Van Allen belts. Unlike the lower atmosphere, on which humans and other life forms rely for warmth and protection, much of the magnetosphere is subject to intense bombardment by large numbers of high-velocity, and potentially very harmful, charged particles. Colliding violently with an unprotected human body, these particles would deposit large amounts of energy wherever they made contact, causing severe damage to living organisms. Without sufficient shielding on the Apollo spacecraft, for example, the astronauts might not have survived the passage through the magnetosphere on their journey to the Moon.

Particles from the Van Allen belts often escape from the magnetosphere near Earth's north and south magnetic poles, where the field lines intersect the atmosphere. Their collisions with air molecules create a spectacular light show called an aurora (Figure 7.11). This colorful display results when atmospheric molecules, excited upon collision with the charged particles, fall back to their ground states and emit visible light. Many different colors are produced because each type of atom or molecule can take one of several possible paths as it returns to its ground state. (Sec. 4.2) Aurorae are most brilliant at high latitudes, especially inside the Arctic and Antarctic circles. In the north, the spectacle is called the aurora borealis, or Northern Lights. In the south, it is called the aurora australis, or Southern Lights.

Figure 7.11 (a) A colorful aurora rapidly flashes across the sky like huge wind-blown curtains glowing in the dark. The aurora is created by the emission of light radiation after magnetospheric particles collide with atmospheric molecules. The colors are produced as excited atoms and molecules return to their ground states. (b) The aurora high above Earth, as photographed from a space shuttle (visible at left).

Occasionally, particularly after a storm on the Sun (see Chapter 16), the Van Allen belts can become distorted by the solar wind and overloaded with many more particles than normal, allowing some particles to escape prematurely and at lower latitudes. For example, in North America, the aurora borealis is normally seen with any regularity only in northern Canada and Alaska. However, at times of greatest solar activity, the display has occasionally been seen as far south as the southern United States.

In reality, Earth's magnetosphere is not nearly as symmetrical as depicted in Figure 7.8. Satellites have mapped its true shape. As shown in Figure 7.12, the entire region of trapped particles is quite distorted, forming a teardrop-shaped cavity. On the sunlit (daytime) side of Earth, the magnetosphere is compressed by the flow of high-energy particles in the solar wind. The boundary between the magnetosphere and this flow is known as the magnetopause. It is found at about 10 Earth radii from our planet. On the side opposite the Sun, the field lines are extended away from Earth, with a long tail often reaching beyond the orbit of the Moon.

Figure 7.12 Earth's real magnetosphere is actually greatly distorted by the solar wind, with a long tail extending from the nighttime side of Earth well into space.

What is the origin of the magnetosphere and the Van Allen belts within it? Earth's magnetism is not really the result of a huge bar magnet lying within our planet. In fact, geophysicists believe that Earth's magnetic field is not a "permanent" part of our planet at all. Instead, it is thought to be continuously generated within the outer core and exists only because Earth is rotating. As in the dynamos that run industrial machines, Earth's magnetism is produced by the spinning, electrically conducting, liquid metal core deep within our planet. The theory that explains planetary (and other) magnetic fields in terms of rotating, conducting material flowing in the planet's interior is known as dynamo theory. Both rapid rotation and a conducting liquid core are needed for this mechanism to work. This connection between internal structure and magnetism is very important for studies of the other planets in the solar system: we can tell a lot about a planet's interior simply by measuring its magnetic field.

Earth's magnetic field plays an important role in controlling many of the potentially destructive charged particles that venture near our planet. Without the magnetosphere, Earth's atmosphere—and perhaps the surface, too—would be bombarded by harmful particles, possibly damaging many forms of life on our planet. Some researchers have even suggested that had the magnetosphere not existed in the first place, life might never have arisen at all on our planet.