The Earth's Magnetosphere

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The ancient Greeks, originally those near the city of Magnesia, and also the early Chinese knew about strange and rare stones (possibly chunks of iron ore struck by lightning) with the power to attract iron. A steel needle stroked with such a "lodestone" became "magnetic" as well, and around 1000 the Chinese found that such a needle, when freely suspended, pointed north-south. The magnetic compass soon spread to Europe. Columbus used it when he crossed the Atlantic ocean, noting not only that the needle deviated slightly from exact north (as indicated by the stars) but also that the deviation changed during the voyage. Around 1600 William Gilbert, physician to Queen Elizabeth I of England, proposed an explanation: the Earth itself was a giant magnet, with its magnetic poles some distance away from its geographic ones (i.e. near the points defining the axis around which the Earth turns). The Earth is a huge magnet, and its magnetic influence extends far into space. In our everyday environment, magnetic forces are of no importance and a sensitive instrument, the compass needle, is needed to detect them. That is because we, the materials we encounter in everyday life, even the oxygen and nitrogen which we breathe, are all electrically neutral. The atoms of oxygen, for instance, contain electrons with negative electric charges and protons which are positive, but the two charges balance each other andthe electric and magnetic forces cancel. Magnetic forces have almost no effect on neutral atoms. However, 60 miles (100 km) or more above the surface of the Earth, the natural environment is quite different. The fringes of the atmosphere at these heights are strongly heated by the Sun's x-rays and ultra-violet light (and by other causes as well), causing negative electrons to be torn off atoms and leaving the remainder of the atoms as

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