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 positively charged "ions". These electrified fragments react strongly to the magnetic forces and can be steered and trapped by them.

    It can be shown that such free electrons and ions will be guided by the magnetic field lines (or "lines of force") which rise from near the southern (magnetic) pole and enter the Earth again near the northern pole. Electrons and ions tend to remain attached to field lines like beads on wires, though unlike beads they also slowly migrate ("drift") to neighboring "wires."

    It follows that the structure of field lines near Earth determines much of the motion and behavior of the free electrons and ions found there. Satellites observing magnetic forces in space have found (figure on right) that lines from most points on Earth are confined inside a fairly well-defined cavity, the magnetosphere of the Earth. The space outside it is dominated by the Sun, and by the fast "solar wind" of free ions and electrons emitted by the Sun.