The earth has a core made up largely of iron, heated to the extent that some of it is liquid. As the earth rotates, the iron flows in complex ways. It is thought that this flow is responsible for the magnetic field that surrounds the earth. Some other planets, notably Jupiter, have magnetic fields as well. Even the sun has one.
The Poles and Axis
The geomagnetic field, as it is called, has poles, just as a bar magnet does. The geomagnetic poles are near, but not at, the geographic poles. The north geomagnetic pole is located in far northern Canada. The south geomagnetic pole is near Antarctica. The geomagnetic axis is therefore tilted relative to the axis on which the earth rotates.
The Solar Wind
Charged subatomic particles from the sun, streaming outward through the solar system, distort the geomagnetic lines of flux above figure. This stream of particles is called the solar wind. That’s a good name for it, because the fast-moving particles produce measurable forces on sensitive instruments in space. This force has actually been suggested as a possible means to drive space ships, equipped with solar sails, out of the solar system! At and near the earth’s surface, the geomagnetic field is not affected very much by the solar wind, so the geomagnetic field is nearly symmetrical. As the distance from the earth increases, the distortion of the field also increases, particularly on the side of the earth away from the sun.
The Magnetic Compass
The presence of the geomagnetic field was first noticed in ancient times. Some rocks, called lodestones, when hung by strings, would always orient themselves a certain way. This was correctly attributed to the presence of a “force” in the air. This effect was put to use by early seafarers and land explorers. Today, a magnetic compass can still be a valuable navigation aid, used by mariners, backpackers, and others who travel far from familiar landmarks. The geomagnetic field interacts with the magnetic field around a compass needle, and a force is thus exerted on the needle. This force works not only in a horizontal plane (parallel to the earth’s surface), but vertically at most latitudes. The vertical component is zero only at the geomagnetic equator, a line running around the globe equidistant from both geomagnetic poles.
As the geomagnetic latitude increases, toward either the north or the south geomagnetic pole, the magnetic force pulls up and down on the compass needle more and more. One end of the needle seems to insist on touching the compass face, while the other end tilts up toward the glass. The needle tries to align itself parallel to the geomagnetic lines of flux. The vertical angle, in degrees, at which the geomagnetic lines of flux intersect the earth’s surface at any given location is called the geomagnetic inclination.
Because geomagnetic north is not the same as geographic north in most places on the earth’s surface, there is an angular difference between the two. This horizontal angle, in degrees, is called geomagnetic declination. It, like inclination, varies with location.