Magnetic field

Earth acts as though it were a huge dipole magnet with the positive and negative poles near the North and South Poles. This does not mean that Earth is literally a dipole magnet—there are too many variations in the field—but that the best fit for a model of the field is two poles of a magnet, rather than a quadrupole, or other shape. The magnetic field of the earth allows magnetic compasses to work, making navigation much easier. It also molds the configuration of Van Allen belts, bands of high-energy charged particles around the earth's atmosphere.

Most of Earth's magnetic field (90%) occurs below the surface and possibly exists because Earth's core doesn't move at the same rate as the earth's mantle (the layer between the earth's core and its crust ). The external 10% of the field is generated by movement of ions in the upper atmosphere.

The earth's magnetic field may help some animals navigate as they migrate. People have been using magnetic compasses for navigation since the fifteenth century. Because it has been so important for navigation, the magnetic field has been mapped all over the surface of the earth.

The magnetic field can also be used in other ways. For example, an instrument called a geomagnetic electrokinetograph determines the direction and speed of ocean currents while a ship is moving by measuring the voltage induced in the moving conductive sea water by the magnetic field of the earth.

The earth's magnetic field can change quickly and temporarily or slowly and permanently, depending on the cause of the change. The magnetic field can change very quickly, within an hour, in magnetic storms . These occur when the magnetic field is disturbed by sunspots, which send clouds of charged particles into Earth's atmosphere. (These same protons and electrons excite oxygen , nitrogen, and hydrogen atoms in the upper atmosphere, causing the aurora borealis and aurora australis.) These disturbances can be measured all over the globe and can cause static on radio stations.

The orientation of the magnetic field also changes slowly over centuries. In the planet's lifetime, the magnetic field has changed and even reversed (north pole becomes south and vice-versa) several times. Evidence for this is seen in reversed paleomagnetism of some sedimentary and igneous rock . In the 1960s, scientists showed that rocks formed at a particular interval in geologic time all indicate a magnetic field with the same orientation; older or younger rocks may show a reversed orientation. The cause of these paleomagnetic reversals is not yet known.

Today, the magnetic poles are not at the same place as the poles of the earth's rotational axis. Therefore, "magnetic north" is not quite the direction of "true north." The difference is known as the magnetic declination. Accordingly, scientists have established a series of geomagnetic coordinates, including latitude and longitude . These are centered on the magnetic dipole of the earth and designed (like geographic latitude and longitude ) as though the earth were a perfect sphere.

See also Earth, interior structure; Earth (planet); Polar axis and tilt

magnetic field Region surrounding a magnet, or a conductor through which a current is flowing, in which magnetic effects, such as the deflection of a compass needle, can be detected. A magnetic field can be represented by a set of lines of force (flux lines) spreading out from the poles of a magnet or running around a current-carrying conductor. The direction of a magnetic field is the direction a tiny magnet takes when placed in the field. Magnetic poles are the field regions in which magnetism appears to be concentrated. If a bar magnet is suspended to swing freely in the horizontal plane, one pole will point north; this is called the north-seeking or north pole. The other pole, the south-seeking or south pole, will point south. Unlike poles attract each other; like poles repel each other. The Earth's magnetic poles are the ends of the huge ‘magnet’ that is Earth.

mag·net·ic field • n. a region around a magnetic material or a moving electric charge within which the force of magnetism acts.

magnetic field See GEOMAGNETIC FIELD.