A bar magnet placed in a magnetic field will rotate until it lines up with the field's direction. The same thing happens when one suspends a magnet from a string: it lines up with Earth's magnetic field and points in a north-south direction. The Chinese of the first century B.C. discovered that a strip of magnetic metal always tends to point toward geographic north, though they were unaware of the electromagnetic force that causes this to happen.
This led ultimately to the development of the magnetic compass, which typically consists of a magnetized iron needle suspended over a card marked with the four cardinal directions (north, south, east, and west). The needle is attached to a pivoting mechanism at its center, which allows it to move freely so that the tip of the needle will always point the user northward. The magnetic compass proved so important that it typically is ranked alongside paper, printing, and gunpowder as one of premodern China's four great gifts to the West. Before the compass, mariners had to depend purely on the position of the Sun and other, less reliable means of determining direction; hence, the invention quite literally helped open up the world.
The compass, in fact, helped make possible the historic voyage of Christopher Columbus (1451-1506) in 1492. While sailing across the Atlantic, Columbus noticed something odd: his compass did not always point toward what he knew, based on the Sun's position, to be geographic north. The further he traveled, the more he noticed this phenomenon, which came to be known as magnetic declination.
When Columbus returned to Europe and reported on his observations of magnetic declination (along with the much bigger news of his landing in the New World, which he thought was Asia), his story perplexed mariners. Eventually, European scientists worked out tables of magnetic declination, showing the amount of deviation at various points on Earth, and this seemed to allay sailors' concerns.
Then, in 1544, the German astronomer Georg Hartmann (1489-1564) observed that a freely floating magnetized needle did not always stay perfectly horizontal and actually dipped more and more strongly as he traveled north. When he was moving south, on the other hand, the needle tended to become more closely horizontal. For many years, this phenomenon, along with magnetic declination, remained perplexing. Nor, for that matter, did scientists understand exactly how or why a compass works. Then, in 1600, the English physicist William Gilbert (1540-1603) became the first to suggest a reason.
Gilbert coined the terms electric attraction, electric force, and magnetic pole. In De magnete (On the magnet), he became the first thinker to introduce the idea, now commonly accepted, that Earth itself is a giant magnet. Not only does it have north and south magnetic poles, but it also is surrounded by vast arcs of magnetic force, called the geomagnetic field. (The term geomagnetism, as opposed to magnetism, refers to the magnetic properties of Earth as a whole rather than those possessed by a single object or place on Earth.)
In the paragraphs that follow, we discuss the shape of this magnetic field, including the positions of the magnetic north and south poles; the origins of the field, primarily in terms of the known or suspected physical forces that sustain it (as opposed to the original cause of Earth's magnetic field, a more complicated and speculative subject); as well as changes in the magnetic field. Those changes, along with techniques for measuring the geomagnetic field, also are discussed at other places in this book.
Hartmann's compass phenomenon can be explained by the fact that Earth is a magnet and that its north and south magnetic poles are close to the geographic north and south. As for the phenomenon observed by Columbus, it is a result of the difference between magnetic and geographic north. If one continued to follow a compass northward, it would lead not to Earth's North Pole but to a point identified in 1984 as 77°N, 102°18′ W—that is, in the Queen Elizabeth Islands of far northern Canada.
Earlier we described the lines that make up the magnetic force field around a bar magnet. A field of similar shape, though, of course, of much larger size (yet still invisible), also surrounds Earth. From the magnetic north and south poles, lines of magnetic force rise into space and form giant curves that come back around and reenter Earth at the opposing pole, so that the planet is surrounded by a vast series of concentric loops. If one could draw a straight line through the center of all these loops, it would reach Earth at a point 11° from the equator. Likewise the north and south magnetic poles—which are on a plane perpendicular to that of Earth's magnetic field—are 11° off the planet's axis.
Surrounding the planet is a vast region called the magnetosphere, an area in which ionized particles (i.e., ones that have lost or gained electrons so as to acquire a net electric charge) are affected by Earth's magnetic field. The magnetosphere is formed by the interaction between our planet's magnetic field and the solar wind, a stream of particles from the Sun. (See Sun, Moon, and Earth for more about the solar wind.) Its shape would be akin to that of Earth's magnetic field, as described earlier, were it not for the Sun's influence.
The side of the magnetosphere closer to the Sun does indeed resemble the giant series of concentric loops described earlier. These loops are enormous, such that the forward, or sunlit, edge of the magnetosphere is located at a distance of some 10 Earth radii (about 35,000 mi., or 65,000 km). On the other side from the Sun—the rear, or dark, side—the shape of the magnetosphere is quite different. Instead of forming relatively small loops that curve right back around into Earth's poles, the lines of magnetic force on this side shoot straight out into space a distance of some 40 Earth radii (about 140,000 mi., or 260,000 km).
The shape of the magnetosphere, then, is a bit like that of a comet moving toward the Sun. Surrounding it is the magnetopause, a sort of magnetic dead zone about 62 mi. (100 km) thick, which shields Earth from most of the solar wind. In front of it (toward the Sun) is an area of magnetic turbulence known as the magnetosheath, and still closer to the Sun is a boundary called the bow shock, a shock-wave front that slows particles of solar wind considerably. Since Earth is turning, the side of the planet away from the Sun is continually changing, of course, but the shape of the magnetosphere remains more or less intact. It is, however, highly affected by solar activity, such that an increase in solar wind can cause a depression in the magnetosphere. (See Sun, Moon, and Earth for a discussion of auroras, produced by an interaction between the solar wind and the magnetosphere.)
As noted earlier, scientists at present understand little with regard to the origins of Earth's magnetic field—that is, the original action or actions that resulted in the creation of a geomagnetic field that has remained active for billions of years. No less a figure than Albert Einstein (1879-1955) identified this question as one of the great unsolved problems of science. On the other hand, scientists do have a good understanding of the geomagnetic field's source, in terms of the physical conditions that make it possible. (This distinction is rather like that between efficient cause and material cause, as discussed in Earth, Science, and Nonscience.)
It is believed that the source of Earth's magnetism lies in a core of molten iron some 4,320 mi. (6,940 km) across, constituting half the planet's diameter. Within this core run powerful electric currents that create the geomagnetic field. Actually, the field seems to originate in the outer core, consisting of an iron-nickel alloy that is kept fluid owing to the exceedingly great temperatures. The materials of the outer core undergo convection, vertical circulation that results from variations in density brought about by differences in temperature.
This process of convection (imagine giant spirals moving vertically through the molten metal) creates the equivalent of a solenoid, described earlier. Even so, there had to be an original source for the magnetic field, and it is possible that it came from the Sun. In any case, the magnetic field could not continue to exist if the fluid of the outer core were not in constant convective motion. If this convection stopped, within about 10,000 years (which, in terms of Earth's life span, is like a few seconds to a human being), the geomagnetic field would decay and cease to operate. Likewise, if Earth's core ever cooled and solidified, Earth would become like the Moon, a body whose magnetic field has disappeared, leaving only the faintest traces of magnetism in its rocks.
Though there is no reason to believe that anything so dramatic will happen, there are curious and perhaps troubling signs that Earth's magnetic field is changing. According to data recorded by the U.S. Geological Survey, which updates information on magnetic declination, the field is shifting—and weakening. Over the course of about a century, scientists have recorded data suggesting a reduction of about 6% in the strength of the magnetic field.
The behavior, in terms of both weakening and movement, appears to be similar to changes taking place in the magnetic field of the Sun. Indeed, as we have seen already, Earth's magnetism is heavily affected by the Sun, and it is possible that a period of strong solar-flare activity could shut down Earth's magnetic field. Even the present trend of weakening, if it were to continue for just 1,500 years, would wipe out the magnetic field. Some scientists believe that the planet is simply experiencing a fluctuation, however, and that the geomagnetic field will recover. Others maintain that the geomagnetic field is on its way to a reversal.
A reversal? Odd as it may sound, the direction of the geomagnetic field has reversed itself many, many times in the past. Furthermore, the planet has attempted unsuccessfully to reverse its geomagnetic field many more times—as recently as 30,000 to 40,000 years ago. These reversals are among the interests of paleomagnetism, the area of geology devoted to the direction and intensity of magnetic fields in the past, as discerned from the residual magnetization of rocks.
A compass works, of course, because the metal points toward Earth's magnetic north pole, which is close to its geographic north pole. Likewise, the magnetic materials in the rocks of Earth point north—or rather, they would point north if the direction of the magnetic field had not changed over time. Around the turn of the nineteenth century, geologists noticed that whereas some magnetic rocks pointed toward Earth's current North Pole, some were pointing in the opposite direction. This led to the realization that the magnetic field had reversed and to the development of paleomagnetism as a field of study. Studies in paleomagnetism, in turn, have provided confirmation of the powerful theory known as plate tectonics. (See Plate Tectonics for more on paleomagnetism and the shifting of plates beneath Earth's surface.)
Campbell, Wallace Hall. Earth Magnetism: A Guided Tour Through Magnetic Fields. New York: Harcourt Academic Press, 2001.
Dispezio, Michael A., and Catherine Leary. Awesome Experiments in Electricity and Magnetism. New York: Sterling, 1998.
Geomagnetism (Web site). <http://ub.nmh.ac.uk/> .
The Great Magnet, the Earth (Web site). <http://istp.gsfc.nasa.gov/earthmag/dmglist.htm> .
Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.
Laboratory of Earth's Magnetism (Web site). <http://geo.phys.spbu.ru/Magnetism/> .
Lafferty, Peter. Magnets to Generators. Illus. Alex Pang and Galina Zolfanghari. New York: Gloucester Press, 1989.
Lefkowitz, R. J., and J. Carol Yake. Forces in the Earth: A Book About Gravity and Magnetism. New York: Parents' Magazine Press, 1976.
Smith, David G. The Cambridge Encyclopedia of Earth Sciences. New York: Cambridge University Press, 1981.
Stern, David P. 400 Years of "De magnete" (Web site). <http://www-istp.gsfc.nasa.gov/earthmag/demagint.htm> .