Earth's Magnetic Field
Earth's magnetic field, though observed by humans for centuries, has been explained only recently by science. Most of the field is the result of electricity flowing through the molten metal outer core of the Earth as it rotates. The remainder is caused by the interaction of the main magnetic field and the solar wind and by electric currents in the ionosphere. As a result of the magnetic field, the Earth acts like a permanent bar magnet with its poles tilted eleven degrees away from its axis of rotation.
Although the Chinese did not understand the nature of the Earth 's magnetic field, they were the first to use it. They devised a compass with magnetized needle suspended by a thread. They did notice, however, that their compass needles did not exactly parallel a given geographic meridian and pointed slightly away from true north, a fact also noted in Europe during the 15th century. The degree of the departure was known as magnetic declination.
Another mysterious variable was known as magnetic dip. During the late 16th century, it was noticed that the freely suspended needles would dip downward with increased northward latitude. At the north magnetic pole (76 degrees north latitude, 100 degrees west longitude in 1975), the needle pointed straight down. At the south magnetic pole (66 degrees south latitude, 139 degrees east longitude in 1975), the needle pointed straight up.
Early navigators hoped that magnetic declination and dip could be used to indicate geographical longitude, but during the 18th century, even more variations were discovered. The needle was affected by displays of the aurora borealis (northern lights), time of day or year, and its own geographical location.
One of the earliest investigators into magnetic phenomena was William Gilbert. He mounted magnetized needles horizontally and vertically, determining that the magnetic force came from the Earth itself, and not electrical influence from the sky.
In 1736 Charles Augustin Coulomb used a torsion balance to measure the very small forces acting on electrically-charged spheres. He investigated the forces between the poles of magnets and discovered both electric and magnetic forces obeyed the same inverse square law. This linked electricity with magnetism, a theory that Hans Christian Oersted (1777-1851) proved in 1821. Edmond Halley, the astronomer and discoverer of a comet which bears his name, also investigated magnetism. While he was a student he determined the variation of a magnetized needle in London. He also discovered that the aurora borealis was magnetic in origin and traveled extensively, measuring variations in the magnetic field.
Following Halley's lead, Friedrich von Humboldt voyaged to North and South America in a five-year exploration that began in 1799. He discovered that magnetic intensity declined as he drew closer to the equator and initiated an international geomagnetic survey of the Earth. The results, compiled with the help of Carl Gauss and Edward Sabine (1788-1883), indicated the Earth was itself a magnet, with variations in declination and dip caused by geo-electric currents and short-term variations from electric charges in the atmosphere.
In 1852 Sabine discovered that the number of disturbances in the Earth's magnetic field followed a ten-or eleven-year cycle, causing magnetic "storms" in which the magnetized needle deviated from its norm more than usual. Finding that his cycle corresponded with Samuel Schwabe's (1789-1857) cycle for varying numbers of sunspots, Sabine revealed that solar activity accounted for the daily and annual variations in the magnetic field.
In 1906 French physicist Bernard Brunhes found magnetized rocks whose magnetic orientation was opposite the direction of the Earth's magnetic field and proposed that the Earth's magnetic field had reversed itself over time. This set off a debate that raged for over fifty years.
Brunhes was joined by Paul L. Mercanton, who, in 1926, suggested that the Earth's magnetic poles were in different places during the Permo-Carboniferous and Tertiary Periods. Three years later, Japanese geophysicist Motonori Matuyama (1884-1958), studying rock specimens for remnant magnetization, concluded Mercanton 's theory was correct. He was able to determine when the field had reversed, and proposed there had been reversals in even shorter reversals in the Miocene and Quaternary Epochs.
In the early 1960s, a detailed study of paleomagnetism was published, indicating nine major reversals in the past 3.6 million years, thus supporting the Matuyama-Brunhes theory. Today it is accepted that Earth's magnetic field can exist in "normal" and "reversed" states, with a transition period of 2,000 to 10,000 years to change polarity.
More evidence was presented in 1965 by Richard Doell (b.1923) and Brent Dalrymple (b.1937). They gave a complete picture of reversals over the previous five million years, linking the reversals worldwide. Their work also cemented arguments for continental drift and plate tectonics.
Further research in the Earth's magnetic field centered on the mechanism for its origin, known as magnetohydrodynamic dynamo. Developed by British physicists W. M. Elasser and Edward Bullard (1907-1980), the theory compares the field to that of a dynamo, or generator, which uses a magnet to convert mechanical energy into electrical energy.
Some scientists have suggested that a period of strong solar-flare activity could wipe out the Earth's magnetic field. At the conclusion of the activity, the magnetohydrodynamic dynamo would restore the magnetic field, but it would be reversed.
During the last century, scientists have noticed a steady reduction of about 6 percent in the strength of the magnetic field. Should this continue, the magnetic field will be gone in only 1,500 years. Some scientists believe this is only a fluctuation, and that it will recover its strength in time. Others believe it indicates a reversal on its way.
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