Earth's Magnetic Field
Earth acts as a dipole magnet with the positive and negative magnetic poles near, but not aligned exactly with, the north and south geographic poles. Because of this difference, detailed maps commonly distinguish between true north and magnetic north. The difference, known as magnetic declination, must be taken into account when navigating with a magnetic compass. Its magnetic field also molds the configuration of Van Allen radiation belts, which are bands of high-energy charged particles around Earth, and helps to shield Earth from a stream of charged particles known as the solar wind.
The origin of Earth's magnetic field is not fully understood, but it is generally thought to be a result of electrical currents generated by movement of molten iron and nickel within Earth's outer core. This is known as the dynamo effect. The Moon, in contrast, has no magnetic field and is thought to have a small core that is only partially molten. A small portion of Earth's magnetic field is generated by movement of ions in the upper atmosphere.
Magnetic field lines produced by the dynamo effect radiate far into space. Unlike the field lines around a simple dipole magnetic, however, the field lines around Earth are asymmetric. Those nearest the Sun are compressed by the solar wind whereas those shielded from the Sun by Earth are highly elongated. The effect is similar to that of water passing around the bow of a ship. Charged particles become trapped along magnetic field lines, just as iron filings around a simple dipole magnet, and form Earth's magnetosphere.
Earth's magnetic field can change for short or long periods of time. The magnetic field can change quickly, within an hour, in magnetic storms. These short-term changes occur when the magnetic field is disturbed by sunspots, which send clouds of charged particles into Earth's atmosphere. The same particles excite oxygen, nitrogen, and hydrogen atoms in the upper atmosphere, causing the aurora borealis and aurora australis.
Long-term changes occur when Earth's magnetic field reverses itself, which occurs on average every 500,000 years. The last known reversal was about 700,000 years ago. Evidence of past reversals was discovered in the 1950s and 1960s, when magnetometers towed behind ships showed that the magnetic polarity of volcanic rocks comprising the oceanic crust alternated in strips parallel to mid-ocean ridges. Further research showed that the strips occur because oceanic plates grow outward from mid-ocean ridges. Grains of magnetite within volcanic rocks such as basalt, which constitutes most of the oceanic crust, align themselves with the prevailing magnetic field when the temperature of the rock falls below its Curie point (about 1,075°F [580°C] for basalt). Thus, the alternating strips represent changes in the polarity of Earth's magnetic field as oceans grow through time. The existence of magnetic strips was one of the key pieces of evidence that led to widespread acceptance of plate tectonics among scientists.