Earth's magnetic field

Earth's magnetic field is generated by rotation, convection, and electrical currents in the earth's outer core, and it acts as a protective shield against solar wind, which stripped away the atmospheres of other planetary bodies, like Venus and Mars. As electromagnetism is one of nature's four fundamental interactions, the magnetic field has wide-ranging applications to our lives and to the future of the planet.

Properties of Earth's Magnetic Field

With any electrical current comes a magnetic field. Electromagnetism, defined as the relationship between electrical and magnetic fields, is a fundamental interaction in nature, one of four such phenomena. (The other fundamental interactions are strong interactions, weak interactions, and gravitation.) French physicist and mathematician André-Marie Ampère derived an equation to describe this relationship; the equation became known as Ampère's law and, in recognition of Ampère's work in electromagnetism, the standard unit for measuring electrical current (the ampere) was named for him.

Like Mercury, Saturn, and several other planets in the solar system, Earth has its own magnetic field. Generated by Earth's outer core, the magnetic field is an estimated 3.5 billion years old, according to a 1980 paleomagnetic study of basalt found in Australia. The shape of Earth's magnetic field can be conceptualized by imagining the field produced by a standard magnetic dipole, such as a basic bar magnet with a “north” end and a “south” end. If one were to draw a diagram with field lines, the lines would curve from the South Pole to the North Pole, becoming virtually vertical at the poles.

Earth's magnetic field is generally tilted approximately 11 degrees from the planet's rotational axis, although it should be noted that the field is not stationary: It moves slowly and even reverses direction completely every few hundred thousand years. At the surface of the earth, the magnetic field strength averages 5.0 × 10⁻⁵ tesla (0.5 gauss), varying locally between 3.0 × 10⁻⁵ and 6.0 × 10⁻⁵ tesla (0.3 and 0.6 gauss). Measurements suggest that the strength has decreased about 10 percent in the past 150 years.

Like other planets that generate their own magnetic fields, Earth is surrounded by a magnetosphere, a region that encompasses the area of influence of the magnetic field. The magnetosphere is present above Earth's ionosphere. The shape of the magnetosphere is formed by the interactions between Earth's magnetic field and the magnetic field embedded in the solar winds that bombard it. The solar wind is a plasma stream pushed from the sun's upper atmosphere, and it is filled with charged particles, mostly protons. The magnetic field that travels with it is called the interplanetary magnetic field, or IMF, and it has a strength of approximately 2.0 × 10⁻⁹ to 5.0 × 10⁻⁹ tesla. As solar wind approaches the magnetosphere, it abruptly loses velocity when it hits a region called the bow shock. One can imagine the bow shock as a sort of invisible armor that cushions the blow of the solar wind hitting the magnetosphere.

Because of the influence of the sun, the outer edge of the “sunny” side of the magnetosphere is much closer to Earth's surface than is the magnetosphere's opposite side (about six to ten times Earth's radii compared with an estimated two hundred or more times Earth's radii on the opposite side. The longer side is referred to as the magnetotail because it extends from the planet in a tail-like manner. The magnetosphere's border, which takes on a bullet-like shape, is called the magnetopause.

Earth's Magnetic Field: A Protective Barrier

Earth's magnetic field serves an important protective purpose: It partially blocks the solar wind, a stream of charged particles from the sun, from stripping away Earth's upper atmosphere. The magnetic field cannot deflect everything, though; some particles do make their way in. Of these, some become trapped within the Van Allen radiation belt, others cause geomagnetic storms within the magnetosphere, and others reach the earth's thermosphere, causing beautiful aurorae, such as the aurora borealis, or northern lights.

Mars and Venus are good models for the harmful effects of solar wind on a planet's atmosphere. Mars shows signs of having had water billions of years ago, but it is now an empty “desert” with a low-density atmosphere. Evidence from the National Aeronautics and Space Administration's Mars Global Surveyor and older probes suggests that because Mars lacks a full protective magnetosphere, solar wind has eroded the planet's atmosphere over time. It seems that Mars had a dynamo-powered magnetic field 4 billion years ago, but for reasons unknown, that field collapsed.

Venus, a younger planet, faces a similar problem. The Venus Express, an orbiter sent by the European Space Agency in 2005, has been analyzing Venus's atmospheric ions, which are being swept away by the solar wind; many of these ions are hydrogen and oxygen. In effect, Venus is losing water to the solar wind. Earth, however, is mostly protected by its magnetosphere, which is able to deflect many of the solar wind's incoming charged particles.

The Van Allen radiation belt (actually two belts, an inner and an outer) catches charged particles from solar wind and from cosmic rays. The belts can be a nuisance, as satellites must be shielded appropriately from radiation if they will be orbiting within a belt for too long. The inner belt is located within 1.5 Earth radii of Earth's surface, whereas the outer belt spans from approximately 3 to 10 Earth radii.

Geomagnetic storms can result from coronal mass ejections (a sudden flare-up of solar wind) or other disturbances beyond and within the magnetosphere. These storms occur regularly, and depending on their severity, they can affect Earth in a variety of ways. For example, the radiation produced during a geomagnetic storm is hypothetically a lethal hazard to humans, but realistically, the radiation could affect only astronauts and, to a much lesser extent, the crews of high-altitude airplane flights. The storms also appear to affect animals, particularly pigeons, dolphins, and whales, which use magnetoception for navigation.

Some communication and navigation systems, particularly those that send signals through the ionosphere, also can be disrupted by geomagnetic storms, a particular hazard for airliners. In the days of telegraph communication, these signals could be disrupted. During some extreme geomagnetic storms, telegraph operators were shocked and receivers caught fire. When the sun is at a peak in its solar cycle, phone, television, radio, and Internet signals that depend on satellites could undergo significant service disruptions. The most extreme geomagnetic storm in recorded history was in September 1859, resulting in aurorae visible through much of the world; the storm disrupted telegraph lines and had other effects.

The solar wind does provide one arguably positive phenomenon: the notoriously beautiful northern lights and other aurorae, displays of colored lights in the sky made of charged solar wind particles interacting with the earth's magnetic field. Most aurorae span from about 100 kilometers (60 miles) to 200 or 300 kilometers (120-200 miles) above Earth's surface, although some are higher or lower. The best-known example is perhaps the aurora borealis, which appears in the skies of the Northern Hemisphere.

Dynamo Theory: and Magnetic Field Formation

Earth's molten outer core forms and maintains Earth's magnetic field. This process is explained by the dynamo theory, which was set forth in 1946 by German-born American physicist Walter M. Elsasser. In the dynamo theory, a rotating, convecting, electricity-conducting fluid generates a long-lasting magnetic field. Convection is key; without it, a magnetic field would collapse from ohmic decay in just tens of thousands of years. Good evidence also exists for the necessity of speedy rotation. Venus's core is thought to have an iron content similar to that of Earth's core, but Venus's core does not produce a magnetic field, likely because it just does not rotate fast enough. (One Venus day equals 243 Earth days.)

The dynamo theory sets forth three requirements for a fluid to generate a magnetic field: Planetary rotation must occur to create kinetic energy; an internal energy source must cause convection; and the fluid must conduct electricity. The outer core of Earth fulfills all of the necessary requirements. First, Earth's rotation powers the core's rotation through the Coriolis effect. (The outer core is a turbulent, molten sea.) Second, convection occurs within the outer core due to several heat sources, including the radioactive decay of trace elements and the presence of residual heat that is still being slowly released after the formation of the core billions of years earlier. (Compositional and thermal convection are both thought to play a role.) Third, the fluid conducts electricity.

The inner core is unable to create a magnetic field on its own. The temperature is too high, prohibiting magnetization by causing the molecules in the core's iron to adopt a randomized, rather than orderly, orientation. Some scientists believe that the inner core does play a stabilizing role to support the magnetic field.

Earth's Magnetic, Geomagnetic, and Geographic Poles: Location, Movement, and Reversal

The terms “North Pole” and “South Pole” are vague, as one must clarify whether one is referring to Earth's north and south geographic poles, the magnetic poles, or the geomagnetic poles. The geographic poles are (mostly) fixed, representing the spot on either end of Earth's rotational axis where the longitude lines meet. Because of some “wobbling” in the earth's axis, the geographic poles occasionally shift slightly on the order of a few meters. In simple terms, the geographic North Pole and the geographic South Pole are the top and bottom of the Earth, respectively.

Magnetic poles are located in the two spots where Earth's magnetic field becomes perfectly vertical. The magnetic North Pole is located in the geographic north, but it is actually the south pole of Earth's magnetic field if one looks at the directionality of the imaginary field lines. The same applies to the magnetic South Pole: It is located in the south, but it is the north pole of the magnetic field. This has important implications for the workings of compasses. Magnetic poles are not stationary poles. Because the magnetic field is generated by the turbulent convection of Earth's outer core, the field is always moving, and the poles follow.

Geomagnetic poles can be thought of as hypothetical versions of the magnetic poles. Earth's magnetic field approximates the shape of a field created by a dipole (such as an ordinary bar magnet). The geomagnetic poles are located at the two hypothetical points where the imaginary axis of the dipole-like field intersects with the earth's surface. Because the earth's field is not caused by a true dipole, there are anomalies in the field that cause the magnetic poles to vary from the hypothetical geomagnetic pole locations.

The magnetic and geomagnetic poles wander, sometimes substantially, with the turbulent and ever-changing motion of the magnetic field. In recent years, for example, the magnetic North Pole has been moving in the range of 40 kilometers (25 miles) per year, while the magnetic South Pole has been moving about 15 kilometers (9 miles) per year. On rare occasions (meaning every hundreds of thousands of years), Earth's magnetic field, and thus the pole orientations, spontaneously reverses in a process that takes several thousand years.

The last pole flip (dubbed the Brunhes-Matuyama reversal) occurred about 780,000 years ago, and geologic research shows evidence of 171 such reversals during the past 71 million years. These reversals are thought to simply indicate the chaos of the earth's churning outer core, which can cause a virtual tangled mess in the magnetic field. Some scientists theorize instead that reversals are caused by external triggers such as the impact of large objects, such as a meteorite, which could potentially disrupt the core's dynamo. The sun's magnetic field has been observed to reverse on a much faster scale, approximately every nine to twelve years.

Standard magnetic compasses have been in common use since about 250 B.C.E in ancient China. These compasses indicate the direction of the magnetic North Pole, which is typically near, but not at, “true” or geographic north. The compass needle is a magnetized bar, and its north pole is attracted to the south pole of the earth's magnetic field, which, as previously mentioned, is located in the earth's geographic north region. Magnetic compasses become virtually useless when one is near a magnetic pole; the error between the magnetic pole and corresponding geographic pole is too great.

Animals and Earth's Magnetic Field

It has long been assumed that humans do not innately sense the presence or effects of the earth's magnetic field, although research now suggests that this assumption might be worth reconsidering. Cryptochrome, a plant and animal protein that plays a role in circadian rhythms, has been demonstrated to have something to do with some animals' sense of the earth's magnetic field. Researchers were able to create transgenic Drosophila (fruit flies) that express the human version of the protein, human cryptochrome 2, rather than their own native cryptochrome. (Human cryptochrome 2 is found in human retinas.) The transgenic flies were able to detect and respond to a magnetic field. While this suggests that humans have a protein that can act as a magnetic sensor, it is still a long way from showing that human bodies can actually make use of this sensor.

There is, however, ample evidence that a variety of animals sense and use the magnetic field, particularly for migratory purposes. Turtles, pigeons, and cows are just a few species that exhibit this “extra” sense. The process is still somewhat mysterious, although several theories dominate. It is possible that some or all of these animals have receptors in their heads or elsewhere in their bodies containing a mineral called magnetite, which aligns itself with the earth's magnetic field. Magnetite has been found in the noses of some migratory fish, such as rainbow trout and salmon. Another possibility involves cryptochrome protein, a photopigment present in animals' eyes, which could react chemically with the magnetic field to provide a visual map to the animal. Research does indicate a possible connection between cryptochrome and magnetoception in some migratory birds.

Cattle and deer also apparently sense the earth's magnetic field. One study observed that when grazing or resting, they tend to align themselves with the magnetic field, facing either the magnetic North Pole or the magnetic South Pole. Two of science's most oft-studied animals, fruit flies and zebrafish, have been found to have magnetoception, so the door to future research in this area is wide open.

Principal Terms

coronal mass ejection: a sudden, large burst of solar wind that can cause geomagnetic storms and aurorae in Earth's upper atmosphere

dynamo theory: a set of three conditions that allow for a body of fluid, such as Earth's outer core, to generate a magnetic field that does not collapse from ohmic decay over time

electromagnetism: the relationship between electric fields and magnetic fields; one of four fundamental interactions in nature

geographic poles: the north and south “ends” of Earth; the spot on either end of Earth's rotational axis where the longitude lines meet

geomagnetic poles: hypothetical magnetic poles located at the points where the axis in the simplified dipole-like model of Earth's magnetic field intersects with Earth's surface; not to be confused with magnetic poles

geomagnetic storm: a type of space weather that occurs when solar wind particles penetrate Earth's magnetosphere

interplanetary magnetic field: embedded bits of magnetic field that are carried with charged particles in the solar wind

magnetic dipole: a pair of magnetic poles with equal magnitude and opposite signs (generally referred to as “north” and “south”)

magnetic field: an invisible area produced by an electric field that can exert a magnetic force on certain things in or around it

magnetic poles: the two spots where Earth's magnetic field becomes vertical; not to be confused with geomagnetic poles

magnetosphere: the area around a planetary body where the influence of a planet's magnetic field is felt

solar wind: a stream of charged particles (mostly protons and electrons) that are pushed out of the sun's upper atmosphere; can affect Earth's atmosphere when not fully blocked by the magnetosphere

Bibliography

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