Earth's magnetic field (the surface magnetic field) is approximately a magnetic dipole, with one pole near the geographic north pole and the other near the geographic south pole. An imaginary line joining the magnetic poles would be inclined by approximately 11.3° from the planet's axis of rotation. The cause of the field is probably explained by dynamo theory. The magnetic field extends several tens of thousands of kilometres into space as the magnetosphere. This article is about the electromagnetic phenomenon. ... Magnetic lines of force of a bar magnet shown by iron filings on paper A magnet is an object that has a magnetic field. ... This is about the geographic meaning of North Pole. ... Location of the South Pole in the Antarctic continent. ... Dynamo theory proposes a mechanism by which a celestial body such as the earth generates a magnetic field. ... A kilometre (American spelling: kilometer) (symbol: km) is a unit of length equal to 1000 metres (from the Greek words khilia = thousand and metro = count/measure). ... For other meanings of the term space, see space. ... A magnetosphere is the region around an astronomical object, in which phenomena are dominated by its magnetic field. ...

The magnetosphere shields the surface of the Earth from the charged particles of the solar wind. It is compressed on the day (Sun) side due to the force of the arriving particles, and extended on the night side.

Magnetic poles

Magnetic declination from true north in 2000.

The location of the magnetic poles is not static but wanders as much as several miles a year. The two poles wander independently of each other and are not at directly opposite positions on the globe. Currently the south magnetic pole is further from the geographic south pole than the north magnetic pole is from the north geographic pole.

Magnetic pole positions

Field characteristics

The field is similar to that of a bar magnet, but this similarity is superficial. The magnetic field of a bar magnet, or any other type of permanent magnet, is created by the coordinated motions of electrons (negatively charged particles) within iron atoms. The Earth's core, however, is hotter than 1043 K, the Curie point temperature at which the orientations of electron orbits within iron become randomized. Such randomization tends to cause the substance to lose its magnetic field. Therefore the Earth's magnetic field is caused not by magnetised iron deposits, but mostly by electric currents (known as telluric currents). Magnetic lines of force of a bar magnet shown by iron filings on paper A magnet is an object that has a magnetic field. ... Properties The electron (also called negatron, commonly represented as e−) is a subatomic particle. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metal Group, Period, Block 8 (VIIIB), 4 , d Density, Hardness 7874 kg/m3, 4. ... Properties For alternative meanings see atom (disambiguation). ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... In physics, the Curie point, or Curie temperature, is the temperature above which a ferromagnet loses its ferromagnetic ability to possess a net (spontaneous) magnetization in the absence of an external magnetic field. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... A telluric current (sometimes referred to as Magnetotelluric) is a extremely low frequency electrical current that occurs naturally over large underground areas at or near the surface of the Earth. ...

Another feature that distinguishes the Earth magnetically from a bar magnet is its magnetosphere. At large distances from the planet, this dominates the surface magnetic field. In addition, the magnetized elements within the planetary core are undergoing rotation and are not static. A magnetosphere is the region around an astronomical object, in which phenomena are dominated by its magnetic field. ... Generally, an element is a basic part that is the foundation of something. ... The planetary core consists of the innermost layers of a planet. ... This article is about rotation as a movement of a physical body. ...

Electric currents induced in the ionosphere also generate magnetic fields. Such a field is always generated near where the atmosphere is closest to the Sun, causing daily alterations which can deflect surface magnetic fields by as much as one degree. The ionosphere is the part of the atmosphere that is ionized by solar radiation, and too tenuous to be cooled by contact with other air. ...

Magnetic field variations

Geomagnetic variations since last reversal.

The strength of the field at the Earth's surface at this time ranges from less than 30 microteslas (0.3 gauss) in an area including most of South America and South Africa to over 60 microteslas (0.6 gauss) around the magnetic poles in northern Canada and south of Australia, and in part of Siberia.

Magnetic field reversals

Recent geomagnetic reversals.

The Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 250,000 years. It is believed that this last occurred some 780,000 years ago, referred to as the Brunhes-Matuyama reversal. Past field reversals are recorded in the "frozen" magnetic domains of solidified lava that has welled up along spreading ocean floor ridges; since the sea floor spreads at a fairly constant rate, this results in broad "stripes" of sea floor from which the past magnetic field direction can be read. At least once in Earth's history, the magnetic field held a constant direction for as long as 30 million years (see: Cretaceous long normal). A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... The Cretaceous long normal was the long period of stability in the Earths magnetic field when no field reversals occurred. ...

The mechanism responsible for geomagnetic reversals is not well understood. Some scientists have produced models for the core of the Earth wherein the magnetic field is only quasi-stable and the poles can spontaneously migrate from one orientation to the other over the course of a few hundred to a few thousand years. Other scientists propose that the geodynamo first turns itself off, either spontaneously or through some external action like a comet impact, and then restarts itself with the "North" pole pointing either North or South. When the "North" reappears in the opposite direction, we would interpret this as a reversal, whereas turning off and returning in the same direction is called a geomagnetic excursion. Artists impression of a major impact event. ...

At present, the overall geomagnetic field is becoming weaker at a rate which would, if it continues, cause the field to disappear, albeit temporarily, by about 4000 AD.¹ Other sources have put the date of field collapse as early as 3000 AD. The deterioration began at least 150 years ago and has accelerated in the past several years. So far the strength of the earth's field has decreased by 10 to 15 percent. The present decrease and strength are in the normal range of variation, as shown by study of magnetic fields in rocks. Introduction to Geomagnetically Trapped Radiation was written by Martin Walt in 1994. ...

One should note that no one knows if field decay will continue in the future. Also, since a magnetic field reversal has never been observed by humans and the mechanism of field generation is not well understood, it is difficult to say what the characteristics of the magnetic field might be leading up to such a reversal. Some speculate that a greatly diminished magnetic field during a reversal period will expose the surface of the earth to a substantial and potentially damaging increase in solar radiation. However homo erectus and their ancestors certainly survived many previous reversals. Solar radiation is radiant energy emitted by the sun, particularly electromagnetic energy. ... Binomial name Homo erectus Dubois, 1894 Homo erectus (upright man) is a hominid species that is believed to be an ancestor of modern humans. ...

See also

• Dynamo theory
• Edward Sabine
• Geophysics
• Ionosphere
• Magnetohydrodynamics
• Schumann resonance
• Sherwood machine
• Solar variation
• South Atlantic Magnetic Anomaly.

Dynamo theory proposes a mechanism by which a celestial body such as the earth generates a magnetic field. ... Sir Edward Sabine (October 14, 1788 – May 26, 1883) was an Irish astronomer, scientist, ornithologist and explorer. ... Geophysics, the study of the earth by quantitative physical methods, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods. ... The ionosphere is the part of the atmosphere that is ionized by solar radiation, and too tenuous to be cooled by contact with other air. ... MHD Simulation of Solar Wind Magnetohydrodynamics (MHD), or magnetofluiddynamics, is the academic discipline which studies the dynamics of electrically-conducting fluids. ... The Schumann Resonance is a set of spectrum peaks in the ELF portion of the Earths electromagnetic field spectrum. ... 20 years of solar irradiance data from satellites Solar variation refers to fluctuation in the amount of energy emitted by the Sun. ... The South Atlantic Anomaly (or SAA) is the region where Earths inner van Allen radiation belt makes its closest approach to the planets surface. ...

References

• Discovering the Essential Universe by Neil F. Comins (2001)
• Introduction to Geomagnetically Trapped Radiation by Martin Walt (1994)

2001 is a common year starting on Monday of the Gregorian calendar. ... Introduction to Geomagnetically Trapped Radiation was written by Martin Walt in 1994. ... One might be looking for Stephen Martin Walt (Political Science). ... 1994 was a common year starting on Saturday of the Gregorian calendar, and was designated the International year of the Family. ...

External links

• USGS Geomagnetism Program (http://geomag.usgs.gov)
• NOAA page on geomagnetism (http://www.ngdc.noaa.gov/seg/geomag)
• "Will Compasses Point South?", New York Times article published July 13, 2004 (http://www.nytimes.com/2004/07/13/science/13magn.html?ex=1247457600&en=e8f37e14d213ba16&ei=5090&partner=rssuserland)
• "Why Does Earth's Magnetic Field Flip?", National Geographic (http://news.nationalgeographic.com/news/2004/09/0927_040927_field_flip.html)
• PBS NOVA: Magnetic Storm (about pole reversals) (http://www.pbs.org/wgbh/nova/magnetic/)