Earth's magnetic field

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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. (Image not to scale.)

Earth's magnetic field (and the surface magnetic field) is approximately a magnetic dipole, with one pole near the 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.

[edit] Magnetic poles

Magnetic declination from true north in 2000.

Two different types of magnetic poles must be distinguished. There are the "magnetic poles" and the "geomagnetic poles". The magnetic poles are the two positions on the Earth's surface where the magnetic field is entirely vertical. Another way of saying this is that the inclination of the Earth's field is 90° at the North Magnetic Pole and -90° at the South Magnetic Pole. A typical compass that is allowed to swing only in the horizontal plane will point in random directions at either the South or North Magnetic Poles.

The Earth's field is closely approximated by the field of a dipole positioned at the centre of the Earth. A dipole defines an axis. The two positions where the axis of the dipole that best fits the Earth's field intersect the Earth's surface are called the North and South geomagnetic poles. If the Earth's field were perfectly dipolar, the geomagnetic and magnetic poles would coincide. However, there are significant non-dipolar terms which cause the position of the two types of poles to be in different places.

The locations of the magnetic poles are not static but wander as much as 15 km every year (Dr. David P. Stern, emeritus Goddard Space Flight Center, NASA ^{[citation needed]}). The pole position is usually not that which is indicated on many charts. The Geomagnetic Pole positions are usually not close to the position that commercial cartographers place "Magnetic Poles." "Geomagnetic Dipole Poles", "IGRF Model Dip Poles", and "Magnetic Dip Poles" are variously used to denote the magnetic poles. ^[1]

The Earth's field is changing in size and position. The two poles wander independently of each other and are not at directly opposite positions on the globe. Currently the magnetic south pole is farther from the geographic south pole than the magnetic north pole is from the geographic north pole.

Magnetic pole positions

[edit] 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 spins of electrons and nuclei within iron atoms. The Earth's core, however, is hotter than 1043 K, the Curie point temperature at which the orientations of spins within iron become randomized. Such randomization causes the substance to lose its magnetic field. Therefore the Earth's magnetic field is caused not by magnetized iron deposits, but mostly by electric currents in the liquid outer core (as it spins, electrons tend to stay behind, thus producing a small current).

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. 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.

[edit] Magnetic field variations

Geomagnetic variations since last reversal.

The strength of the field at the Earth's surface 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.

Magnetometers detect minute deviations in the Earth's magnetic field caused by iron artifacts, kilns, some types of stone structures, and even ditches and middens in archaeological geophysics. Using the magnetic instruments adapted from airborne devices developed during World War II to detect submarines, the magnetic variations across the ocean floor have been mapped. The basalt -- the iron-rich, volcanic rock making up the ocean floor -- contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. The distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations have provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials record the Earth's magnetic field.

Frequently, the Earth's magnetosphere is hit by solar flares causing geomagnetic storms, provoking displays of aurorae. The short-term instability of the magnetic field is measured with the K-index.

[edit] Magnetic field reversals

Main article: geomagnetic reversal

Based upon the study of lava flows of basalt throughout the world, it has been proposed that 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. The last such event, called the Brunhes-Matuyama reversal, is theorized to have occurred some 780,000 years ago.

There is no clear theory as to how the geomagnetic reversals might have occurred. 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 magnetic "North" pole pointing either North or South. External events are not likely to be routine causes of magnetic field reversals due to the lack of a correlation between the age of impact craters and the timing of reversals. Regardless of the cause, when magnetic "North" reappears in the opposite direction this is a reversal, whereas turning off and returning in the same direction is called a geomagnetic excursion.

One theory does contend that the core of the Earth is not iron^{[citation needed]} but much denser atoms. Nuclear reactions as replicated in a fast breeder reactor are suggested to take place and this accounts for the change in the Earth's magnetic field.^{[citation needed]}

Using a magnetic detector (a variant of a compass), scientists have measured the historical direction of the Earth's magnetic field, by studying sequences of relatively iron-rich lava flows. Typically such layers have been found to record the direction of Earth's magnetic field when they cool (see paleomagnetism). They have found that the poles have shifted a number of times throughout the past.

[edit] Magnetic field electrogenerators

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Some free-energy enthusiasts claim that the Earth's magnetic field could be used to generate power^[4], but such claims are regarded as pseudoscience by many skeptics. Many designs for using the Earth's electromagnetic field and atmospheric electricity have been researched, but have failed to gain any widespread acknowledgement in the scientific community. There is also some energy stored in the form of separated electrical charges, which can provide low direct currents at high voltages. However, ordinary electric motors cannot use this energy directly as a prime mover. Benjamin Franklin developed several motors that used the Earth's fields. Oleg D. Jefimenko has researched several machine designs for tapping the Earth's electromagnetic field.

The Earth's magnetic field can be used as the starting field for a self-excited electric generator. Cromwell Varley discovered in 1867 that an electric generator did not need to be started with a conventional prime mover. He used the Earth's magnetic field to induce enough field strength in the stator windings to get a generator running. ^[5]

Electrodynamic tethers can induce a current by moving through the planet's magnetic field. When the conductive tether is trailed in a planetary or solar magnetosphere (magnetic field), the tether cuts the field, generates a current, and thereby slows the spacecraft into a lower orbit. The tether's end can be left bare, and this is sufficient to make contact with the ionosphere and allow a current to flow through a phantom loop. A cathode tube may also be placed at the end of the tether. The cathode tube will interact with the planet's magnetic field^{[citation needed]} in the vacuum of space. A double-ended cathode tube tether will allow alternating currents.^{[citation needed]}

[edit] Magnetic field detection

The earth's magnetic field strength was measured by Carl Friedrich Gauss in 1835 and has been repeatedly measured since then, showing an exponential decay with a half-life of about 1400 years.^{[citation needed]} This indicates a relative decay of about 7 % over the last 150 years.

Animals including birds and turtles can detect the Earth's magnetic field, and use the field to navigate during migration.

[edit] References and further readings

General

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

Field characteristics

• Temperature of the Earth's core, US Dept of Energy.

Citations

1. ^ "Problem with the "MAGNETIC" Pole Locations on Global Charts". Eos Vol. 77, No. 36, American Geophysical Union, 1996.
2. ^ Geomagnetism, North Magnetic Pole. Natural Resources Canada, 2005-03-13.
3. ^ South Magnetic Pole. Commonwealth of Australia, Australian Antarctic Division, 2002.
4. ^ C. L. Stong, "Electrostatic motors are powered by electric field of the Earth". October, 1974. (PDF)
5. ^ Bunch, Bryan, and Alexander Hellemans, "The History of Science and Technology: A Browser's Guide to the Great Discoveries, Inventions, and the People Who Made Them from the Dawn of Time to Today". ISBN 0-618-22123-9

Further readings

• Wait, J.R., "On the relation between telluric currents and the earth’s magnetic field", Geophysics, 19, 281-289, 1954.
• Towle, J. N.,"The Anomalous Geomagnetic Variation Field and Geoelectric Structure Associated with the Mesa Butte Fault System, Arizona". Geological Society of America, Bulletin, 95:221, 1984.

[edit] See also

• Dip circle
• Earth radiation
• Earth battery
• Van Allen radiation belt

[edit] External links

Wikimedia Commons has media related to: Earth's magnetic field

• USGS Geomagnetism Program. Real time monitoring of the Earth's magnetic field. U.S. Department of the Interior, U.S. Geological Survey, February 17, 2005.
• Geomagnetism. National Geophysical Data Center, NOAA. Apr-2005.
• BGS Geomagnetism. Information on monitoring and modelling the geomagnetic field. British Geological Survey, August 2005.
• William J. Broad, "Will Compasses Point South?". New York Times, July 13, 2004.
• John Roach, "Why Does Earth's Magnetic Field Flip?". National Geographic, September 27, 2004.
• "Magnetic Storm". PBS NOVA, 2003. (ed. about pole reversals)
• "When North Goes South". Projects in Scientific Computing, 1996.
• "3D Earth Magnetic Field Charged-Particle Simulator" Tool dedicated to the 3d simulation of charged particles in the magnetosphere.. [VRML Plug-in Required]

[edit] References

• Herndon, J. Marvin (1996) Substructure of the inner core of the Earth Vol. 93, Issue 2, 646-648, January 23, 1996, PNAS
• Hollenbach, D. F. ,dagger and J. M. HerndonDagger (2001) Deep-Earth reactor: Nuclear fission, helium, and the geomagnetic field Published online before print September 18, 2001, 10.1073/pnas.201393998, September 25, 2001, vol. 98, no. 20, PNAS