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Magnetism of planets

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Magnetic field of earthEdit

File:Dipole field.svg
Main article: Earth's magnetic field

The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[1][2]

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.[3]

Magnetic field of MoonEdit

Main article: Magnetic field of the Moon
Moon ER magnetic field

Total magnetic field strength at the surface of the Moon as derived from the Lunar Prospector electron reflectometer experiment

The Moon has an external magnetic field of the order of one to a hundred nanotesla—less than one hundredth that of the Earth, which is 30–60 microtesla. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo in its core), and the magnetizations that are present are almost entirely crustal in origin.[4] One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the giant impact basins. It has been proposed that such a phenomenon could result from the free expansion of an impact generated plasma cloud around the Moon in the presence of an ambient magnetic field.[5]

Magnetic field and core of MarsEdit

In 1980, The Pioneer Venus Orbiter found that Venus's magnetic field is both weaker and smaller (i.e. closer to the planet) than Earth's. What small magnetic field is present is induced by an interaction between the ionosphere and the solar wind,[6] rather than by an internal dynamo in the core like the one inside the Earth. Venus's magnetosphere is too weak to protect the atmosphere from cosmic radiation.

The lack of an intrinsic magnetic field at Venus was surprising given that it is similar to Earth in size, and was expected also to contain a dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, while its rotation is often thought to be too slow, simulations show that it is adequate to produce a dynamo.[7][8] This implies that the dynamo is missing because of a lack of convection in Venus's core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much hotter than the top. Since Venus has no plate tectonics to let off heat, it is possible that it has no solid inner core, or that its core is not currently cooling, so that the entire liquid part of the core is at approximately the same temperature.

See also: Error: Template must be given at least one article name Another possibility is that its core has already completely solidified.
See also: Error: Template must be given at least one article name

Magnetic field and magnetosphere of MercuryEdit

Mercury Magnetic Field NASA

Graph showing relative strength of Mercury's magnetic field

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s. The magnetic field strength at the Mercurian equator is about 300 nT.[9][10] Like that of Earth, Mercury's magnetic field is dipolar in nature.[11] Unlike Earth, however, Mercury's poles are nearly aligned with the planet's spin axis.[12] Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.[12]

It is likely that this magnetic field is generated by way of a dynamo effect, in a manner similar to the magnetic field of Earth.[13][14] This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal effects caused by the planet's high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.[15]

Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within the Earth,[11] is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface.[12] Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles were detected in the planet's magnetotail, which indicates a dynamic quality to the planet's magnetosphere.[11]

References Edit

  1. Fitzpatrick, Richard (2006-02-16). "MHD dynamo theory". NASA WMAP. Retrieved on 2007-02-27.
  2. Campbell, Wallace Hall (2003). Introduction to Geomagnetic Fields. New York: Cambridge University Press. pp. 57. ISBN 0521822068. 
  3. Stern, David P. (2005-07-08). "Exploration of the Earth's Magnetosphere". NASA. Retrieved on 2007-03-21.
  4. "Magnetometer / Electron Reflectometer Results". Lunar Prospector (NASA) (2001). Retrieved on 2007-04-12.
  5. Hood, L.L.; Huang, Z. (1991). "Formation of magnetic anomalies antipodal to lunar impact basins: Two-dimensional model calculations". J. Geophys. Res. 96: 9837–9846. doi:10.1029/91JB00308. 
  6. Kivelson G. M., Russell, C. T. Introduction to Space Physics, Cambridge University Press, 1995.
  7. Luhmann J. G., Russell C. T. Venus: Magnetic Field and Magnetosphere in Encyclopedia of Planetary Sciences, ed. J. H. Shirley and R. W. Fainbridge, 905–907, Chapman and Hall, New York, 1997.
  8. Stevenson, D. J., (2003). Planetary magnetic fields, Earth and Planetary Science Letters, 208, 1–11.
  9. Seeds, Michael A. (2004). Astronomy: The Solar System and Beyond (4th ed.), Brooks Cole. ISBN 0534421113. 
  10. Williams, David R. (January 6, 2005). "Planetary Fact Sheets". NASA National Space Science Data Center. Retrieved on 2006-08-10.
  11. 11.0 11.1 11.2 Beatty, J. Kelly; Petersen, Carolyn Collins; Chaikin, Andrew (1999). The New Solar System, Cambridge University Press. ISBN 0521645875. 
  12. 12.0 12.1 12.2 Staff (January 30, 2008). "Mercury’s Internal Magnetic Field". NASA. Retrieved on 2008-04-07.
  13. Gold, Lauren (May 3, 2007). "Mercury has molten core, Cornell researcher shows". Cornell University. Retrieved on 2008-04-07.
  14. Christensen, Ulrich R. (2006). "A deep dynamo generating Mercury's magnetic field". Nature 444: 1056–1058. doi:10.1038/nature05342. 
  15. Spohn, T.; Sohl, F.; Wieczerkowski, K.; Conzelmann, V. (2001). "The interior structure of Mercury: what we know, what we expect from BepiColombo". Planetary and Space Science 49 (14–15): 1561–1570. doi:10.1016/S0032-0633(01)00093-9. 

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