Earth's magnetic field

From Wikipedia, the free encyclopedia
Jump to: navigation, search

The Earth’s magnetic field is the magnetic field that surrounds the Earth. It is sometimes called the geomagnetic field.

The Earth’s magnetic field is created by the rotation of the Earth and Earth's core.[1] It shields the Earth against harmful particles in space. The field is unstable and has changed often in the history of the Earth.[2] The magnetic field creates magnetic poles that are near the geographical poles. A compass uses the geomagnetic field to find directions. Many migratory animals also use the field when they travel long distances each spring and fall. The magnetic poles will trade places during a magnetic reversal.[3]

Characteristics[change | change source]

The Earth’s geomagnetic field is created because of two things. The convective motions in the liquid conducting core inside the center of the Earth are important for making the magnetic field.[2] When the convective motions occur with the electrical currents around the Earth, the magnetic field is created.[2] The Earth’s rotation is what keeps the magnetic field up. The interaction between the convective motions and the electrical currents creates a dynamo effect.

The intensity of the magnetic field is greatest near the magnetic poles[1] where it is vertical. The intensity of the field is weakest near the equator where it is horizontal. The magnetic field’s intensity is measured in gauss.[1]

The magnetic field has decreased in strength through recent years. In the past twenty-two years, the field has decreased its strength 1.7%, on average.[2] In some areas of the field, the strength has decreased up to 10%.[2] The fast strength decrease of the field is a sign that the magnetic field might be reversing. The reversal might happen in the next few thousand years. It has been shown that the movement of the magnetic poles is related to the decreasing strength of the magnetic field.[2]

A geomagnetic reversal is when the north magnetic pole and south magnetic pole trade places. The Earth’s magnetic poles have reversed a few times throughout the Earth’s history. The magnetic reversal happens after the strength of the field reaches zero.[3] When the strength begins to increase again, it will increase in the opposite direction, causing a reversal of the magnetic poles.[3] The time it takes the magnetic field to undergo a reversal is unknown, but can last up to ten thousand years.[3] All of the Earth’s magnetic reversals are recorded in volcanic rocks also called basalt. Scientists believed that the last geomagnetic reversal occurred 780,000 years ago.[3]

Magnetosphere[change | change source]

This figure shows the magnetosphere blocking solar wind caused by the sun.

The magnetosphere is created by the magnetic field. It is the area around the Earth that acts as a shield against the harmful particles in solar wind.[4] The magnetosphere has many different layers and structures, and solar wind shapes each of these layers.[4] The interaction of solar wind and the magnetosphere also causes the Northern and Southern Lights to appear.[5] The magnetosphere is very important in protecting the Earth against solar storms[4] which increase solar wind activity. Solar storms can cause geomagnetic storms which sometimes have serious affects on the Earth.

Movement of the north magnetic pole. It is expected to pass near the north geographic pole and continue its path to Siberia

The areas in between the north and south magnetic poles are the magnetic field lines. These lines leave the Earth from the vertical point of the South and reenters through the vertical point of the North. These two vertical points are called magnetic dip poles.[1] The magnetic dip poles are commonly referred to as the magnetic poles. The magnetic poles intersect the earth at two points. The north magnetic pole intersects the Earth at 78.3 N latitude and 100 W longitude.[6] This places the north magnetic pole in the Arctic Circle. The south magnetic pole intersects the Earth at 78.3 S latitude and 142 E longitude.[6] This places the south magnetic pole in Antarctica. The magnetic poles are also where the magnetic fields are the strongest.[2]

Earth's magnetic poles[change | change source]

North magnetic pole[change | change source]

The North Magnetic Pole is the point on the surface of Earth's northern hemisphere where the planet's magnetic field points vertically downwards. There is only one place where this occurs, near to (but distinct from) the Geographic North Pole.

Its southern hemisphere counterpart is the South Magnetic Pole. Since the Earth's magnetic field is not exactly symmetrical, a line drawn from one to the other does not pass through the geometric centre of the Earth.

The North Magnetic Pole moves over time due to magnetic changes in the Earth's core.[7] In 2001, it was near Ellesmere Island in northern Canada at 81°18′N 110°48′W﻿ / ﻿81.3°N 110.8°W﻿ / 81.3; -110.8﻿ (Magnetic North Pole 2001). As of 2015, the pole is thought to have moved east beyond the Canadian Arctic territorial claim to 86°18′N 160°00′W﻿ / ﻿86.3°N 160.0°W﻿ / 86.3; -160.0﻿ (Magnetic North Pole 2012 est).[8]

The Earth's North and South Magnetic Poles are also known as Magnetic Dip Poles, referring to the vertical "dip" of the magnetic field lines at those points.[9]

Reversals of Earth's magnetic field[change | change source]

Geomagnetic polarity during the late Cainozoic era. Dark areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed

Earth does change its magnetic poles every million years (plus or minus 200,000 years). Before a change of magnetic field, the Earth's magnetic field becomes weaker and moves around, like a spinning top would before it falls. The Earth has already had hundreds of changes (flip flops). Scientists know this as a result of studies of magnetism on the sea floor, near the mid-Atlantic ridge. The lava slowly moves out of this crevasse (gap in the sea floor) and then it cools with its iron oxide molecules all pointing in the new direction of the Earth's magnetic field. We can look at the history of this magnetic field today to look back at the many flips in the past.[10]

Reversals occur at intervals from less than 0.1 million years to as much as 50 million years. The most recent geomagnetic reversal, called the Brunhes–Matuyama reversal, occurred about 780,000 years ago.[11][7] Another global reversal of the Earth's field, called the Laschamp event, occurred during the last ice age (41,000 years ago). However, because of its brief duration it is called an "excursion".[12][7]

Migratory animals[change | change source]

Animals which take long migrations may depend on the magnetic field for a guide.[5]

Some migratory animals know their locations by the intensity of the field.[13] They know the time because of circadian rhythms the field produces. Migratory animals are born with a magnetic map in their head that allows them to migrate great distances safely.[14] Their ability to sense the magnetic field is because of magnetic particles. Other animals have a chemical compass based on a radical pair mechanism.[13][15]

References[change | change source]

  1. 1.0 1.1 1.2 1.3 Zvereva T.I. (2012). "Motion of the Earth's magnetic poles in the last decade". Geomagnetism and Aeronomy 52 (2): 278-286.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Dergachev V.A. et al (2012). "Impact of the geomagnetic field and solar radiation on climate change". Geomagnetism and Aeronomy 52 (8): 959-976.
  3. 3.0 3.1 3.2 3.3 3.4 Markove, Marko S. (2011). "How living systems recognize applied electromagnetic fields". The Environmentalist 31 (2): 89-96.
  4. 4.0 4.1 4.2 Dergachev V.A. et al (2011). "The connection between cosmic rays and changes in the geomagnetic field and the Earth’s climate". Bulletin of the Russian Academy of Sciences:Physics 75 (6): 847-850.
  5. 5.0 5.1 Mikhailova G.A. & Smirnov S.E. (2011). "Effects of geomagnetic disturbances in the near Earth’s atmosphere and possible biophysical mechanism of their influence on the human cardiovascular system". Izvestiya, Atmospheric and Oceanic Physics 47 (7): 805-818.
  6. 6.0 6.1 Bertolotti, Mario. "The Earth’s magnetic field and the geomagnetic effects". Celestial messengers: cosmic rays: the story of a scientific adventure. Astronomers' Universe. Springer. pp. 75-103. ISBN 978-3642283703.
  7. 7.0 7.1 7.2 Merrill, Ronald T.; McElhinny, Michael W.; McFadden, Phillip L. (1996). "Chapter 8". The magnetic field of the earth: paleomagnetism, the core, and the deep mantle. Academic Pres]. ISBN 978-0-12-491246-5.
  8. World Data Center for Geomagnetism, Kyoto. "Magnetic North, Geomagnetic and Magnetic Poles". Retrieved 2012-07-03.
  9. "The Magnetic North Pole". Ocean bottom magnetology laboratory. Woods Hole Oceanographic Institution. Retrieved June 2012.
  10. Vacquier, Victor (1972). Geomagnetism in marine geology (2nd ed.). Amsterdam: Elsevier Science. p. 38. ISBN 9780080870427.
  11. Phillips, Tony (2003). "Earth's inconstant magnetic field". Science@Nasa. Retrieved 27 December 2009.
  12. "Ice Age polarity reversal was global event: extremely brief reversal of geomagnetic field, climate variability, and super volcano". ScienceDaily. 2012. doi:10.1016/j.epsl.2012.06.050. Retrieved 21 March 2013.
  13. 13.0 13.1 Scott, Rebecca, Robert Marsh, and Graeme C. Hays (2012). "A little movement orientated to the geomagnetic field makes a big difference in strong flows". Marine Biology 159 (3): 481-488. Invalid <ref> tag; name "r" defined multiple times with different content
  14. Wiltschicko, Wolfgang and Roswitha Witschko (2005). "Magnetic orientation and magnetoreception in birds and other animals". Journal of Comparative Physiology 191 (8): 675-693.
  15. Lehikoinen, Aleksi and Kim Jaatinen (2011). "Delayed Autumn migration in northern european waterfowl". Journal of Ornithology 153 (2): 563-570. Retrieved 26 February 2013.