Magnetic Field of Earth-Earth՚s Magnetism, Dynamo Effect (For CBSE, ICSE, IAS, NET, NRA 2022)

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Magnetic Field of Earth - Earth՚s Magnetism

  • Earth՚s magnetic field is approximately a magnetic dipole, with the magnetic field S pole near the Earth՚s geographic north pole and the other magnetic field N pole near the Earth՚s geographic south pole
  • This makes the compass usable for navigation.
  • The earth՚s magnetic field extends millions of kilometers into outer space and looks very much like a bar magnet.
  • The cause of the field can be explained by dynamo theory.
  • A magnetic field extends infinitely, though it weakens with distance from its source.
  • The earth՚s south magnetic pole is actually near the North Pole and the magnetic north pole is in Antarctica! This is why a compass magnet՚s north pole actually points north.
  • The Earth՚s magnetic field extends far and wide but is very weak in terms of field strength. A mere 40,000 nT compared to a refrigerator magnet which has a strength of 10
  • A paleomagnetic study of Australian red dacite and pillow basalt has estimated the magnetic field to be at least 3.5 billion years old.

Theory of Earth՚s Magnetism: Dynamo Effect

Dynamo Effect
  • The Earth՚s magnetic field is attributed to a dynamo effect of circulating electric current, but it is not constant in direction. Rock specimens of different age in similar locations have different directions of permanent magnetization.
  • The earth gets its own magnetic field lines because of the presence of the metallic fluids that are present at the outer core as well as in the inner core.
  • The outer core consists of molten iron while the inner core has the solidified elements.
  • Evidence for 171 magnetic field reversals during the past 71 million years has been reported.
  • Although the details of the dynamo effect are not known in detail, the rotation of the Earth plays a part in generating the currents which are presumed to be the source of the magnetic field.
  • Mariner 2 found that Venus does not have such a magnetic field although its core iron content must be similar to that of the Earth.
  • Venus՚s rotation period of 243 Earth days is just too slow to produce the dynamo effect.

Causes of Earth՚s Magnetism

  • Earth՚s Magnetism is generated by convection currents of molten iron and nickel in the earth՚s core.
  • These currents carry streams of charged particles and generate magnetic fields.
  • This magnetic field deflects ionizing charged particles coming from the sun (called solar wind) and prevents them from entering our atmosphere.
  • The solar wind is responsible for the overall shape of Earth՚s magnetosphere, and fluctuations in its speed, density, direction, and entrained magnetic field strongly affect Earth՚s local space environment.
  • These phenomena are collectively called space weather. The mechanism of atmospheric stripping is caused by gas being caught in bubbles of magnetic field, which are ripped off by solar winds. Variations in the magnetic field strength have been correlated to rainfall variation within the tropics
  • Without this magnetic shield, the solar wind could have slowly destroyed our atmosphere preventing life on earth to exist.
  • Mars does not have a strong atmosphere that can sustain life because it does not have a magnetic field protecting it.
  • The earth՚s magnetic poles are not aligned to the actual geographic north and south poles. Instead, the magnetic South Pole is in Canada while the magnetic north pole lies in Antarctica.
  • The magnetic poles are inclined by about 10 degrees to the earth՚s rotational axis. So, all this time your compass was really pointing to Canada, not the true North!
  • The magnetic North pole is located to the south in Northern Canada; the geographic South pole is at the center of the Antarctic continent, but the magnetic pole is hundreds of miles away, near the coast.
  • In regions near the magnetic poles, compasses are virtually useless.

Components of Earth՚s Magnetic Field

  • Magnetic declination
  • Magnetic inclination or the angle of dip
  • Horizontal component of the earth՚s magnetic field

Magnetic Declination

  • It is defined as the angle between the true north and the magnetic north.
  • The true north is never at a constant position and keeps varying depending upon the position on the earth՚s surface and time.

Magnetic Inclination

  • It is also known as the angle of dip.
  • It is the angle made the horizontal plane on the earth՚s surface.
  • At the magnetic equator, the angle of dip is and at the magnetic poles, the angle of dip is .

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