Geomagnetic Reversal, Magnetic Declination & Inclination (Dip)

Magnetic Poles

  • A magnet’s North pole is thought as the pole that is attracted by the Earth’s North Magnetic Pole when the magnet is suspended so it can turn freely.
  • Since opposite poles attract, the North Magnetic Pole of the Earth is the south pole of its magnetic field.

Earth’s Magnetic field (Credits: universe-review.ca)

  • Magnetic dipole field (simple north-south field like that of a simple bar magnet) is usually aligned fairly closely with the Earth’s rotation axis; in other words, the magnetic poles are usually fairly close to the geographic poles (earth’s axis passes through these poles), which is why a compass works.
  • However, the dipole part of the field reverses after a few thousand years causing the locations of the north and south magnetic poles to switch.

The terms magnetic north and magnetic south are not to be confused with geographic north and geographic south, and geomagnetic north and geomagnetic south.

Geomagnetic Poles

  • The Geomagnetic poles (dipole poles) are the intersections of the Earth’s surface and the axis of a bar magnet hypothetically placed at the centre the Earth.
  • There is such a pole in each hemisphere, and the poles are called as “the geomagnetic north pole” and “the geomagnetic south pole”, respectively.
  • Approximately, geomagnetic dipole is currently tilted at an angle of about 11 degrees to Earth’s rotational axis.
  • On the other hand, the magnetic poles (the magnetic north pole and the magnetic south pole) are the points at which magnetic needles become vertical.
  • The difference in the position of magnetic poles and geomagnetic poles is due to the uneven and complex distribution of the earth’s magnetic field.

Wikipedia

Geomagnetic Reversal

  • A geomagnetic reversal or a reversal in earth’s magnetic field is a change in a planet’s magnetic field such that the positions of magnetic north and magnetic south are interchanged.
  • Based on palaeomagnetism (magnetism in rocks that was induced by the earth’s magnetic field at the time of their formation), it is observed that over the last 20 million years, magnetic north and south have flipped roughly every 200,000 to 300,000 years.
  • The reversal is not literally ‘periodic’ as it is on the sun, whose magnetic field reverses every 11 years.
  • The time between magnetic reversals on the Earth is sometimes as short as 10,000 years and sometimes as long as 25 million years.
  • And the time it takes to reverse could be about a few hundred or a few thousand years.
  • The magnetic poles emerge at odd latitudes throughout the process of the reversal.

Normal and Reversed Field

  • The Earth’s field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite.

Normal and Reversed field (The bar magnet at the centre represents earth’s magnetic field)

In Normal Polarity, Earth’s North Magnetic Pole is the South Pole of its Magnetic Field.

In Reverse Polarity, Earth’s North Magnetic Pole is the North Pole of its Magnetic Field.

The current location of the Magnetic Poles

  • The North and South Magnetic Poles wander (Polar Shift Theory) due to changes in Earth’s magnetic field.
  • The North Magnetic Pole (86֯ N, 172֯ W) lie to the north of Ellesmere Island in northern Canada and is rapidly drifting towards Siberia.
  • The location of the South Magnetic Pole is currently off the coast of Antarctica and even outside the Antarctic Circle.
  • Scientists suggest that the north magnetic pole migrates about 10 kilometres per year.
  • Lately, the speed has accelerated to about 40 kilometres per year and could reach Siberia in a few decades.
  • Since the Earth’s magnetic field is not exactly symmetrical, the North and South Magnetic Poles are not antipodal (a straight line drawn from one to the other does not pass through the centre of the Earth).
  • The Earth’s North and South Magnetic Poles are also known as Magnetic Dip Poles because of the vertical “dip” of the magnetic field lines at those points.
  • That is, if a magnetic compass needle is suspended freely at the magnetic poles then it will point straight down at the north magnetic pole (south pole of earth’s magnetic field) and straight up at the south magnetic pole (north pole of earth’s magnetic field).

Compass

  • A compass point north because all magnets have two poles, a north pole and a south pole, and the north pole of one magnet is attracted to the south pole of another magnet.
  • The Earth is a magnet that can interact with other magnets in this way, so the north end of a compass magnet is drawn to align with the Earth’s magnetic field.
  • Because the Earth’s Magnetic North Pole attracts the “north” ends of other magnets, it is technically the “South Pole” of our planet’s magnetic field.
  • While a compass is a great tool for navigation, it doesn’t always point exactly north. This is because the Earth’s magnetic North Pole is not the same as “true north (Earth’s Geographic North Pole).”
  • Although the magnetic declination (deviation from true north) does shift with time, this wandering is slow enough that a simple compass remains useful for navigation.

Using magnetoreception various organisms, ranging from some types of bacteria, sea turtles, some migratory birds, pigeons, etc. use the Earth’s magnetic field for orientation and navigation.

Magnetic Declination

  • Magnetic declination is the angle between magnetic north and true north.
  • It is positive when the angle derived is east of the true north, and it is considered negative when the angle measured is west of the true north.
  • In which direction would a compass needle point if you were standing on the true North Pole?

Magnetic Declination (Odder or GPL, via Wikimedia Commons)

Magnetic Declination

  • Importance: Ships and other long-distance means of transport that rely on the compass for navigation should do necessary corrections to account for magnetic declination at different latitudes and longitudes to stay in the right course.

Magnetic deviation is the error of a compass needle due to the influence of nearby metallic objects.

Magnetic Inclination or Magnetic Dip

  • Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the Earth’s magnetic field lines.
  • In simple terms, magnetic inclination is the angle made by a compass needle when the compass is held in a vertical orientation.
  • The magnetic equator is the irregular imaginary line, passing round the earth near the equator, on which a magnetic needle has no dip (because magnetic field lines are parallel to the horizontal at the equator).
  • Again, the magnetic equator, like the magnetic field and poles, is not fixed.

From Marshak, S., 2001, Earth: Portrait of a Planet: New York, W.W. Norton. via Rutgers.edu

  • Magnetic dip at the magnetic equator is 0֯, and at the magnetic poles, it is 90֯.
  • Importance: The phenomenon of magnetic dip is important in aviation, as it causes the aeroplane’s compass to give erroneous readings during banked turns and airspeed changes. Necessary corrections need to be made to the compass reading to stay in the right course.
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