Magnetic Field Of Earth - Earth's Magnetism

If you have ever used a compass (either the traditional mechanical one or the one built in your Smartphone), then you know it always points north. If you suspend a refrigerator magnet, it will also point north. This suggests that there is a magnetic field all around us generated by the earth under your feet!

The earth’s magnetic field extends millions of kilometres into outer space and looks very much like a bar magnet. 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 (north and south poles attract). 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

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 107 nT!


Theory of Earth’s Magnetism

There is one theory that explains how the earth’s magnetism is caused:

  1. Dynamo effect: 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.

What Causes 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 ionising charged particles coming from the sun (called solar wind) and prevents them from entering our atmosphere. 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!

Magnetic Field

Components of Earth’s Magnetic Field

There are three components that are responsible for the magnitude as well as the direction of the earth’s magnetic field:

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

Earth's Magnetic Field

Magnetic Declination

The magnetic declination is defined as the angle between the true north and the magnetic north. On the horizontal plane, 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

The magnetic inclination 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 0° and at the magnetic poles, the angle of dip is 90°.

Horizontal Component of the Earth’s Magnetic Field

There are two components to explain the intensity of the earth’s magnetic field:

  • Horizontal component (H)
  • Vertical component (v)
\(tan \delta =\frac{B_{v}}{B_{H}}\)
 
\(sin \delta =\frac{B_{v}}{B}\)
 
\(cos \delta =\frac{B_{H}}{B}\)
 
\(sin^{2}\delta +cos^{2}\delta =\frac{B_{H}^{2}}{B^{2}}+\frac{B_{v}^{2}}{B^{2}}\)
 
\(1=\frac{B_{H}^{2}}{B^{2}}+\frac{B_{v}^{2}}{B^{2}}\)
 
\(B=\sqrt{B_{H}^{2}}+B_{v}^{2}\)
 

Summary

Component Definition Description
B Total Magnetic-field Strength vector B=(X2 + Y2 + Z2)
X Magnetic field component along Geographic North direction X = H cos α
Y Magnetic field component along Geographic East direction Y = H sin α
Z Magnetic field component pointing vertically downwards
H Magnetic field component parallel to the earth’s surface (Points towards the magnetic south pole) H = (X2 + Y2)
α Magnetic declination: Angle between true north and magnetic north α = tan1YX
θ Magnetic Inclination: Angle measured from horizontal to magnetic field vector. It is 90 deg at magnetic poles. θ = tan1ZH

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