Earth’s Magnetism

The Earth’s magnetic field is identical to a bar magnet angled 11 degrees from the Earth’s rotational axis. The difficulty with that depiction is that iron’s Curie temperature is around 770 degrees Celsius. Because the Earth’s core is hotter, it is not magnetic. So, where did the Earth’s magnetism come from? Keep reading to know more about what is earth’s magnetism in detail. 

What is Earth’s Magnetism? 

The earth’s magnetic field lines mimic a (hypothetical) magnetic dipole positioned at its centre. The dipole’s axis does not correspond with the earth’s axis of rotation, but it is now titled by approximately 11.3 degrees concerning the latter. The magnetic poles, in this view, are the points at which the dipole’s magnetic field lines enter or depart the earth. 

The magnetic South pole of the Earth is located at the geographical North pole, while the magnetic North pole is located at the geographical South pole. The earth’s magnetic South pole tends to attract the compass’s North pole, which is why the compass’s magnet faces north.

Theories of Earth’s Magnetism

There are multiple theories regarding Earth’s magnetism. However, two are more prominent- 

1. The dynamo theory 

2. The ionisation theory

Dynamo Theory of Earth’s Magnetism 

The dynamo theory describes how a celestial entity, such as the Earth or a star, generates and maintains a magnetic field over long time scales (millions of years). Key points describing the earth’s magnetism are:

• Convection in the outer core, together with the Coriolis effect (caused by the earth’s rotation), produces a self-sustaining (geodynamo) magnetic field, according to dynamo theory.

• The Earth’s magnetic field is formed in the planet’s outer core. The metal in the outer core is fluid because the outer core has a lower pressure than the inner core. The temperature of the outer core varies between 4400 and 6000 degrees Celsius near the inner core.

• Variations in temperature, pressure and composition inside the outer core generate convection currents in the molten iron, as the cool, dense matter sinks and heated, less dense matter rises

• The movement of liquid iron generates electric currents, which eventually form magnetic fields

• Charged metals passing across these fields generate electric currents, which keep the cycle going. Geodynamo is the name given to this self-sustaining cycle.

• Independent magnetic fields are virtually aligned in the same direction due to the spiral movement of charged particles caused by the Coriolis force, culminating in the planet’s single massive magnetic field. 

Ionisation of the Outer Layer  

Earth revolves both on its axis and around the Sun, as we all know. As the Earth’s outer layer gets ionised, this spinning generates an electric current. Because these ions are moving, they produce magnetic field. However, the Dynamo effect is a more plausible explanation because this magnetic field is so faint. 

Meaning of True north

True north is the direction along the earth’s surface that leads to the true North Pole, also known as the geographical North Pole. It’s also known as geodetic north and it’s distinct from magnetic north, the direction shown by a compass and grid north, which is the direction indicated by grid lines pointing north. 

Meaning of  Magnetic North 

The north direction, shown by a compass needle, runs parallel to the earth’s magnetic field.

Declination

On Earth, magnetic and geographical north do not correspond. Magnetic declination is the angle formed by magnetic North and geographical North. Geographic North is never fixed in the horizontal plane; it shifts with the location of the Earth’s surface and magnetic declination follows . Thus, the angle between magnetic north (the direction the northern end of a compass needle points) and true north is known as magnetic declination. When magnetic north is east of true north, the declination is positive.

Inclination

The angle formed by a compass needle held vertically is known as magnetic inclination. At the place of measurement, positive inclination values imply that the field is pointing downward into the Earth. Thus, the magnetic inclination is the angle formed by a horizontal plane on the Earth’s surface. It’s also called the dip angle. The angle of dip is 0 degrees at the magnetic equator and 90 degrees at the magnetic poles.

The Horizontal Component of Earth’s Magnetic Field

The earth’s magnetic intensity is composed of a horizontal component and a vertical component.

The horizontal component of the earth’s magnetic field, BH, represents the magnetic field on the earth’s surface. The magnetic field of the Earth changes with longitude and latitude.

tan= B(v)B(h) 

sin= B(v)B 

cos= B(h)B 

sin2 +cos2 =B(v) 

2B2+ B(h)2B2 

B=B(h)2 + B(v)2 

Where B is a vector component and indicates the entire magnetic field intensity or strength. 

B can also be written in the following way: 

B=X2+Y2+Z2 

Where X, Y, and Z indicate the magnetic field components along the geographic north, 

Geographically east and vertically downward are the two directions. 

The formula below can be used to determine X and Y. 

Y=Hsin and X=Hcos 

H is equal to (X2+Y2) and indicates the part of the magnetic field that is parallel to the earth’s surface. 

𝛂 represents the magnetic declination angle (the angle between true and magnetic north). 

It is equal to tan-1(XY). 

Also, θ is the magnetic inclination or the angle of the horizontal component of the magnetic field. It is equal to tan-1(ZH) 

Factors affecting Magnetic Field of Earth  

The Earth’s magnetism is described by seven factors. 

• The North (X) and East (Y) element 

• Declination (D) 

• Inclination (I)

• Horizontal intensity (H)

• Vertical intensity (Z) 

• Total intensity (F)

• Declination (D)

D and I are both graded on a scale of one to ten. The units of measurement for all other rudiments are nanoteslas (nT; 1 nT = 10-9 Tesla).  

The following simple formulations link the seven elements together:  

• Declination (D) 

D = tan − 1 (YX)  

• Inclination (I) 

 I = tan − 1 (ZH)  

• Horizontal (H) 

H = √ X2 Y2 

• North (X) 

 X = Hcos (D) 

• East (Y) 

Y = Hsin (D) 

• Intensity (F) 

 F = √ X2 Y2 Z2 

Changes in the Earth’s Magnetic Field 

• Secular Variation – The magnetic axis of the earth’s magnetism changes every 960 times due to the earth’s rotation on its axis.

• Daily and Annual Variation– The Sun’s ultraviolet radiation ionises the Earth’s atmosphere daily and as a result, electricity is created, which causes magnetic fields. This varies on a day-to-day and periodic basis.  

• Eleven-year Sunspot Cycle– On the Sun, there’s a zone with a strong magnetic field on the Sun. Earth rotates about the position every eleven times due to oscillations in the Earth’s magnetism.  

• Lunar Variations-The Moon, like the Sun, has a significant impact on Earth’s magnetic field. During a lunar eclipse, the earth’s ionised stratification undergoes tidal movements, causing oscillations in the earth’s magnetic field. 

Conclusion 

Because magnetic fields surround electric currents, we believe the Earth’s magnetism is created by circulating electric currents in the Earth’s molten metallic core. A current ring produces a field that resembles that of the earth. The magnitude varies depending on where you’re on the surface of the earth.