Earth's Magnetic Field - Definition, Causes, Components

If you've ever used a compass (either a classic mechanical one or one incorporated into your smartphone), you'll know that it always points north. If you hang a refrigerator magnet from the ceiling, it will also point north. This implies that the ground beneath your feet generates a magnetic field all around us!

What is a Magnet?

Magnet is any material that has its own magnetic field. The magnetic field is a vector in nature and is present in the neighborhood of a magnet, electric current, or a changing electric field. All the magnetic properties are present inside the magnetic field and it’s the component of the electromagnetic force, one of the fundamental forces of nature. The magnetic field is invisible but it’s the only thing responsible for the most notable property of a magnet, pulling objects towards it.

The magnet is mainly divided into three categories that are:

1. Permanent magnet - A magnet made from a material that is permanently magnetized and creates its own consistent magnetic field. Materials that can be magnetized are called ferromagnetic. Examples of such materials are iron, nickel, and cobalt.  
2. Temporary magnet - Magnet that remains magnetized for a short period of time. Materials used to make these magnets are generally soft materials having low magnetic properties. When a strong or permanent magnet attracts them, they become magnetized for a short duration.
3. Electromagnet - A magnet in which a magnetic field is produced by the current flowing in it. The most common electromagnet is the wire wound into a coil; when current flows through the wire, a magnetic field is created inside the coil. It disappears as soon as the current is cut off. Electromagnets are mainly used in devices such as motors, generators, and hard disks.

Magnetism is the force of pull or push when two magnets are kept in each other’s vicinity.

Earth's Magnetism

As we all know, the pin of a compass always points in the North direction. All compasses have a magnet inside them, and if a magnet is moving on its own, that means it's under the influence of some magnetic field. This suggests we have a magnetic field around us all the time. This field is generated by the Earth.  

The Earth’s magnetic South pole is at the geographical North pole, and the magnetic North pole is at the geographical South pole. This is the reason why the compass's magnet points towards the North direction, as the earth magnet’s South pole, attracts the compass's North pole.

The Earth’s magnetic field goes up to millions of kilometers into deep space, and it appears more like a bar magnet. This magnetic field is extended to very long distances and is very wide, but it’s very weak in terms of strength, it's only 40000 nT, whereas a common magnet is of 10000000 nT. Earth’s magnetic field is very necessary for us, as it works as a protective layer for Earth, it also repels and traps Sun’s charged particles.  

Causes of Earth’s Magnetism

• Earth’s core is very hot and contains so many minerals and metals in a molten state. Such minerals are molten iron and nickel. Earth’s core is very hot and it keeps boiling these minerals continuously, this continuous heating causes convection in them, which produces a process known as convection currents. These currents carry the charged particles and produce the magnetic field.
• The magnetic field deflects the ionized charged particles coming from the sun, called the solar wind. These winds are capable of entering our atmosphere and slowly destroying it. Magnetic fields prevent them from entering into Earth’s atmosphere and make life possible on Earth. Mars does not have its magnetic field, so life is not possible there.
• As stated above, Earth’s magnetic poles do not coincide with geographical poles. The Magnetic South pole lies in Canada whereas the Magnetic North pole lies in Antarctica. Magnetic poles are inclined by 10 degrees to Earth’s rotational axis.  

Theory of Earth’s Magnetism

There are two theories present for the possible reason of earth’s magnetism, they are:

1. The Dynamo Effect - The outer part of Earth’s core has Iron and other heavy metals in a molten state. The influence of gravity solidifies the inner core. The molten state metals in the outer layer move, causing an electric current. This electric current produces magnetic field lines. This effect is known as the Dynamo effect.
2. Ionization of outer layer - As we know, Earth rotates on its own axis as well as around the Sun. This rotation produces electric current as the outer layer of Earth is ionized. These ions move, so they produce magnetism. But, this magnetic field is very weak, so the Dynamo effect is a more acceptable theory.

Components of Earth’s Magnetism

The direction and magnitude of Earth’s magnetic field depends on three components:

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1. Magnetic declination - As we know, magnetic North and geographical North do not coincide on Earth. The angle between magnetic North and geographical North is called Magnetic declination. On the horizontal plane, geographical North is never fixed, it changes depending upon the position on Earth’s surface, so Magnetic declination also changes according to the position on Earth.

2. Magnetic inclination - The angle made by the horizontal plane on Earth’s surface is known as Magnetic inclination. It is also known as the angle of dip. At the magnetic equator, the angle of dip is 0 degrees, and at magnetic poles, it’s 90 degrees.

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3. Horizontal component of the Earth’s magnetic field - There are two components that explain the magnitude of Earth’s magnetic field:

• Horizontal Component (H), and
• Vertical Component (V).

Formula for Earth’s Magnetic Field

The magnetic intensity of the earth's magnetic field forms an angle with the horizontal axis known as the Angle of Dip (δ). The strength of the Earth's magnetic field may be divided into two components:

• tan A = B_V/B_H,
• sin A = B_V/B, and 
• cos A = B_H/B

As we know, sin²A + cos²A = 1

⇒ B_H²/ B² + B_V²/B²  = 1

Thus, 

B = (B_H² + B_V²)^1/2

Variations in Earth’s Magnetic field

1. Secular Variation: In every 960 years, a change occurs on the magnetic axis, because of its spin around its own axis.
2. Daily and Annual Variation - On a daily basis, Sun’s Ultraviolet rays ionize the Earth’s atmosphere. As a result, the current is generated and this current produces magnetic fields. This is a daily and annual variation.
3. Eleven-year Sunspot cycle - There’s a region on the Sun where a strong Magnetic field is present. Every eleven years, Earth faces that region, due to this Earth’s Magnetic field faces variations.
4. Lunar Variations - Along with the Sun, the Moon also has a great influence on Earth’s Magnetic activity. During a lunar eclipse, the earth’s ionized layer gets into tidal motions, this causes variations in Earth’s magnetic field. This variation is called Lunar variation.  
5. Irregular and Aperiodic Variation - Sometimes, the Solar activity of the Sun is more active and the radiation reaching during this time, ionizes the atmosphere even more. This causes current when Earth revolves and results in the magnetic field.

Sample Problems

Problem 1: Why does the Earth have a magnetic field?

Solution:

There are several theories present for this however, only two are major theories. Current generated due to motion of molten elements inside Earth is responsible for Magnetic field. The movement of ionized particles in Earth’s atmosphere is responsible for producing current, which produces a magnetic field.

Problem 2: Does the Earth’s Magnetic field varies at different locations?

Solution:

Yes, Earth’s magnetic field depends on the location and time. The magnetic field is observed using satellites and observatories around the world.

Problem 3: What is geomagnetic reversal?

Solution:

The geomagnetic reversal occurs when the magnetic North pole and magnetic South pole are interchanged.

Problem 4: What are the horizontal components of the earth’s magnetic field and angle of dip at the place where a magnetic needle is kept free to rotate in a vertical plane?

Solution:

As the magnetic needle is kept free to rotate, the horizontal component is 0 degrees. The angle of dip at such a place would be 90 degrees.

Problem 5: When is the angle of dip equal to 45 degrees at a place?

Solution:

Angle of dip is 45 degrees at the place where vertical and horizontal components of Earth’s magnetic field are equal.