The Earth is a rather amazing thing. It possesses all the beauty there is to offer, and also the deadliest forces which can wipe out entire countries in a trice (instant).

Then, there is a force which is not quite dangerous as many others, but still plays a vital role when it comes to our existence on this planet. This is the Earth's magnetic field.

You are sure to have found the behaviour of the compass and images of Northern Lights extremely interesting. These phenomena (occurrences) are directly related to the Earth's magnetic field. Is there a giant magnet inside the Earth's core? Let's dig in to find out more details.

The Earth's magnetosphere

Solar winds are occurrences which can bring considerable harm to planets nearby. A solar wind is a stream of ionised (converted into electrically charged particles) gases blown outward from the Sun at about 400 kilometres per second, and that varies in intensity with the amount of surface activity on the Sun. If this is the case, how has Earth survived up to now?

The Earth's magnetic field is the shield that has protected us from much of the solar wind. When the solar wind encounters the Earth's magnetic field, since it is made of charged particles, it is deflected (turned aside), like water around the bow of a ship.

The imaginary surface at which the solar wind is first deflected is called the 'bow shock'. The

 corresponding region of space sitting behind the bow shock and surrounding the Earth is called the 'magnetosphere'.

It represents a region of space dominated by the Earth's magnetic field where it largely prevents the solar wind from entering.

However, some high energy charged particles from the solar wind leak into the magnetosphere and these are trapped in the Van Allen belts.

The regions of the Earth that are surrounded by two regions of particularly high concentrations of trapped charged particles are called the Van Allen radiation belts.

There are two types - inner and outer Van Allen belts. These charged particles trapped in the Earth's magnetic field create the aurora or Northern and Southern Lights.

Structure of the field

In time to come, when you are progressing with your studies, you children will get to know more about magnetic fields and how the flux (flow) lines are organised. The particular fields can be different depending on the type and shape of a particular magnet.

However, it has been identified that the Earth's magnetic field is similar to that of a bar magnet. But, it has also been found that the axis which the Earth revolves around and the axis of the magnetic field do not overlap. Instead, they are tilted 11 degrees from each other.

We have spoken about magnets and we have also seen them. But, they are found in a solid state and are said to be within the 'Curie Temperature', which is the temperature above which a magnetic material loses its permanent magnetism.

This temperature is about 770 degrees Celsius for iron, and the Earth's core is hotter than that, and is in a molten (hot liquid) state.

How did the Earth get its magnetism?

It might be simple to ask the question 'How does the Earth get its magnetic field?' However, it does not have a simple answer.

Generally, magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. Therefore, the Earth's magnetic field should also be associated with electrical currents. But, how these currents are produced is another question.

It certainly seems likely that electrical currents are produced by the combination of convective effects and rotation in the spinning molten outer core of iron and nickel. This is known as the 'dynamo effect' or 'geodynamo', and is similar to the functionality of an electric generator.

Although the details of this dynamo effect are not known, the rotation of the Earth is believed to play a part in generating the currents. A good piece of evidence is the test results sent by the Mariner 2 spacecraft.

They have revealed that Venus does not have such a magnetic field although its core iron content is quite similar to that of the Earth. The difference lies in the rotation periods of the Earth and Venus. Venus's rotation period of 243 Earth days seems just too slow to produce the dynamo effect.

Another stunning discovery has been made by analysing rocks that have been formed from the molten state. They contain indicators of the magnetic field at the time of their solidification(becomes hard).

The study of such rocks have revealed that the Earth's magnetic field reverses itself every million years or so. This simply means that the north and south magnetic poles switch, and this is one mystery that is yet to be solved. It has been reported that evidence for 171 magnetic field reversals during the past 71 million years have been found.

Aravinda Dassanayake