The Earth's Magnetism

A magnetic compass was used to help the sailors for navigational purpose. But recently it has been discovered that some migrant birds have magnetic sensors in their heads, which help to guide them using the Earth's magnetic field.

William Gilbert suggested that Earth itself is a huge magnet from various observations he had made:

On disturbing a freely suspended magnet it returns quickly to N-S direction. The north pole of this huge magnet must be towards geographic south as to attract South Pole of the suspended magnet.

Soft iron pieces buried under surface of Earth are found to acquire magnetic properties.

On mapping magnetic field lines due to bar magnet, we come across neutral points. These points are those where magnetic field of the bar magnet cancel with that of Earth's field. But for the latter, we cannot obtain neutral points.

The exact cause of magnetic field of Earth is not yet known but some important postulates are:

Magnetic field of Earth may be due to molten charged metallic fluid in core. This rotating fluid results in currents thus magnetising the Earth.

Since every substance is made up of charged particles, these substances rotating about an axis is equivalent to a circulating current and hence is responsible for the Earth's magnetisation.

As the earth rotates, strong electric currents are set up due to movement of charged iron (due to showers of cosmic ray). These moving ions magnetise the Earth.

Sub Topics

Features of Earth's Magnetic Field
Magnetic Elements
Magnetic declination (q)
Magnetic field around the earth
Magnetic dip or Inclination
Horizontal component
Global and Temporal Variation in Earth's Magnetic Field

Features of Earth's Magnetic Field

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The earth's magnetic field has an axis which is inclined 20^o west of the axis of rotation of earth. The point where this huge earth's magnet cuts the earth's surface are the magnetic poles. A freely suspended magnet has its north pole pointing towards geographic north; we therefore designate the earth's magnetic pole close to geographic north as magnetic south. The same argument follows for the south pole of the freely suspended magnet.

The magnetic equator divides the earth's surface into two. The lines of force enter geographic north and come out of the geographic south.

Magnetic Elements

To describe the magnetic field of earth at any place three quantities or elements are required. They are:

Magnetic declination (q)

Magnetic inclination (d)

Horizontal component (B_H)

Magnetic declination (q)

Magnetic declination is the angle between magnetic axis and the geographic axis.

Magnetic field around the earth

Magnetic dip or Inclination

The angle between the direction of total intensity of Earth's field with the horizontal line in magnetic meridian. It is represented as d.

At poles, the angle of dip = 90^o and at the equator, the angle of dip = 0^o.

The dip at a place can be determined by an apparatus known as dipcircle as shown below. The needle rotates freely in the vertical plane of scale. The pointed ends move over the graduations on the scale, which are marked 0-0 in the horizontal and 90-90 in the vertical directions.

Simple Dip Needle

Horizontal component

Horizontal component is the component of the total intensity of Earth's magnetic field in the horizontal direction in magnetic meridian.

Global and Temporal Variation in Earth's Magnetic Field

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The dipole pattern of earth's magnetic field is disturbed due to solar winds. Solar winds are a stream of charged particles coming from the sun. These particles ionise the atmosphere above these poles which display a light high up in the atmosphere. This phenomenon occurring in the arctic region is called aurora borealis or northern lights and in south it is called aurora australis.

The earth's magnetic field is found to change with time. The magnetic poles of earth keep shifting their position which is short term change. Detailed charts are maintained and revised periodically. The changes occurring over long term come from the evidence of basalt. The basalt from volcano cools and solidifies and provides the picture of earth's magnetic field. As the basalt can be dated back, a clear picture of the earth's magnetic field has emerged. The currents in the earth's core slow down, stop and pick up in the opposite direction.