Earth’s Magnetic Field

Earth produces a magnetic field around it. For many purposes, this field is approximately represented by the field of a magnetic dipole placed near Earth’s centre.

A freely suspended bar magnet aligns approximately along the north–south direction. This observation indicates the presence of Earth’s magnetic field.

Key Idea: The N-marked end of a compass needle points approximately toward geographic north. Therefore, the region near the geographic North Pole behaves like Earth's magnetic south pole.

• A freely suspended magnetic needle settles approximately along the geographic north–south direction.
• A soft iron rod kept or hammered along the magnetic north–south direction can become magnetised.
• Near a bar magnet, neutral points occur where the field due to the magnet cancels Earth’s horizontal magnetic field.

Analogy

Imagine a very large bar magnet placed inside Earth, with its axis slightly tilted relative to Earth’s rotation axis. A compass needle behaves like a tiny magnet that tries to align with this large-scale magnetic field.

Earth’s rotational axis meets its surface at the geographical North Pole and geographical South Pole.

Earth’s magnetic field does not exactly coincide with this rotational axis. The points where a dip needle becomes vertical are called the geomagnetic poles.

Magnetic Axis

The straight line joining the geomagnetic north and south poles is Earth's magnetic axis.

• It is inclined by approximately 11.5° to Earth’s rotational axis.
• Earth’s magnetic field is therefore not perfectly symmetric about the geographical poles.

Magnetic Equator

The magnetic equator is the imaginary circle on Earth’s surface where the magnetic field is horizontal.

• At the magnetic equator, a dip needle rests horizontally.
• The angle of dip is zero: I = 0°.
• It divides Earth into magnetic northern and southern hemispheres.

At a given place, Earth’s magnetic field is specified by three magnetic elements:

1. Magnetic declination D
2. Angle of dip (inclination) I
3. Horizontal component of Earth’s field B_H

Magnetic Declination D: Magnetic declination is the angle between the geographical meridian and the magnetic meridian at a place.

• The geographical meridian is the vertical plane containing the geographic north–south line.
• The magnetic meridian is the vertical plane containing the direction of Earth’s horizontal magnetic field.

Use: Declination explains why a compass does not always point exactly towards true geographic north.

Angle of Dip I: The angle of dip is the angle made by Earth’s resultant magnetic field B with the horizontal in the magnetic meridian.

• In the northern magnetic hemisphere, the north-seeking end of a dip needle dips downward.
• In the southern magnetic hemisphere, it dips upward.
• At the magnetic equator: I = 0°.
• At geomagnetic poles: ∣I∣ = 90°.

Horizontal Component B_H: The horizontal component B_H​ is the component of Earth’s magnetic field in the horizontal plane.

• It causes the horizontal north–south alignment of a compass needle.
• It is maximum at the magnetic equator.
• It is zero at the geomagnetic poles.