Elements of the Earth's Magnetic Field > Horizontal Component of Earth's Magnetic Field

The Earth’s magnetic field is generally inclined to the horizontal direction. In the magnetic meridian, it can be resolved into two mutually perpendicular components:

• Horizontal component B_H
• Vertical component B_V

The horizontal component B_H is the component of Earth’s magnetic field along the horizontal direction in the magnetic meridian.

Let:

• B_E = total Earth’s magnetic field
• θ = angle of dip
• B_H​ = horizontal component
• B_V​ = vertical component

From the right-angled triangle:

Therefore,

and

• tan⁡θ = \[\frac {B_V}{B_H}\]

The three elements of Earth’s magnetism are:

These elements vary from place to place and may also vary with time at the same place.

Key observations

• At the magnetic equator, Earth’s magnetic field is horizontal.
• At the magnetic poles, Earth’s magnetic field is vertical.

A dip needle gives the true angle of dip only when its vertical plane lies in the magnetic meridian.

If the vertical plane of the dip needle makes an angle αα with the magnetic meridian, the angle shown by the needle is called the apparent angle of dip θ′.

Apparent dip: tan⁡θ′ = \[\frac {tan ⁡θ}{cos ⁡α}\]

where:

• θ = true angle of dip
• θ′ = apparent angle of dip
• α = angle between the vertical plane of the dip needle and the magnetic meridian

• When α = 0°: θ′ = θ
  The apparent dip equals the true dip.
• When α ≠ 0°, the apparent dip is greater than the true dip.
• When α = 90°: θ′ = 90°
  The dip needle becomes vertical.
• If θ₁ and θ₂​ are apparent angles of dip in two mutually perpendicular vertical planes, then: cot⁡²θ = cot⁡²θ₁ + cot⁡²θ₂