Earth's Magnetic Field

The Earth behaves like a giant bar magnet. Its magnetic field surrounds the planet and reaches far out into space. This is why a compass needle always points roughly north.

Where does the field come from?

The outer core is liquid iron and nickel. As the Earth spins and heat escapes from the core, this liquid metal moves and swirls. Moving metal carrying electric currents produces a magnetic field. This natural generator is called the geodynamo.

Magnetic poles and geographic poles

• The geographic poles are the ends of the axis the Earth spins around.
• The magnetic poles are where the magnetic field points straight down or straight up. They are not in the same place as the geographic poles, and they slowly move every year.
• The angle between true (geographic) north and the direction a compass points is called declination. In Pakistan it is small, only a degree or two.

Dip (inclination): O Level extra

Class 9 students only need to know that a compass points along the field lines. O Level: A needle that can also turn up and down, called a dip needle, does not stay level. It lines up with the field lines, which slope into the ground in the northern hemisphere.

• At the magnetic equator, the needle is level (dip = 0°).
• Moving towards the magnetic poles, the dip increases.
• At a magnetic pole, the needle points straight down (dip = 90°).

Why the magnetic field matters

• It protects life. The field (the magnetosphere) turns aside most of the charged particles streaming from the Sun (the solar wind).
• Auroras. Some particles are guided towards the poles, where they make the air glow: the northern and southern lights.
• Navigation. Compasses have helped sailors and travellers for about 1,000 years.
• Animals. Some birds, sea turtles and fish sense the field to find their way on long journeys.