Earth Magnetism:

The earth act like a strong Magnet, the north Pole of earth’s magnet lies near the geographical south and south pole of earth’s magnet lies near the geographical north. This causes a freely suspended magnet to point towards north south direction. It has been established that earth’s magnetic field is fairly uniform such that its value on the surface is 10^-5 Tesla and this field extends up to a height nearly 5 times the radius of earth.

Reasons or Causes of Earth Magnetic Field:

The exact cause of Earth Magnetic Field is not yet known but different theories have been put forward from time to time and these are:

1. Charged Metallic fluid inside the core of earth rotates and as the earth rotates about its axis. This rotating charge produces electric current which ultimately produced earth magnetism.
2. The various substances are made up of protons and electron. These charges rotate as earth rotate about its axis. So these charges produce a current and hence magnetism.
3. The ionosphere above the surface of earth is ionized due to cosmic rays. These ions rotates with the rotation of earth producing current and hence magnetism.

These are the reasons of earth magnetic field, if you know more then please share.