From the generator theory: the outer core of nickel, cobalt, iron, etc. rotate with the rotation of the Earth, and then produce a current, which is the Earth as a whole to produce a magnetic field theory.

The Earth is like a huge magnet. Because the North Pole and the South Pole have the S pole and the North Pole, respectively, in fact the N pole of the "magnet" of the Earth is not in the North Pole but is about 100 degrees west of northern Canada. About 75 degrees east longitude, so Seoul is about 6 degrees west of the north direction.

The Earth's center contains substances that easily become magnets, such as nickel, cobalt and iron, which rotate with the rotation of the Earth.

They also form convection due to partial differences in temperature.

Current Flowing material that rotates inside a magnetic field will generate current like a generator, initially a very small magnetic field. However, when a current is generated, the current will create a new magnetic field.

Earth's rotation is always in one direction, the magnetic field and current continue to combine strongly, and eventually the entire earth to form a huge magnetic field.