When the liquid metal that creates the outer core of the earth passes through a magnetic field, it begins to carry the electric current through the liquid. This current makes its own, much stronger magnetic field.
Earth's outer core is liquid and it has a lot of highly conductor melted iron that is flowing, making spiral roads of electric current that generates a magnetic field, that in this case works like a magnetic dipole with a north and a south pole.
The earth's magnetic field protects the earth from the solar vents.
