When the magnetic field  in the centre stops moving, so there is more interactions with  the top layers on the surface, because the magnetic fields  of the centre of the planet, or the star, are created through, hydrogen atoms, and the surface of the hydrogen atom  to a plasma of the same... they create a balanced fields,  or they create what they call the "Dark Spots", which means in reality, there is less fragments of light  left to create. 