Good question, and a difficult one too. I guess you mean that on the Earth the north and south magnetic poles switch position after several hundred thousand years, because on the Sun this only takes 11 years. The magnetic field on the Earth (or on the Sun) is not 'fixed' or 'frozen' like it is with a permanent bar magnet such as you might meet in a science lab, but instead it is being regenerated constantly by the huge electrical currents flowing inside the liquid interior of the Earth (or Sun - although the interior of the Sun is a gas rather than a liquid, they behave more-or-less the same way). This is all complicated also by the fact that the liquid is spinning as the planet/Sun spins. So nothing is fixed! The interplay between these currents and the conducting fluid inside the Earth/Sun is complicated, and described by a branch of physics called dynamo theory, and I think that it is fair to say that no-one knows exactly why the fields behave as they do although some huge computer simulations have managed to make models of the process. But in the case of the Sun we do know a couple of things. The magnetic field in Sunspots is concentrated in 2 bands around the equator, but this slowly drifts up towards the poles of the Sun under the influence of something called the meridional flow, and this can deliver new magnetic field of the opposite sign to the pole, helping in the reversal process. How this compares to what happens on Earth, I am just not sure! Lyndsay