Hi Dan,

This is a very good question and one I was just discussing with some other solar scientists. Perhaps the simplest way of saying why is that intense magnetic fields, 1000 times stonger than the magnetic field of Earth, embedded within the surface of the Sun prevent hot plasma from rising to the surface. But in other locations, hot plasma rises continuously from below to the surface. We see the surface of the Sun in light that our eyes can detect, known as "visible light." In this kind of light, hot plasma at the surface of the Sun is brighter than cooler plasma and so the Sun looks dimmer in the regions of cooler areas, which are the magnetic sun spot areas.

Laura