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On the Sun, moving hot gas sets up magnetic fields. In particular, in sunspots, a very strong magnetic field appears. That magnetic field holds gas in place, since the gas can move only along the direction of the magnetic field lines and not across them. In this way, the magnetic field acts as a giant hand, tamping down motions. It cools the gas. And when the gas is cooler, it is darker. We can actually see the shape of magnetic field lines on the Sun, as we could see the lines from a refrigerator magnet if we used it to attract iron filings. A wonderful NASA spacecraft called Transition Region and Coronal Explorer (TRACE) can see details on the Sun so fine that the magnetic lines of force show. We are actually seeing the glowing gas that is following along the shapes of the magnetic lines of force.

The Complete Idiot's Guide to the Sun
By Jay M. Pasachoff
Chapter 1: What The Sun Looks Like
Page 16