The magnetic activity doesn't come from the fusion reaction directly. The sun has a differential rotation profile. Some layers rotate faster than others. This s

The magnetic activity doesn't come from the fusion reaction directly. The sun has a differential rotation profile. Some layers rotate faster than others. This stretches the magnetic flux tubes as they pass through these layers, increasing their strength. If you drop some oil in a river which is not moving at a uniform speed, the oil spot will get deformed and stretched. (it sounds like voodoo magic, I hate that explanation, but it's the textbook version as far as I can remember)