For centuries, scientists have measured the sun's activity by counting how many sunspots dapple its surface. That number peaked early last fall, says Dr. Hairston. Yet some of the hottest activity hasn't happened until recently. It reversed its magnetic field. Loops of highly magnetized gas usually form in one direction in the sun's northern hemisphere and another in the southern.
Sunspots appear as dim blobs because they are relatively cooler than the rest of the sun's surface.
They form where the sun's magnetic field lines have knotted themselves together just beneath the surface. The disturbed magnetism can also produce solar flares, or looping arches of gas, and giant ejections of material. As the sun rotated, Active Region 9393 eventually slipped onto the far side of the sun. But before it did, it let loose the biggest solar flare seen in a quarter-century. Because sunspots can persist up to three months, the giant sunspot might still exist on the far side of the sun, says Dr. Reiff.