Mushrooming Sunspot Group A small sunspot group quickly mushroomed into a substantial sunspot group in less than 2.5 days (Feb. 9-11, 2012). With its high resolution imagers, SDO could follow the action in exquisite detail. Sunspots are cooler, darker regions generated by magnetic forces beneath the Sun's surface. They are often the source for solar storms, although this particular group did not produce any major storms. For a rough size comparison, the larger, more developed spots are larger than Earth.