The umbra is dark because it is cooler around 3,500 degrees Celsius. Spots occur singly and in groups ranging from two to over one hundred in number. There is a tendency for spots to occur in pairs. When they do appear in groups they are usually more complex and there also is a leader who is longer lived than the other companion spots. The most exciting and interesting phenomenon is the chromospheric flare.
Flares are short lived explosive brightening of plages.
Solar flares produce seismic waves in the Sun's interior that resemble those created by earthquakes. A flare-generated solar quake that contained about forty thousand times the energy released in the great earthquake that devastated San Francisco in 1906. That amount of energy released was enough to power the United States for twenty years at its current level of consumption. That is equivalent to an 11.3 magnitude earthquake. That flare released enough energy to complete ly cover the Earth's continents with a yard of dynamite and detonating it all at once.