This STEREO image and video clip of the Sun in extreme ultraviolet light (June 9-10, 2007) showcases a string of active regions near the Sun's equator over about 36 hours. We see several active regions (brighter areas with the loops above them) that were lined up as they approached the edge of the Sun. With frames being taken every two and a half minutes, scientists can observe the activity along these magnetic field lines in exquisite detail. Active regions are areas of intense magnetic activity that appear brighter in extreme UV light, in this case the wavelength of 171 Angstroms. The images were captured by the Behind spacecraft.
The most powerful solar flare in the last 500 years occurred on September 2, 1859 and was sighted by two astronomers who happened to be looking at the sun at exactly the right time!