Solar corona. Artwork, based on X-ray imagery, of the Sun showing the activity of the corona, the outer layer of the solar atmosphere. The corona is formed by gas at very low density but at an extremely high temperature (1 to 5 million degrees Celsius) which causes the intense X-ray emission. The brightest areas are solar active regions where sunspots would be seen in visible light; the darkest regions are coronal holes where the gas density is extremely low and from which the solar wind, a stream of charged particles, is ejected into interplanetary space. Strong magnetic fields bend some flows of coronal material into loops.