Solar corona. X-ray image taken by the YOHKOH satellite showing the activity of the Sun's corona, the outer layer of the solar atmosphere. An eruptive filament which lasted about 3 hours is also seen at centre right near the Sun's limb. The corona is formed by gas at very low density but an extremely high temperature (1 to 5 million degrees Kelvin) which causes the intense emission in the X-ray band. 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.