Solar corona. X-ray image of the Sun taken by the YOHKOH satellite 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 Kelvin) 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. The image was taken on 12 November 1991 in the 3-40 Angstrom wavelength interval.