Total solar eclipse. Inner corona of the Sun, seen during a total solar eclipse. The corona is a cloud of low-density plasma (hot ions) surrounding the Sun. Normally it is not visible, being obscured by the brightness of the Sun's visible surface. It is revealed during a total solar eclipse, which occurs when the Moon blocks the light from the rest of the Sun. Also seen here are prominences (red), which are cooler regions of the corona. This moment of totality lasts for only a few minutes. Total solar eclipses usually occur less than once a year, and can only be seen from a small area of the Earth's surface. Photographed from Turkey on 29 March 2006.