Total solar eclipse. Outer corona of the Sun, seen during a total solar eclipse. The corona is shaped by the Sun's magnetic field, with the magnetic poles seen at upper right and lower left. 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. 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.