Solar corona during a total eclipse with minimal sunspot activity. 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.