X-ray image showing a coronal hole on the Sun, photographed by Skylab, 1970s.

The corona is the outer part of the solar atmosphere. It consists of highly ionised gas superheated to temperatures in exces of 1 million degrees Celsius and is visible during a total solar eclipse as a halo or cloud of white light. The corona is a strong source of X-rays, making X-ray astronomy useful for observing variations in its structure, which changes according to the solar activity cycle. Coronal holes were first discovered by observing the Sun at X-ray wavelengths. They are features often found at the solar poles which appear dark as they emit les X-rays than other areas of the corona. Coronal holes have open magnetic fields, allowing particles to escape from the Sun at high speed, forming the solar wind which causes aurorae in the Earth's atmosphere.