Coronal holes are areas where the Sun's corona is darker, and colder, and has lower-density plasma than average. These were found when X-raytelescopes in the Skylab mission were flown above the Earth's atmosphere to reveal the structure of the corona. Coronal holes are linked to unipolar concentrations of open magnetic field lines. During solar minimum, coronal holes are mainly found at the Sun's polar regions, but they can be located anywhere on the sun during solar maximum. The fast-moving component of the solar wind is known to travel along open magnetic field lines that pass through coronal holes..