'Coronal holes' are areas where the Sun's
corona is darker, colder, and has lower-density plasma than average. These were found when X-ray telescopes 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.
References
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Spaceweather.com
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Physics of the Space Environment, , Tamas, Gombosi, Cambridge University Press, , ISBN 0-521-59264-X