Coronal hole

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

1. Spaceweather.com
2. Gombosi, Tamas. Physics of the Space Environment. New York: Cambridge University Press. ISBN 0-521-59264-X. 
3. Jiang, Y., Chen, H., Shen, Y., Yang, L., & Li, K. (2007, January). Hα dimming associated with the eruption of a coronal sigmoid in the quiet Sun. Solar Physics, 240(1), 77-87.