Coronal holes are areas in the sun’s corona that cooler; therefore darker, and from where a large portion of high speed the solar wind comes from.
Coronal holes were first discovered when X-ray space telescopes on board the Skylab were first flown above the earth’s atmosphere. Coronal holes are also detected where open or uni-polar solar magnetic field lines are present or mostly at the sun’s poles during solar minimum, but they can be detected anywhere on the sun during solar maximum. Although coronal holes were frequent observed all over the sun during the abnormal solar minimum phase of 2001–2012.
CREDIT: NASA- Solar Dynamics Observatory (SDO)