冕洞是太阳日冕上黑暗、低温、和电浆密度低于平均的地区，寿命最长可达一年。它们是天空实验室在地球大气层之上的太空中使用X射线望远镜以X射线观察日冕的构造时发现的。冕洞与开放磁场线的单极强度有关，当太阳极小期时，冕洞主要都在太阳的极区被发现，但是在太阳极大期时，在太阳的任何地区都可以发现冕洞。已知太阳风中快速运动的组件会沿着冕洞的开放性磁场线发散出来。

冕洞依位置可分为两种，极区冕洞（polar hole）和孤立冕洞（isolated hole）。其中，亦有位于高纬地区的极区冕洞移动至低纬而成孤立冕洞的观察。另外亦有延伸冕洞（extension hole）和短暂冕洞（transient hole）的分法，但较少在文献中看到。

Coronal holes are part of the Sun's corona and are constantly changing and reshaping because the corona is not uniform. Coronal holes are areas where the Sun's corona is darker, and colder, and has lower-density plasma than average because there is lower energy and gas levels. The sun contains magnetic fields that arch away from areas in the corona that are very thin due to the lower levels of energy and gas, which cause coronal holes to appear when they do not fall back. Thus, solar particles or solar wind escape and create a lower density and lower temperature in that area. The aurora borealis that is seen at the northern and southern poles are the result of solar wind entering the Earth’s atmosphere.