A region of the Sun’s corona that is cooler and less dense than its surroundings. Coronal holes can last for several solar rotations before dissipating; the record duration is nearly two years. They show up as apparent dark voids in X-ray and ultraviolet images or, at the limb, by an absence of white-light emission in coronagraph images. A large coronal hole is present at each of the Sun’s poles around the time of solar minimum, while holes extending down to lower latitudes are most frequent on the declining half of the solar cycle. The magnetic field in coronal holes is in the form of open field lines stretching out into space, along which plasma flows to produce the high-speed streams in the solar wind. If these high-speed streams encounter the Earth, they cause major auroral displays.

http://sdo.gsfc.nasa.gov/gallery/potw/item/272