A region in the Sun's corona of very low density, about 100 times less than that of coronal active regions. Coronal holes show up as apparent voids in X-ray images or, at the limb, by an absence of white-light emission in coronagraph images. A large coronal hole is always present at each of the Sun's poles. Low-latitude holes appear shortly before solar minimum, increasing in size over several months and sometimes coalescing with one of the polar holes. Other holes may contract and disappear. 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.
Subjects: Astronomy and Astrophysics — Meteorology and Climatology.