A sample of solar coronal holes around the time of the maximum of sunspot activity (April 2014). Note the polar regions are devoid of coronal holes but a large hole appears in the southern hemisphere.
Coronal holes are large dark regions in the solar atmosphere that appear in extreme ultraviolet (EUV) and X-ray wavelength imagery.
These holes are created by large regions of the solar magnetic field that have a single polarity (unipolar) and extend far out into the solar system. These are sometimes called 'open' magnetic field lines since they don't connect back to the Sun. The regions appear dark because they have a lower density of the hot coronal plasma. The open magnetic field lines make it easy for the charged plasma to stream away from the Sun, forming the fast component (about 700 kilometers per second) of the solar wind.
Coronal holes can form at any time during the 11-year solar cycle but are more prominent near solar minimum when persistent coronal holes can form near the Sun's polar regions. Coronal holes can grow and migrate to lower solar latitudes.
A sample of solar coronal holes around the time of the minimum of sunspot activity (October 2019). Note the coronal holes in the solar polar regions (near the top and bottom of the solar disk) and the large coronal hole across the Sun's equator.