  As the observational signature of the footprints of solar magnetic field
lines open into the heliosphere, coronal holes provide a critical measure of
the structure and evolution of these lines. Using a combination of Solar and
Heliospheric Observatory / Extreme ultraviolet Imaging Telescope (SOHO/EIT),
Solar Dynamics Observatory / Atmospheric Imaging Assembly (SDO/AIA) and Solar
Terrestrial Relations Observatory / Extreme Ultraviolet Imager (STEREO/EUVI
A/B) extreme ultraviolet (EUV) observations spanning 1996-2015 (nearly two
solar cycles), coronal holes are automatically detected and characterized.
Coronal hole area distributions show distinct behavior in latitude, defining
the domain of polar and low-latitude coronal holes. The northern and southern
polar regions show a clear asymmetry, with a lag between hemispheres in the
appearance and disappearance of polar coronal holes.
