Coronal Holes and Open Magnetic Flux over Cycles 23 and 24.

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 (SOHO/EIT), (SDO/AIA), and (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.