Coronal hole boundaries and their interactions with adjacent regions

Coronal hole boundaries are the interfaces between regions where the corona l magnetic field contains a significant component which is open into the he liosphere and regions where the field is primarily closed. It is pointed ou t that there are constraints on the magnetic field which opens into the hel iosphere that must be satisfied in the corona: it must come into pressure e quilibrium in the high corona. and the component of the held which connects to the polar regions of the Sun must differentially rotate. A model is pre sented in which satisfying these constraints determines which field lines a re open and which are closed, and thus where the polar coronal hole boundar ies occur. Some of the consequences of this model are discussed.