The open-closed boundary (OCB) defines the region where geomagnetic field lines transition from being closed to open. Closed field lines have both foot points at or near Earth in opposing hemispheres. Open field lines have one foot point at Earth; the other maps to the interplanetary magnetic field (IMF). Charged particles are able to follow these open field lines into Earth's upper atmosphere. The OCB also defines the polar cap boundary. Being able to identify and track the OCB allows study of several components of the geomagnetic system. Among them are the electrodynamics of the geomagnetic field and the reconnection balance between the dayside and nightside of the geomagnetic field. Furthermore, the OCB can provide interesting insights into the precipitation of energetic protons into the ionosphere. Using the Tsyganenko model of the geomagnetic field (T96) we have demonstrated a diurnal fluctuation of the OCB, independent of all other inputs. Also, we have found that this UT effect has an effect on the energy cutoff latitude of energetic protons. We anticipate that the UT effect could have important consequences to modeling the OCB and other polar cap-related events, especially polar cap absorption events affecting high frequency radio wave propagation via polar paths.