This work describes an application of incoherent scatter (IS) radar and all-sky imaging techniques to the identification of signatures for the nightside auroral zone/polar cap boundary. This identification is made by estimating the characteristic energy of precipitating electrons across all latitudes measured by the IS radar. The primary method used to discern the boundary is based on an algorithm wherein the detailed shape of E region ionization profiles is mapped to the required number flux from a library of monoenergetic ionization profiles. This procedure results in a reconstructed energy distribution for the precipitating electrons from which a characteristic energy is derived. The latitudinal gradient of the characteristic energy is subsequently used to identify the boundary. Monochromatic all-sky imagers are used to establish the time history of the boundary and estimate characteristic energy (at magnetic zenith). Additional plasma signatures such as auroral ionospheric cavities and localized F region Te enhancements are shown to provide contextual clues for the location of the boundary. Three in-depth case study periods are used to qualify the various boundary identification methods. A detailed study of 30 IS radar scans in the geomagnetic meridian is used to conclude that a latitudinal gradient in characteristic energy of -5 to -11 keV per degree, when measured adjacent to an extended region of low-energy precipitation, can locate the boundary with a latitudinal precision of 0.2¿.¿ 1997 American Geophysical Union