We examine ground-based observations of the meridional profile of 6300 ¿ atmospheric emission from 67.3¿ to 80.7¿ invariant latitude for the signature of the polar cap boundary, the ionospheric boundary between open and closed magnetic field lines. The open-closed field line boundary is assumed to lie at the boundary between polar rain and plasma sheet precipitation. We assume that nonprecipitation-dependent sources of 6300 ¿ emission cause a spatially uniform luminosity in the polar cap and that auroral zone luminosity is also spatially uniform. Therefore we determine the location of the polar cap boundary from the auroral emission data at each time by finding the best fit of the observations to a step function in latitude. Thus we produce a time series of the location of the polar cap boundary. We have developed criteria on the step function fit that identify when a reliable boundary identification has been obtained. Generally, where these criteria are not satisfied, the boundary is outside the latitudinal range of the optical observations. We compare the boundary identified from the emissions to the boundary in precipitating particle observations made by DMSP as it passes along a meridian within 1 1/2 hours of local time of the photometer. The latitudes of the two boundaries are highly correlated. During the expansion phase of substorms, however, there are large discrepancies apparently arising from longitudinal structure of the polar cap boundary associated with auroral surges. We conclude that 6300 ¿ emissions provide a good means for monitoring the polar cap boundary continuously with an estimated precision of ¿0.9¿ invariant latitude. ¿ American Geophysical Union 1995