LOCATING THE POLAR-CAP BOUNDARY FROM OBSERVATIONS OF 6300 ANGSTROM AURORAL EMISSION

We examine ground-based observations of the meridional profile of 6300 Angstrom atmospheric emission from 67.3 degrees to 80.7 degrees invar iant latitude for the signature of the polar cap boundary, the ionosph eric boundary between open and closed magnetic field lines. The open-c losed field line boundary is assumed to lie at the boundary between po lar rain and plasma sheet precipitation. We assume that nonprecipitati on-dependent sources of 6300 Angstrom emission cause a spatially unifo rm luminosity in the polar cap and that auroral zone luminosity is als o spatially uniform. Therefore we determine the location of the polar cap boundary from the auroral emission data at each time by finding th e 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 h ave developed criteria on the step function fit that identify when a r eliable boundary identification has been obtained. Generally, where th ese criteria are not satisfied, the boundary is outside the latitudina l range of the optical observations. We compare the boundary identifie d from the emissions to the boundary in precipitating particle observa tions 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 str ucture of the polar cap boundary associated with auroral surges. We co nclude that 6300 Angstrom emissions provide a good means for monitorin g the polar cap boundary continuously with an estimated precision of /- 0.9 degrees invariant latitude.