ACCURACY OF USING 6300 ANGSTROM AURORAL EMISSION TO IDENTIFY THE MAGNETIC SEPARATRIX ON THE NIGHTSIDE OF EARTH

Ground observations of 6300 Angstrom auroral emission at the polar cap boundary are studied to determine the accuracy with which the latitud inal profile of emission intensity can be used to identify the separat rix. Meridian scanning photometers at Rankin Inlet and Gillam provide the observations of 6300 Angstrom emission, and the separatrix determi nations obtained from the photometer data are compared with those dete rmined from DMSP F9 precipitating particle data obtained within 1.5 ho urs in MLT of the ground stations. We assume that the separatrix lies at the poleward edge of boundary plasma sheet precipitation. We find t hat the average intensity of 6300 Angstrom in the polar cap is fairly uniform at 60 R. In the auroral zone the average emission is fairly un iform at 170 R. On the basis of the efficiency and accuracy of separat rix identification, choosing a threshold of 110 R provides the best id entification of the polar cap boundary. The rms error in this identifi cation is 1.2 degrees, and the boundary is located in 54% of the cases . The latitudinal gradient of the emission intensity is also investiga ted as a possible identifier of the polar cap boundary. However, using the intensity gradient to identify the polar cap boundary is less acc urate, with a minimum uncertainty of 2.4 degrees. Finally, fitting the measurements of the 6300 Angstrom emission to a latitudinal step func tion, which represents an idealized emission profile, reduces the rms error in the identification of the separatrix to 1.0 degrees while sti ll identifying the boundary in 54% of the cases.