The nightside poleward boundary of the auroral oval as seen by DMSP and the Ultraviolet Imager

A lack of reliable error estimates for poleward auroral emission (PAE) boun daries derived from satellite-borne auroral imagers has hampered the applic ation of these instruments in quantitative magnetospheric energy balance an d substorm analysis. In this study, PAE boundaries from Polar Ultraviolet I mager (UVI) images are compared with precipitation boundaries from Defense Meteorological Satellite Program (DMSP) satellite spectrograms. In particul ar, the study quantifies the accuracy with which UVI images can be used to reproduce the DMSP poleward auroral oval (b5e) boundary. Most of the DMSP b 5e boundaries were obtained in the evening sector. It has been determined t hat a UVI PAE boundary defined by a fixed ratio to the maximum in the auror al oval at each magnetic local time correlates better with the DMSP b5e bou ndary than one defined by a constant brightness threshold (0.90 versus 0.80 maximum correlation). The optimal threshold and ratio values are found to be 4.3 photons cm(-2)s(-1) and 0.30 normalized flux, respectively. The stud y also reveals a systematic latitudinal offset between UVI and DMSP in the evening sector with a magnitude of approximately 1 degrees. This offset mig ht represent a real height-dependent geomagnetic influence (e.g., active ma gnetic topology) or result from systematic errors in the analysis (e.g,, re moval of the UVI platform wobble). It is demonstrated that the offset can b e partially removed with a linear calibration model, allowing the reproduct ion of DMSP b5e boundaries from UVI images with a standard deviation error of approximately 1 degrees.