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) boundaries derived from satellite‐borne auroral imagers has hampered the application of these instruments in quantitative magnetospheric energy balance and substorm analysis. In this study, PAE boundaries from Polar Ultraviolet Imager (UVI) images are compared with precipitation boundaries from Defense Meteorological Satellite Program (DMSP) satellite spectrograms. In particular, 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 b5e boundaries were obtained in the evening sector. It has been determined that a UVI PAE boundary defined by a fixed ratio to the maximum in the auroral oval at each magnetic local time correlates better with the DMSP b5e boundary 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−2S−1 and 0.30 normalized flux, respectively. The study also reveals a systematic latitudinal offset between UVI and DMSP in the evening sector with a magnitude of approximately 1°. This offset might represent a real height‐dependent geomagnetic influence (e.g., active magnetic topology) or result from systematic errors in the analysis (e.g., removal of the UVI platform wobble). It is demonstrated that the offset can be partially removed with a linear calibration model, allowing the reproduction of DMSP b5e boundaries from UVI images with a standard deviation error of approximately 1°.