New technique for aurora / dayglow separation in UV imagery: IMF B_y dependence of seasonal differences in auroral oval location

Abstract. We investigate how the location, size, and intensity of the auroral oval is affected by the combination of a strong dawn/dusk component of the interplanetary magnetic field (IMF B_y ) during northward IMF and the tilt of the Earth's magnetic dipole axis. Sunlit auroral observations are contaminated by dayglow, and its impact on average intensity estimates remains unclear. Dayglow modelling is also accompanied by significant uncertainties that increase with increasing sunlight intensity. These difficulties motivate us to develop a new technique for separating dayglow and aurora, where we assume the observed distribution of intensities consists of a convoluted distribution with separate contributions from dayglow and aurora. By performing a nonlinear fit to observed count distributions one may extract the best-fit parameters that describe the dayglow and auroral sources separately. We apply this separation method to data obtained by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard the Defense Meteorological Satellite Program's (DMSP) F16–19 satellites. We demonstrate to what extent the heteroskedasticity of distributions of dayglow intensity can influence quantitative estimates of the average auroral intensities. By applying our separation method to SSUSI observations made during stable and strong IMF B_y conditions over many years, the auroral component is isolated, and estimates of auroral intensities during sunlit conditions are improved. The separation method produces 30–40 % higher auroral intensities than a simple calculation of the distribution mean. Statistics of the auroral component show a clear and substantial (~500 km in NH, ~430 km in SH) dawn-dusk shift in the polar cap location depending on the sign of IMF B_y during local summer in both Hemispheres. This shift is absent during local winter. We propose that the cause of this seasonally dependent shift in the polar cap location is likely to be related to seasonal differences in lobe reconnection rates. To our knowledge, the seasonal dependence of polar cap location has not been previously reported.