New technique for isolating the auroral contribution in UV imagery: IMF <i>B</i> _<i>y</i> dependence of seasonal differences in auroral oval location during positive IMF <i>B</i> _<i>z</i>

Abstract. We investigate how the location, size, and intensity of the auroral oval is affected by the combination of the tilt of the Earth's magnetic dipole axis and a strong, stable dawn/dusk component of the interplanetary magnetic field (IMF By) during northward IMF. 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 isolating the auroral contribution in dayglow-subtracted UV images. This technique assumes the observed distribution of intensities consists of separate contributions from dayglow and aurora, and that the dayglow subtraction process to which the observations are subjected is imperfect and leaves a residual error. By performing a nonlinear fit to dayglow-subtracted count distributions one may extract the best-fit parameters that describe the dayglow residual error and auroral sources separately. We apply this isolation technique to dayglow-subtracted UV images derived from measurements made by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard the Defense Meteorological Satellite Program’s F16–19 satellites during 2005–2018. The isolation technique 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 By 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. We also demonstrate how the heteroskedasticity of distributions of dayglow residual intensity can influence quantitative estimates of the average auroral intensities.