| 08 Oct 2025
New technique for aurora / dayglow separation in UV imagery: IMF By 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 By ) 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 By 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 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. To our knowledge, the seasonal dependence of polar cap location has not been previously reported.
New technique for aurora / dayglow separation in UV imagery: IMF By dependence of seasonal differences in auroral oval location
Hessen et al.
This paper develops a new technique to separate contributions to auroral images from the auroras and dayglow. The technique is applied to images from the DMSP/SSUSI instrument. The technique assumes that the distributions of luminosity contributions from the two sources are different, and uses a probabilistic approach to separate the two. The technique cannot be applied to individual images, but to the sum of multiple images collected over several months. Hence seasonal variations in the locations of the auroras are studied. The paper concludes that dawn-dusk asymmetries in the location of the auroral oval associated with IMF By dominate in some seasons and not others. In principle the paper is of interest, but is has some major short-fallings which require major corrections to address. This issues are outlined below, on no particular order.
The paper studies dawn-dusk asymmetries in the location of the auroras during northward IMF. It is not stated in the title that the study relates to northward IMF, it is only alluded to a few times within the paper, and it is never (to my knowledge) explained why the authors ignore southward IMF. The rationale for studying northward rather than southward IMF should be clearly articulated and should be clear in the title.
A major issue appears to be with the dataset used itself. The DMSP/SSUSI images have already had a dayglow removal algorithm applied to them (which introduces serious artefacts into them – see below). This paper then tries to separate a dayglow contribution, which has supposedly already been removed, from the auroral contribution. It would seem to make more sense to use the original images, without dayglow removal, and develop a new technique that does the removal better. One of the coauthors is from the DSMP/SSUSI team – is the original data available?
Artefacts in the data include a bias towards negative luminosities (even when the positive auroral contributions are included!) and spikes in the distributions at particular luminosity values. At the very least there should be a discussion in the paper about how the original dayglow removal is done, and why it leads to these artefacts. Otherwise the results from this paper are of little use. Currently, most of the paper is discussing the fact that there are these artefacts, without describing why they arise. The paper would be much better focussed if the original data was used (see previous comment).
I’m not sure that I understand Figures 6 and 7. Three distributions are shown: blue is aurora and yellow is DG-residual. My understanding is that green is the sum of these two distributions, but this is clearly not the case. For instance, if green is the sum then the two bottom distributions should be bimodal. Please explain these figures better.
I think Figures 4 and 8 are the same, but one has more labelling. Just use the second figure.