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Fig. 3.

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Spectrogram of a series of spectra collected during the almost 10 hours of observation in March 2017. Wavelength (in nm) is on the vertical axis and the local time is on the horizontal axis. The red and green lines are easily recognizable at respectively 557 and 630 nm. The two N+ lines at 391 and 428 nm are faint but recognizable. The N2 first positive and N+ Meinel features are present in the red part of the spectra (see also Fig. 4). The dawn at the end of night is also recognizable at the right of the figure. The wavelength scale of the figure is approximate and does not correspond exactly to the fit mentioned in the text. Some strong enhancements of the total intensity are visible. These enhancements seem to concern all wavelengths. This appears to be some combination of different effects: A strong enhancement of all line intensities that is real and due to greater auroral activity. However, since the intensity scale is the same for the entire night, the noise becomes much larger when the total number of photons increases, giving the impression of continuous emissions. This is of course a fake process exacerbated by the strong noise experienced during the ground based demonstration. A third effect is due to the Fourier transform process, despite the apodisation windows used. These data processing effects will be resolved by ground based calibrations.

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