"In the event of weak airglow or a bright auroral display, there is no doubt in anyone’s mind what to call the phenomenon; for the borderline cases of a bright airglow or faint aurora, however, there is considerable doubt! Generally one distinguishes between the two on physiological grounds, calling a structure "aurora” if it is bright enough to be seen. But according to Roach, McCaulley, and Marovich [1959a] there is no abrupt change at the visual threshold in the frequency distribution of intensities of the green line, h5577; thus it is not evident on the basis of brightness alone that there is a physical distinction between auroral and airglow green-line emission. "

Aurora and airglow as reflections from an internal sun, with chromatic dispersion, and airglow as light reflecting off the aurora or "crown" of the Earth, a bit like a lamp-shade.

Synapses

Chapter 9. The Airglow Spectrum. (1961). In International Geophysics (pp. 345–392). Elsevier. https://doi.org/10.1016/s0074-6142(08)60657-9