The composition of the aurora borealis became a subject of scientific interest with the introduction of spectroscopy, but for a long time the aurora refused to reveal its secrets. After fifty years of research, the origin of the dominant green line of wavelength 5577 ร
was still a mystery. Only in 1912 did progress finally begin to occur in the understanding of the aurora, a field of research which appealed in particular to Norwegian scientists. Prominent among them was Lars Vegard (1880โ1963), who in 1923 suggested a new picture of the upper atmosphere and an explanation of the green line in terms of excitations of frozen nitrogen dust particles. Although apparently confirmed by cryogenic experiments, Vegard's theory was challenged by the Canadian physicist John McLellan (1867โ1935) who in 1925, together with his postdoctoral student Gordon Shrum (1896โ1985), reproduced the line in experiments with helium-oxygen mixtures. This is the story of how the enigma of the green auroral line was finally resolved and explained by the quantum theory of atoms, namely as a transition between two metastable states of oxygen. It is also the story of two of the period's leading specialists in auroral spectroscopy, their rivalry, and different approaches to the study of the northern light.