Ground-based photometric observations of the type A aurora of 17-18 December 1971

Ground-based observations have been made during the event of 17-18 Dec. 1971 from northern Norway, using a tilting-filter photometer monitoring the intensities of N2(plus) 1.Neg., H-beta, 6300-A O-I, and 5200-A N-I. The high 6300-A/N2(plus) 1.Neg ratio classes this event among type A auroras. The 6300-A/5200-A ratio varies during the night and remains higher than the usual value of nearly 20. These observations are compatible with a soft electron flux as the origin of the red line enhancement as illustrated by comparison with a model aurora. It is also found that N(2Do) is deactivated by collisions with electrons at a rate foreseen by theoretical calculations. An upper limit of about 40 seconds is derived for the N(2Do) effective lifetime, though no simple relationship gives a good fit to the observations.