Sodium dayglow: Observation and interpretation of a large diurnal variation

International audienceAnalysis of the dayglow of the sodium D lines since September 1960 and the twilight glow since 1954 at Haute Provence Observatory shows a large seasonal maximum of 30 kR for the dayglow in June. The ratio of daytime to twilight abundance then is about 7 to 1. The ratio becomes 2 to 1 in December. Evidence presented for the reality of this effect includes actual line profiles showing an emission peak above Rayleigh-scattered background, observation of the dayglow from a high-altitude airplane, and observation of the dayglow with a rocket-borne photometer. The seasonal maximum is accompanied by a seasonal decrease of 5 km in twilight layer altitude in midsummer. The effect is interpreted as being caused by a diurnal variation in atomic oxygen and ozone enhanced by turbulent diffusion of the sodium and its compounds. Other possible interpretations such as photochemical excitation are rejected. A seasonal variation is also found in the ratio of morning to evening sodium abundance in twilight. The maximum ratio is 1.4 at the equinoxes. At Tromsø, Norway, the dayglow intensity is about 15 kR and shows no seasonal variation