The altitude of green OI 557.7 nm and blue N 2 + 427.8 nm aurora

[摘要] We have performed a large statistical study of the peak emission altitude of green O ( 1 D 2 – 1 S 0 ) (557.7  nm ) and blue N 2 +  1 N (427.8  nm ) aurora using observations from a network of all-sky cameras stationed across northern Finland and Sweden recorded during seven winter seasons from 2000 to 2007. Both emissions were found to typically peak at about 114  km . The distribution of blue peak altitudes is more skewed than that for the green, and the mean peak emission altitudes were 114.84  ±  0.06 and 116.55  ±  0.07  km for green and blue emissions, respectively. We compare simultaneous measurements of the two emissions in combination with auroral modelling to investigate the emission production mechanisms. During low-energy electron precipitation ( ∼  4  keV ), when the two emissions peak above about 110  km , it is more likely for the green emission to peak below the blue emission than vice versa, with the difference between the two heights increasing with their average. Modelling has shown that under these conditions the dominant source of O ( 1 S ), the upper state of the green line, is energy transfer from excited N 2 ( A 3 Σ u + ), with a rate that depends on the product of the N 2 and O number densities. Since both number densities decrease with higher altitude, the production of O ( 1 S ) by energy transfer from N 2 peaks at lower altitude than the N 2 ionisation rate, which depends on the N 2 number density only. Consequently, the green aurora peaks below the blue aurora. When the two emissions peak below about 110  km , they typically peak at very similar altitude.The dominant source of O ( 1 S ) at low altitudes must not be energy transfer from N 2 , since the rate of that process peaks above the N 2 ionisation rate and blue emission due to quenching of the long-lived excited N 2 at low altitudes.Dissociative recombination of O 2 + seems most likely to be a major source at these low altitudes, but our model is unable to reproduce observations fully, suggesting there may be additional sources of O ( 1 S ) unaccounted for.

[学科分类] 化学工程（综合）