02 Sep 2022
02 Sep 2022
Status: this preprint is currently under review for the journal ANGEO.

The altitude of green OI 557.7 nm and blue N2+ 427.8 nm aurora

Daniel K. Whiter1, Noora Partamies2,3, Björn Gustavsson4, and Kirsti Kauristie5 Daniel K. Whiter et al.
  • 1School of Physics & Astronomy, University of Southampton, UK
  • 2University Centre in Svalbard (UNIS), Longyearbyen, Norway
  • 3Birkeland Centre for Space Science, Norway
  • 4Dept. of Physics and Technology, UiT The Arctic University of Norway, Tromsø, Norway
  • 5Finnish Meteorological Institute, Helsinki, Finland

Abstract. We have performed a large statistical study of the peak emission altitude of green O(1D21S0) (557.7 nm) and blue N2+ 1N (427.8 nm) aurora using observations from a network of all-sky cameras stationed across northern Finland and Sweden recorded during 7 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 km 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(1S), the upper state of the green line, is energy transfer from excited N2(A3+u), with a rate that depends on the product of the N2 and O number densities. Since both number densities decrease to higher altitude, the production of O(1S) by energy transfer from N2 peaks at lower altitude than the N2 ionisation rate, which depends on the N2 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(1S) at low altitudes must not be energy transfer from N2, since the rate of that process peaks above the N2 ionisation rate and blue emission due to quenching of the long-lived excited N2 at low altitudes. Dissociative recombination of O2+ seems most likely to a major source at these low altitudes, but our model is unable to reproduce observations fully, suggesting there may be additional sources of O(1S) unaccounted for.

Daniel K. Whiter et al.

Status: open (until 14 Oct 2022)

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  • RC1: 'Comment on angeo-2022-23', Shin-ichiro Oyama, 23 Sep 2022 reply

Daniel K. Whiter et al.

Daniel K. Whiter et al.


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Short summary
We measured the height of green and blue aurora using thousands of camera images recorded over a 7 year period. Both colours are typically brightest at about 114 km altitude. When they peak at higher altitudes the blue aurora is usually higher than the green aurora. This information will help other studies which need an estimate of the auroral height. We used a computer model to explain our observations and to investigate how the green aurora is produced.