The height of green 557.7 nm and blue 427.8 nm aurora

An estimate of the height of the aurora is often required for the derivation or interpretation of other auroral or ionospheric parameters, such as horizontal spatial scales, velocities, neutral temperatures, or electron precipitation energies. We have performed a large statistical study of the peak emission height of coincident green 557.7 nm and blue 427.8 nm aurora using a network of ground-based all-sky cameras stationed in northern Finland and Sweden. We have obtained almost 58000 simultaneous measurements of both emissions between 2000 and 2007, and found that both emissions typically peak at about 114 km, but the distribution of peak emission heights is more skewed for blue aurora than for green aurora.

During low-energy electron precipitation (< 4 keV), when the two emissions peak above about 110 km, it is more likely for the blue emission to peak above the green emission than vice-versa. Modelling has shown that this is because the dominant mechanism producing the O(¹S) upper state of the green line is energy transfer from N₂. The rate of that process depends on the product of the N₂ and O number densities, which both decrease to higher altitude. The blue line is produced through electron impact ionisation of N₂, and so depends on the N₂ number density only, and consequently peaks below the green emission.

During high-energy electron precipitation the two emissions typically peak at very similar altitude. In those circumstances, where the emissions peak below the peak in O number density, energy transfer from N₂ must not be the dominant production mechanism of O(¹S). Dissociative recombination of O₂⁺ seems most likely to be the dominant mechanism, but modelling cannot fully reproduce observations and there may be an additional mechanism which is currently unaccounted for.

The observations are best reproduced using a Maxwellian shaped electron precipitation spectrum at low energies, but a Gaussian shaped electron precipitation spectrum at high energies.