Mechanism for the auroral red lower border

Abstract

A model is proposed for the generation of the red lower border of type B auroras that results from the effect of increased collision frequency at the lower altitudes on the distribution of excitation within the nitrogen molecule. There are two characteristic spectral distributions for the red sequence (Δυ=3) of the molecular nitrogen first‐positive system, one for normal auroras and one for laboratory (higher pressure) discharges. The laboratory spectrum is shifted in wavelength by the intersystem collisional transfer of excitation with respect to the auroral spectrum, and such a shift manifests itself to the human eye as a two‐fold increase in luminous flux in the red. Recent investigations have suggested that it is the λ5577‐Å cutoff that is responsible for the red border and have placed the lower limit of the green line emission at about 100 km. The present enhancement mechanism, however, is consistent with the onset of added bright red emission at 85 km.