Multiwavelength Emission Altitudes Constrain the Role of O₂ Dissociative Excitation in Auroral OI 844.6 nm Emission

Abstract. Multiwavelength imaging of auroral emissions is widely used to diagnose the characteristic energy of precipitating electrons, often through intensity ratios between prompt emission lines such as N₂⁺ 427.8 nm and OI 844.6 nm. The 844.6 nm emission can be produced by both direct excitation of atomic oxygen (O) and dissociative excitation of molecular oxygen (O₂), but their relative contributions cannot be separated from single-site, column-integrated intensity measurements alone. Here, we use emission altitude as an independent observational constraint on the 844.6 nm emission processes. We analyze diffuse aurora observed during one night with five ground-based stations in northern Sweden and continuously estimate the emission altitudes at 427.8 and 844.6 nm. During the 294 min for which emission altitudes were determined at both wavelengths, the two peak altitudes varied together, while the 844.6 nm emission peak was located above the 427.8 nm peak in 83% of the measurements, with a median altitude difference of 2.0 km. We then use the 427.8 nm peak altitude as an observational proxy for the penetration depth of the precipitating electrons and compare the observed relationship between the two peak altitudes with calculations by two different electron transport models. In calculations without O₂ dissociative excitation, the 844.6 nm peak altitude was increasingly overestimated toward lower 427.8 nm peak altitudes. Including O₂ dissociative excitation reduced the altitude discrepancy between the model and observations by approximately 70%. Despite their different electron-transport formulations, both models showed the same energy-dependent response: the downward shift of the 844.6 nm peak due to O₂ became apparent at a Maxwellian characteristic energy of approximately 5 keV and increased progressively with characteristic energy. Together, the observations and model comparisons provide the first altitude-based observational evidence for an O₂ dissociative-excitation contribution to auroral 844.6 nm emission and show that its influence on the peak altitude is not confined to the highest-energy precipitation.