Using spectral characteristics to interpret auroral imaging in the 731.9 nm O⁺ line

Abstract. Simultaneous observations were made of dynamic aurora during substorm activity on 26 January 2006 with three high spatial and temporal resolution instruments: the ASK (Auroral Structure and Kinetics) instrument, SIF (Spectrographic Imaging Facility) and ESR (EISCAT Svalbard Radar), all located on Svalbard (78° N, 16.2° E). One of the narrow field of view ASK cameras is designed to detect O⁺ ion emission at 731.9 nm. From the spectrographic data we have been able to determine the amount of contaminating N₂ and OH emission detected in the same filter. This is of great importance to further studies using the ASK instrument, when the O⁺ ion emission will be used to detect flows and afterglows in active aurora. The ratio of O⁺ to N₂ emission is dependent on the energy spectra of electron precipitation, and was found to be related to changes in the morphology of the small-scale aurora. The ESR measured height profiles of electron densities, which allowed estimates to be made of the energy spectrum of the precipitation during the events studied with optical data from ASK and SIF. It was found that the higher energy precipitation corresponded to discrete and dynamic features, including curls, and low energy precipitation corresponded to auroral signatures that were dominated by rays. The evolution of these changes on time scales of seconds is of importance to theories of auroral acceleration mechanisms.