Auroral Ultraviolet Emissions

Abstract

The identification of most of the important features of the UV spectrum has progressed simultaneously with the development of sensitive rocket-borne and satellite-borne UV spectrometers. It may be considered, at the present time, that our knowledge of the auroral spectrum above 1200 Å is satisfactory as far as identification of important transitions is concerned. Unfortunately, the spectral range covered by such measurements has often been very limited, thus preventing accurate comparisons to be made of intensities of features spread over the whole spectrum. As far as theory is concerned, a great deal of effort has been devoted to the prediction of UV emission intensities (Green and Barth, 1965; Stolarski and Green, 1967). The calculations were generally performed for a range of electron energies and comparison was made with available observational results. Ambiguities remain in the interpretation to be given to the discrepancies which may be due either to the use of inappropriate cross sections or excitation mechanisms, or to large differences between real physical conditions existing at the time of the observation. For the first time, Sharp and Rees (1972) combined satellite-borne and airborne equipment to observe simultaneously the same spot of aurora in the range 1200 to 4000 Å. Below 1200 Å, the presence of strong emissions has been detected photometrically by Landensperger (1971) and Paresce et al. (1972) but the spectral distribution is still unknown. Most of the excitation of UV emission is direct e.g., resulting from impact of primary and secondary electrons on ground state species. This makes interpretation of intensities often easier than in the case of visible emissions. However, radiation trapping complicates analysis of OI λ1304 Å and NI λ1200 Å altitude distribution. This paper reviews the present knowledge on excitation processes and related intensity of the UV auroral emissions. It will also report on new results concerning N₂A³Σ_u ⁺ deactivation as determined by the V-K bands altitude profile. Finally, UV photometric data obtained recently with the ESRO TD-1 satellite concerning early evening polar cap aurora will be reported.