Relative brightness of the O+(2D-2P) doublets in low-energy aurorae

Abstract

The ratio of the emission line doublets from O+ at 732.0 nm (I732) and 733.0 nm (I733) has been measured in auroral conditions of low-energy electron precipitation from Svalbard (78°.20 north, 15°.83 east). Accurate determination of R = I732/I733 provides a powerful method for separating the density of the O+2Po1/2,3/2 levels in modeling of the emissions from the doublets. A total of 383 spectra were included from the winter of 2003-2004. The value obtained is R = I732/I733 = 1.38 ± 0.02, which is higher than theoretical values for thermal equilibrium in fully ionized plasma, but is lower than reported measurements by other authors in similar auroral conditions. The continuity equations for the densities of the two levels are solved for different conditions, in order to estimate the possible variations of R. The results suggest that the production of ions in the two levels from O(3P1) and O(3P2) does not follow the statistical weights, unlike astrophysical calculations for plasmas in nebulae. The physics of auroral impact ionization may account for this difference, and therefore for the raised value of R. In addition, the auroral solution of the densities of the ions, and thus of the value of R, is sensitive to the temperature of the neutral atmosphere. Although the present work is a statistical study, it shows that it is necessary to determine whether there are significant variations in the ratio resulting from non-equilibrium conditions, from auroral energy deposition, large electric fields, and changes in temperature and composition.