Auroral N-2 emissions and the effect of collisional processes on N-2 triplet state vibrational populations

Previous model results have shown that the N-2 triplet vibrational level populations in the aurora are strongly affected by cascade and quenching by atomic and molecular oxygen. As the aurora penetrates to lower altitudes (less than 100 km) the role of quenching by atomic oxygen becomes less important and processes involving N-2 collisions begin to play a more prominent part. We are developing a model which will yield steady state vibrational level populations for both the singlet and triplet valence states of N-2. The model currently provides results for the seven low-lying N-2 triplet states (A(3) Sigma(u)(+), B-3 Pi(g), W-3 Delta(u), B'(3) Sigma(u)-, C-3 Pi(u), D-3 Sigma(u)(+), and E(3) Sigma(g)(+)). These states are responsible for auroral emissions from the UV (Vegard-Kaplan (VK), second positive (2PG)) through the visible to the infrared (first positive (1PG), infrared afterglow (IRA), Wu-Benesch (WB)). We have included two additional collisional processes in the current model which were not treated previously. These are the intersystem collisional transfer of excitation (ICT) between the B state and the A, W, and B' states and vibrational redistribution within the A state vibrational manifold, both due to collisions with ground state N-2. The present work compares our current model results with those of a previous model as well as ground, airborne, and rocket observations. The comparison between N-2(A) (VK) and N-2(B) (1PG) vibrational level populations predicted by our model and a number of auroral observations indicate that the current model achieves a significant improvement in the fit between calculation and observation. In addition, the current model predicts a shift in the band intensity distribution of the 1PG Delta nu = 3 sequence from the infrared into the visible red at the lower altitudes (less than 90 km) as well as an overall enhancement in the entire 1PG system. Consequently, this provides a possible explanation of a dominate feature of type b aurora, the auroral red lower border.