Auroral N₂ emissions and the effect of collisional processes on N₂ triplet state vibrational populations

Abstract

Previous model results have shown that the N2 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 N2 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 N2. The model currently provides results for the seven low‐lying N2 triplet states (A 3Σu+, B 3IIg, W3Δu, B′ 3Σu−, C3IIu, D3Σu+, and E3Σg+). 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 N2. 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 N2(A) (VK) and N2(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 Δν = 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.

Vibrational kinetics of air plasmas induced by sprites

“…The rates associated to vibrational redistribution within N 2 (A 3 S u + ) with Dv = 3 are zero up to v = 17 [Morrill and Benesch, 1996;J. S. Morrill, private communication, 2009].…”

“…which rate coefficients are taken from Morrill and Benesch [1996]. The rates associated to vibrational redistribution within N 2 (A 3 S u + ) with Dv = 3 are zero up to v = 17 [Morrill and Benesch, 1996;J.…”

“…which forward (and backward) rate coefficients (see Table 2) for all the vibrational levels considered in this work for each of the electronic states involved are taken from Morrill and Benesch [1996].…”

“…where a stands for the electronic states A 3 S u [Morrill and Benesch, 1996], for v = 0 through v = 4 of C 3 P u [Simek, 2002], for v = 0 through v = 3 of W 3 D u [Morrill and Benesch, 1996] and for v = 0 and v = 1 of B′ 3 S u − [Morrill and Benesch, 1996]. [19] 6.…”

Electronic kinetics of molecular nitrogen and molecular oxygen in high-latitude lower thermosphere and mesosphere

“…Firstly, high-energetic auroral electrons penetrate to these altitudes causing the type B aurora (Gattinger and Vallance Jones, 1979;Gattinger et al, 1985). Benesch (1981Benesch ( , 1983, Morrill and Benesch (1996), Kirillov (2008a) have shown that vibrational populations of the triplet manifolds of molecular nitrogen are strongly affected by collision processes at the altitudes. This dependence of vibrational populations of Correspondence to: A. S. Kirillov (kirillov@pgia.ru) electronically excited N 2 on the rates of collisional processes causes a redistribution of First Positive Group (1PG) relative intensities with the increase in the density of the atmosphere.…”