The effect on thermospheric chemistry of a resonant charge exchange reaction involving vibrationally excited N₂⁺ ions with atomic oxygen

The charge exchange of O + (² D ) with N 2 + is one of the major scources of N 2 + in the thermosphere. Its rate coefficient k was measured in the laboratory to be ≅ 1×10 −9 cm³ s −1 . Model calculations using this reaction rate yielded N 2 + and NO + concentrations in excess of those measured by the Atmosphere Explorer (AE) satellites. It was deduced that a smaller value of ∼ 1 × 10 −10 cm³ s −1 was necessary in order to provide agreement between calculated and measured values of N 2 + and NO + . Since the laboratory measurements were performed at high ion energies, it was suggested that the charge exchange of O + (² D ) with N 2 proceeded faster in the laboratory than in the thermosphere where only thermal energies are acquired by the ions. However, recent laboratory studies by two independent groups have confirmed a value of 8 ± 3×10 −10 cm³ s −1 for the rate coefficient of the charge exchange of O + (² D ) with N 2 at thermal energies. In this paper we draw the attention to the fact that the resonant reaction O + (² D ) + N 2 ( X ¹Σ g + ) υ = 0 ⇌ N 2 + ( X ²Σ g + ) υ = 5 + O(³ P ) has not been considered in previous studies of N 2 + chemistry in the ionosphere. We show here that the inclusion of the back reaction as a new sink for N 2 + resolves the discrepancy between the laboratory and aeronomically deduced results.