Metastable ²Datomic nitrogen in the mid-latitude nocturnal ionosphere

At night at mid-latitudes the sources of N(²D) in the F region are reduced to the dissociative recombination of NO+. The only important sinks are quenching by atomic oxygen and electrons. As a result, a photochemical study made in the nighttime is greatly simplified in comparison with one made in the daytime. At night, however, the optical emission at 5200 Å resulting from the transition from N(²D) to N(4S) is difficult to measure because of the low intensities. By combining ground-based measurements of I 5200 with coincident satellite observations of neutral and ion densities and temperatures and the shape of the 5200 Å emission profile, we are able to relate the rate of quenching by atomic oxygen and the efficiency of production of N(²D) to the rate of quenching by electrons. Assuming a rate of 1.0 × 10−9 (Te/300)0.5 cm³ s−1 for quenching by electrons, we obtain 1.5–2.5 × 10−12 cm³ s−1 for quenching by atomic oxygen and an efficiency of (0.8–1.0) ± 30% for the production of N(²D) by dissociative recombination of NO+. The recent laboratory determination of the rate coefficient for the recombination of NO+ of 4.3 × 10−7(Te/300)−0.37 cm³ s−1 is found to be in good agreement with the measured ion densities.